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February 2019 Vol. 62 No. 2: 029506https://doi.org/10.1007/s11433-017-9186-1

c⃝ Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018 phys.scichina.com link.springer.com

. Invited Review .Special Issue: The X-ray Timing and Polarimetry Frontier with eXTP

Observatory science with eXTP

Jean J. M. in ’t Zand1*, Enrico Bozzo2, JinLu Qu3, Xiang-Dong Li4, Lorenzo Amati5, Yang Chen4,Immacolata Donnarumma6,7, Victor Doroshenko8, Stephen A. Drake9, Margarita Hernanz10,

Peter A. Jenke11, Thomas J. Maccarone12, Simin Mahmoodifar9, Domitilla de Martino13,Alessandra De Rosa7, Elena M. Rossi14, Antonia Rowlinson15,16, Gloria Sala17, Giulia Stratta18,

Thomas M. Tauris19, Joern Wilms20, XueFeng Wu21, Ping Zhou15,4, Ivan Agudo22, Diego Altamirano23,Jean-Luc Atteia24, Nils A. Andersson25, M. Cristina Baglio26, David R. Ballantyne27, Altan Baykal28,

Ehud Behar29, Tomaso Belloni30, Sudip Bhattacharyya31, Stefano Bianchi32, Anna Bilous15, Pere Blay33,Joao Braga34, Søren Brandt35, Edward F. Brown36, Niccolo Bucciantini37, Luciano Burderi38,

Edward M. Cackett39, Riccardo Campana5, Sergio Campana30, Piergiorgio Casella40, Yuri Cavecchi41,25,Frank Chambers15, Liang Chen42, Yu-Peng Chen3, Jerome Chenevez35, Maria Chernyakova43,

ChiChuan Jin44, Riccardo Ciolfi45,46, Elisa Costantini1,15, Andrew Cumming47, Antonino D’Aı48,Zi-Gao Dai4, Filippo D’Ammando49, Massimiliano De Pasquale50, Nathalie Degenaar15,

Melania Del Santo48, Valerio D’Elia40, Tiziana Di Salvo51, Gerry Doyle52, Maurizio Falanga53,XiLong Fan54,55, Robert D. Ferdman56, Marco Feroci7, Federico Fraschetti57, Duncan K. Galloway58,

Angelo F. Gambino51, Poshak Gandhi59, MingYu Ge3, Bruce Gendre60, Ramandeep Gill61, Diego Gotz62,Christian Gouiffes62, Paola Grandi5, Jonathan Granot61, Manuel Gudel63, Alexander Heger58,64,121,Craig O. Heinke65, Jeroen Homan66,1, Rosario Iaria51, Kazushi Iwasawa67, Luca Izzo68, Long Ji8,

Peter G. Jonker1,69, Jordi Jose17, Jelle S. Kaastra1, Emrah Kalemci70, Oleg Kargaltsev71,Nobuyuki Kawai72, Laurens Keek73, Stefanie Komossa19, Ingo Kreykenbohm20, Lucien Kuiper1,

Devaky Kunneriath74, Gang Li3, En-Wei Liang75, Manuel Linares17, Francesco Longo76,FangJun Lu3, Alexander A. Lutovinov77, Denys Malyshev8, Julien Malzac78, Antonios Manousakis79,

Ian McHardy59, Missagh Mehdipour1, YunPeng Men80, Mariano Mendez81, Roberto P. Mignani82,Romana Mikusincova83, M. Coleman Miller84, Giovanni Miniutti85, Christian Motch86, Joonas Nattila87,

Emanuele Nardini88, Torsten Neubert35, Paul T. O’Brien89, Mauro Orlandini5, Julian P. Osborne89,Luigi Pacciani7, Stephane Paltani2, Maurizio Paolillo90, Iossif E. Papadakis91, Biswajit Paul92,

Alberto Pellizzoni93, Uria Peretz29, Miguel A. Perez Torres94, Emanuele Perinati8,Chanda Prescod-Weinstein95, Pablo Reig96, Alessandro Riggio38, Jerome Rodriguez62,

Pablo Rodrıguez-Gil33,97, Patrizia Romano30, Agata Rozanska98, Takanori Sakamoto99, Tuomo Salmi87,Ruben Salvaterra82, Andrea Sanna38, Andrea Santangelo8, Tuomas Savolainen100,101, Stephane Schanne62,

Hendrik Schatz102, LiJing Shao19, Andy Shearer103, Steven N. Shore104,105, Ben W. Stappers106,Tod E. Strohmayer9, Valery F. Suleimanov8, Jirı Svoboda74, F.-K. Thielemann107, Francesco Tombesi9,

* Corresponding author (email: [email protected])

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https://doi.org/10.1007/s11433-017-9186-1

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J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-2

Diego F. Torres10, Eleonora Torresi5, Sara Turriziani108, Andrea Vacchi109,110, Stefano Vercellone30,Jacco Vink15, Jian-Min Wang3, JunFeng Wang111, Anna L. Watts15, ShanShan Weng112,

Nevin N. Weinberg113, Peter J. Wheatley114, Rudy Wijnands15, Tyrone E. Woods58, Stan E. Woosley115,ShaoLin Xiong3, YuPeng Xu3, Zhen Yan42, George Younes116, WenFei Yu42, Feng Yuan42,

Luca Zampieri117, Silvia Zane118, Andrzej A. Zdziarski79, Shuang-Nan Zhang3, Shu Zhang3,Shuo Zhang119, Xiao Zhang4, and Michael Zingale120

1 SRON Netherlands Institute for Space Research, Utrecht 3584 CA, The Netherlands;2 Department of Astronomy, University of Geneva, Versoix 1290, Switzerland;

3 Key Laboratory for Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China;4 School of Astronomy and Space Science, Nanjing University, Nanjing 210023, China;

5 INAF-IASF Bologna, via P. Gobetti 101, Bologna 40129, Italy;6 ASI, Via del Politecnico snc, Roma 00133, Italy;

7 INAF-Istituto di Astrofisica e Planetologie Spaziali, Roma 00133, Italy;8 Institut fur Astronomie und Astrophysik Tubingen, Universitat Tubingen, Tubingen 72076, Germany;

9 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA;10 ICREA & Institute of Space Sciences (IEEC-CSIC), Campus UAB, Carrer de Can Magrans, Barcelona 08193, Spain;

11 CSPAR, SPA University of Alabama in Huntsville, Huntsville, AL 35805, USA;12 Department of Physics, Texas Tech University, Lubbock, TX 79409, USA;

13 INAF Osservatorio Astronomico di Capodimonte, Napoli 80131, Italy;14 Leiden Observatory, Leiden University, Leiden 2300 RA, The Netherlands;

15 Anton Pannekoek Institute, University of Amsterdam, Amsterdam 1098 XH, The Netherlands;16 ASTRON Netherlands Institute for Radio Astronomy, Dwingeloo 7990 AA, The Netherlands;

17 Universitat Politecnica de Catalunya (UPC-IEEC), Barcelona 08034, Spain;18 Universita degli Studi di Urbino ‘Carlo Bo’, Urbino 61029, Italy;19 Max-Planck-Institut fur Radioastronomie, Bonn 53121, Germany;

20 Dr. Karl-Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics (ECAP),Friedrich Alexander Universitat Erlangen-Nurnberg, Bamberg 96049, Germany;

21 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China;22 Instituto de Astrofysıca de Andalucia - CSIC, Granada 18008, Spain;

23 Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK;24 IRAP, Universite de Toulouse, CNRS, UPS, CNES, Toulouse 31028, France;

25 Mathematical Sciences and STAG Research Centre, University of Southampton, Southampton SO17 1BJ, UK;26 New York University Abu Dhabi, Abu Dhabi, UAE;

27 Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA;28 Physics Department, Middle East Technical University, Ankara 06531, Turkey;

29 Department of Physics, Technion-Israel Institute of Technology, Haifa 32000, Israel;30 INAF Osservatorio Astronomico di Brera, Merate 23807 (LC), Italy;

31 Tata Institute of Fundamental Research, Mumbai 400005, India;32 Dipartimento di Matematica e Fisica, Universita degli Studi Roma Tre, Roma 00146, Italy;

33 Instituto de Astrofısica de Canarias, Tenerife 38205, Spain;34 Instituto Nacional de Pesquisas Espaciais - INPE, S.J. Campos-SP 12227-010, Brazil;

35 National Space Institute, Technical University of Denmark (DTU Space), 2800 Kgs. Lyngby, Denmark;36 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA;

37 INAF Osservatorio Astrofisico di Arcetri, Firenze 50125, Italy;38 Dipartimento di Fisica, Universita degli Studi di Cagliari, Monserrato 09042, Italy;

39 Department of Physics and Astronomy, Wayne State University, Detroit, MI 48201, USA;40 INAF Osservatorio Astronomico di Roma, Roma 00078, Italy;

41 Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA;42 Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences,

Shanghai 200030, China;43 School of Physical Sciences, Dublin City University, Dublin 9, Ireland;

44 Max-Planck-Institut fur Extraterrestrische Physik, Giessenbachstrasse, Garching 85748, Germany;45 INAF, Osservatorio Astronomico di Padova, Padova 35122, Italy;

46 INFN-TIFPA, Trento Institute for Fundamental Physics and Applications, Trento 38123, Italy;47 Department of Physics and McGill Space Institute, McGill University, Montreal, QC H3A 2T8, Canada;

48 INAF/IASF Palermo, Palermo 90146, Italy;49 INAF - Istituto di Radioastronomia, Bologna 40129, Italy;

50 Department of Astronomy and Space Sciences, Istanbul University, Beyazit 34119, Turkey;


51 Universita degli Studi di Palermo, Dipartimento di Fisica e Chimica, Palermo 90123, Italy;52 Armagh Observatory, Armagh BT61 9DG, N. Ireland;

53 International Space Science Institute (ISSI), Bern 3012, Switzerland;54 School of Physics and Electronics Information, Hubei University of Education, Wuhan 430205, China;

55 SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK;56 Faculty of Science, University of East Anglia, Norwich NR4 7TJ, UK;

57 Department of Planetary Sciences and Astronomy, University of Arizona, Tucson, AZ 85721, USA;58 School of Physics and Astronomy, Monash University, Clayton VIC 3800, Australia;

59 Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK;60 University of the Virgin Islands, Virgin Islands 00802-9990, USA;

61 Department of Natural Sciences, The Open University of Israel, Raanana 4353701, Israel;62 CEA Paris-Saclay, DRF/IRFU/Departement d’Astrophysique, Laboratoire AIM (UMR 7158), Gif sur Yvette 91191, France;

63 Department of Astrophysics, University of Vienna, Vienna 1180, Austria;64 School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA;

65 Department of Physics, University of Alberta, Edmonton AB T6G 2E1, Canada;66 Eureka Scientific, Inc., Oakland, CA 94602, USA;

67 ICREA & Institut de Ciencies del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Barcelona 08028, Spain;68 Instituto de Astrofisica de Andalucia (IAA-CSIC), Granada 18008, Spain;

69 Department of Astrophysics, IMAPP, Radboud University, Nijmegen 6500 GL, The Netherlands;70 Faculty of Engineering and Natural Sciences, Sabanci University, Orhanli-Tuzla, Istanbul 34956, Turkey;

71 Department of Physics, The George Washington University, Washington, DC 20052, USA;72 Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan;

73 Department of Astronomy, University of Maryland, Park, MD 20771, USA;74 Astronomical Institute, Academy of Sciences, Prague 14131, Czech Republic;

75 GXU-NAOC Center for Astrophysics and Space Sciences, Department of Physics, Guangxi University, Nanning 530004, China;76 Dip. di Fisica, Universita di Trieste and INFN, Trieste 34127, Italy;

77 Space Research Institute of the Russian Academy of Sciences, Moscow 117997, Russia;78 IRAP, Universite de Toulouse, CNRS, UPS, CNES, Toulouse 31028, France;

79 Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Warszawa 00-716, Poland;80 Department of Astronomy, School of Physics, Peking University, Beijing 100871, China;

81 Kapteyn Astronomical Institute, University of Groningen, Groningen 9700 AV, The Netherlands;82 INAF Istituto di Astrofisica Spaziale e Fisica Cosmica, Milano 20133, Italy;

83 Institute of Theoretical Physics, Faculty of Mathematics and Physics, Charles University, Praha 180 00, Czech Republic;84 Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, Park, MD 20742-2421, USA;

85 Centro de Astrobiologia (CSIC-INTA), Dep. de Astrofsica, ESAC, Madrid 28850, Spain;86 Universite de Strasbourg, CNRS, Observatoire Astronomique, UMR 7550, Strasbourg 67000, France;

87 Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Piikkio 21500, Finland;88 INAF Osservatorio Astrofisico di Arcetri, Firenze 50125, Italy;

89 Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK;90 Dipartimento di Fisica Ettore Pancini, Universita di Napoli Federico II, via Cintia, Napoli 80126, Italy;

91 Department of Physics and Institute of Theoretical and Computational Physics, University of Crete, Heraklion 71003, Greece;92 Raman Research Institute, Sadashivnagar, Bangalore 560080, India;93 INAF, Osservatorio Astronomico di Cagliari, Selargius 09047, Italy;94 Instituto de Astrofısica de Andalucıa - CSIC, Granada 18008, Spain;

95 Department of Physics, University of Washington, Seattle, WA 98195-1560, USA;96 IESL, Foundation for Research and Technology-Hellas, Heraklion 71110, Greece;

97 Departamento de Astrofısica, Universidad de La Laguna, La Laguna 38206, Spain;98 N. Copernicus Astronomical Center, Warsaw 00-716, Poland;

99 Department of Physics and Mathematics, Aoyama Gakuin University, Sagamihara, Kanagawa 252-5258, Japan;100 Aalto University Metsahovi Radio Observatory, Kylmala 02540, Finland;

101 Aalto University Department of Electronics and Nanoengineering, Aalto 00076, Finland;102 National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA;

103 Centre for Astronomy, National University of Ireland, Galway H91 TK33, Ireland;104 Dipartimento di Fisica ‘Enrico Fermi’, University of Pisa, Pisa 56127, Italy;

105 INFN Sezione di Pisa, Pisa 56127, Italy;106 Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK;

107 Department of Physics, University of Basel, Basel 4056, Switzerland;108 Computational Astrophysics Laboratory - RIKEN, Saitama 351-0198, Japan;

109 Department of Mathematics, Computer Science and Physics, University of Udine, Udine 33100, Italy;110 INFN Italian National Institute for Nuclear Physics, Trieste 34012, Italy;

111 Department of Astronomy and Institute of Theoretical Physics and Astrophysics, Xiamen University, Xiamen 361005, China;112 Department of Physics and Institute of Theoretical Physics, Nanjing Normal University, Nanjing 210023, China;

113 Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA;114 Department of Physics, University of Warwick, Coventry CV4 7AL, UK;

115 Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA;


116 Department of Physics, The George Washington University, Washington, DC 20052, USA;117 INAF Astronomical observatory of Padova, Padova 35122, Italy;

118 Mullard Space Science Laboratory, University College London, Surrey RH5 6NT, UK;119 MIT Kavli Institute for Astrophysics and Space Research, Cambridge, MA 02139, USA;

120 Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA;121 Astronomy Department, Shanghai Jiao Tong University, Shanghai 200240, China

Received December 6, 2017; accepted February 9, 2018; published online August 30, 2018

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related toObservatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-raybinaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellentlysuited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium ledby the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in themid 2020s.

space research instruments, nuclear astrophysics, flare stars, accretion and accretion disks, mass loss and stellar winds,cataclysmic binaries, X-ray binaries, supernova remnants, active galactic nuclei, X-ray bursts, gamma-ray bursts, gravi-tational wavesPACS number(s): 07.87.+v, 26.30.ca, 97.30.Nr, 97.10.Gz, 97.10.Me, 97.30.Qt, 97.80.Jp, 98.38.Mz, 98.54.Cm, 98.70.Qy,98.70.rz, 04.30.Db

Citation: J. J. M. in’t Zand, E. Bozzo, J. L. Qu, X.-D. Li, L. Amati, Y. Chen, I. Donnarumma, V. Doroshenko, S. A. Drake, M. Hernanz, P. A. Jenke, T.J. Maccarone, S. Mahmoodifar, D. de Martino, A. De Rosa, E. M. Rossi, A. Rowlinson, G. Sala, G. Stratta, T. M. Tauris, J. Wilms, X. F. Wu, P.Zhou, I. Agudo, D. Altamirano, J.-L. Atteia, N. A. Andersson, M. C. Baglio, D. R. Ballantyne, A. Baykal, E. Behar, T. Belloni, S. Bhattacharyya,S. Bianchi, A. Bilous, P. Blay, J. Braga, S. Brandt, E. F. Brown, N. Bucciantini, L. Burderi, E. M. Cackett, R. Campana, S. Campana, P. Casella, Y.Cavecchi, F. Chambers, L. Chen, Y.-P. Chen, J. Chenevez, M. Chernyakova, C. C Jin, R. Ciolfi, E. Costantini, A. Cumming, A. D’Aı, Z.-G. Dai, F.D’Ammando, M. De Pasquale, N. Degenaar, M. Del Santo, V. D’Elia, T. Di Salvo, G. Doyle, M. Falanga, X. L. Fan, R. D. Ferdman, M. Feroci, F.Fraschetti, D. K. Galloway, A. F. Gambino, P. Gandhi, M. Y. Ge, B. Gendre, R. Gill, D. Gotz, C. Gouiffes, P. Grandi, J. Granot, M. Gudel, A. Heger,C. O. Heinke, J. Homan, R. Iaria, K. Iwasawa, L. Izzo, L. Ji, P. G. Jonker, J. Jose, J. S. Kaastra, E. Kalemci, O. Kargaltsev, N. Kawai, L. Keek, S.Komossa, I. Kreykenbohm, L. Kuiper, D. Kunneriath, G. Li, E.-W. Liang, M. Linares, F. Longo, F. J. Lu, A. A. Lutovinov, D. Malyshev, J. Malzac,A. Manousakis, I. McHardy, M. Mehdipour, Y. P Men, M. Mendez, R. P. Mignani, R. Mikusincova, M. C. Miller, G. Miniutti, C. Motch, J. Nattila,E. Nardini, T. Neubert, P. T. O’Brien, M. Orlandini, J. P. Osborne, L. Pacciani, S. Paltani, M. Paolillo, I. E. Papadakis, B. Paul, A. Pellizzoni, U.Peretz, M. A. P. Torres, E. Perinati, C. Prescod-Weinstein, P. Reig, A. Riggio, J. Rodriguez, P. Rodrıguez-Gil, P. Romano, A. Rozanska, T. Sakamoto,T. Salmi, R. Salvaterra, A. Sanna, A. Santangelo, T. Savolainen, S. Schanne, H. Schatz, L. J. Shao, A. Shearer, S. N. Shore, B. W. Stappers, T. E.Strohmayer, V. F. Suleimanov, J. Svoboda, F.-K. Thielemann, F. Tombesi, D. F. Torres, E. Torresi, S. Turriziani, A. Vacchi, S. Vercellone, J. Vink,J.-M Wang, J. F Wang, A. L. Watts, S. S. Weng, N. N. Weinberg, P. J. Wheatley, R. Wijnands, T. E. Woods, S. E. Woosley, S. L. Xiong, Y. P. Xu, Z.Yan, G. Younes, W. F. Yu, F. Yuan, L. Zampieri, S. Zane, A. A. Zdziarski, S.-N. Zhang, S. Zhang, S. Zhang, X. Zhang, and M. Zingale, Observatoryscience with eXTP, Sci. China-Phys. Mech. Astron. 62, 029506 (2019), https://doi.org/10.1007/s11433-017-9186-1

Contents

1 Introduction 029506-5

2 Flare stars 029506-7

3 Supernova remnants 029506-8

4 Accreting white dwarf binaries 029506-10

5 Binary evolution 029506-12

https://doi.org/10.1007/s11433-017-9186-1


6 Thermonuclear X-ray bursts 029506-14

7 High-mass X-ray binaries 029506-18

8 Radio-quiet active galactic nuclei 029506-21

9 Radio-loud active galactic nuclei 029506-24

10 Tidal disruption events 029506-26

11 Gamma-ray bursts and supernovae 029506-28

12 Objects of uncertain nature 029506-31

12.1 Ultraluminous X-ray sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 029506-31

12.2 Fast radio bursts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 029506-32

1 Introduction

The enhanced X-ray Timing and Polarimetry mission (eXTP)is a mission concept proposed by a consortium led by the In-stitute of High-Energy Physics of the Chinese Academy ofSciences and envisaged for a launch in the mid 2020s. It car-ries 4 instrument packages for the 0.5-30 keV bandpass, withthe primary purpose to study conditions of extreme density,gravity and magnetism in and around compact objects in theuniverse. The mission concept provides for a low-Earth orbitwith a low inclination (< 15◦), incurring earth obscurationsevery one and a half hour for targets near the celestial equatorsuch as the Galactic center region. The scientific payload ofeXTP consists of: the Spectroscopic Focusing Array (SFA),the Polarimetry Focusing Array (PFA), the Large Area De-tector (LAD) and the Wide Field Monitor (WFM).

The eXTP instrumentation is discussed in detail in ref. [1],which includes a comparison with other instruments. Wesummarize as follows. The SFA is an array of nine identi-cal X-ray telescopes covering the energy range 0.5-20 keVwith a spectral resolution of better than 180 eV (full width athalf maximum, FWHM) at 6 keV, and featuring a total effec-tive area from ∼0.5 m2 at 6 keV to ∼0.7 m2 at 1 keV. The SFAangular resolution is required to be less than 1′ (half-powerdiameter, HPD) and its sensitivity reaches 10−14 erg s−1 cm−2

(0.5-20 keV) for an exposure time of 104 ks. In the currentbaseline, the SFA focal plane detectors are silicon drift de-tectors that combine CCD-like spectral resolutions with verysmall dead times, and therefore are excellently suited forstudies of the brightest cosmic X-ray sources at the smallesttime scales.

The PFA consists of four identical X-ray telescopes thatare sensitive between 2 and 8 keV, have an angular resolution

better than 30′′ (HPD) and a total effective area of ∼900 cm2

at 2 keV (including the detector efficiency). The PFA fea-tures Gas Pixel Detectors (GPDs) to allow polarization mea-surements in the X-rays. It reaches a minimum detectablepolarization (MDP) of 5% in 100 ks for a source with a Crab-like spectrum of flux 3 × 10−11 erg s−1 cm−2. The spectralresolution is 1.1 keV at 6 keV (FWHM).

The LAD has a very large effective area of ∼3.4 m2 at8 keV, obtained with non-imaging silicon drift detectors, ac-tive between 2 and 30 keV with a spectral resolution ofabout 260 eV and collimated to a field of view of 1 degree(FWHM). The LAD and the SFA together reach an unprece-dented total effective area of more than 4 m2, compared to0.7 m2 for the largest flown instrument (Proportional CounterArray on the Rossi X-ray Timing Explorer [2]).

The science payload is completed by the WFM, consist-ing of 6 coded-mask cameras covering 4 sr of the sky at asensitivity of 4 mCrab for an exposure time of 1 d in the2 to 50 keV energy range, and for a typical sensitivity of0.2 mCrab combining 1 yr of observations outside the Galac-tic plane. The instrument will feature an angular resolution ofa few arcminutes and will be endowed with an energy reso-lution of about 300 eV (FWHM). The WFM pairs the largestduty cycle of 30% for an imaging monitoring 2-30 keV de-vice (e.g., compared to 8% for BeppoSAX-WFC [3]) withthe largest spectral resolution (usually about 1 keV at 6 keV).

The baseline for the observatory response time to targetsof opportunity within the 50% part of the sky accessible toeXTP is 4-8 h. Depending on the the outcome of missionstudies, this may improve to 1-3 h. The launch is currentlyforeseen in the mid 2020s.

With a uniquely high throughput, good spectral resolution,polarimetric capability and wide sky coverage, eXTP is anobservatory very well suited for a variety of studies comple-


menting the core science of strong gravity [4], dense matter[5] and strong magnetism [6]. The SFA will provide highsensitivity in a soft but wide bandpass, the PFA will pro-vide X-ray polarimetry capability, the LAD will provide thebest timing and spectroscopic studies ever for a wide rangeof high energy sources brighter than ∼1 mCrab in the 2 to30 keV band, and the WFM, with its unprecedented com-bination of field of view and imaging down to 2 keV, makeseXTP a discovery machine of the variable and transient X-raysky. The WFM will reveal many new sources that need to befollowed up with the SFA, PFA, LAD and other facilities. Ex-perience from the past decades shows that newly discovered,unforeseen types of sources will provide unexpected insightsinto fundamental questions. The WFM will also be moni-toring daily hundreds of sources, to catch unexpected eventsand provide long-term records of their variability and spectro-scopic evolution. Several targets of the eXTP core program(e.g., low-mass X-ray binaries) allow one to address scien-tific questions beyond those related to the key mission sciencegoals. Thus, they can be used to fulfill the observatory sci-ence goals without requiring additional exposure time. Other

targets are instead uniquely observed as part of the observa-tory science program (e.g., accreting white dwarfs, blazars,high mass X-ray binaries), in turn providing useful compara-tive insights for the core science objectives.

eXTP will be a unique, powerful X-ray partner of othernew large-scale multi-messenger facilities across the spec-trum likely available in the 2020s, such as advanced grav-itational wave (GW) and neutrino experiments (AdvancedLIGO and Virgo, KAGRA, LIGO-India, Einstein Telescopein GWs and KM3NeT and Icecube in neutrinos), SKA andpathfinders in the radio, LSST and E-ELT in the optical, andCTA at TeV energies (Figure 1). First eXTP and 4 years laterAthena will be the major facilities in the X-ray regime andas such cover the hot and energetic non-thermal phenom-ena of any cosmic object, Athena focusing on diffuse con-stituents in the early Universe and eXTP focusing on bright,variable and transient phenomena. eXTP will be particularlysuitable for electromagnetic (EM) studies of GW sources, anexciting field of research whose birth we witnessed throughGW170817/GRB170817A [7, 8]. The combined EM/GWsignal is expected to provide us new insights of neutron stars

KM3NeTIceCube/PINGU

Einstein TelescopeLIGO-India

KAGRAAdvanced LIGO/Virgo

CTAH.E.S.S.

E-ELTLSST

VLTAPEXALMA

SKAIRAM/NOEMA

FASTMEERKAT

ASKAP/MWALOFAR

M5M4

Lomonosov/MVL-300DAMPE

FermiINTEGRAL

AthenaeXTP

XARMASO-S

Einstein ProbeSVOMIXPE

Kanazawa-Sat3Spectrum-X-Gamma

MVNInsight-HXMT

NICERPOLAR

ASTROSATNuSTAR

MAXISwift

ChandraXMM-Newton

PLATOEuclid

Solar OrbiterTESS

CHEOPSGaia

Kepler/K2HST

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Figure 1 Multi-wavelength and multi-messenger facilities relevant to eXTP with timelines as known in January 2018. The thin line separates space-based(top) from ground-based (bottom) facilities. Colors indicate similar wavebands from the radio (brown) via IR (red) and optical (green) to X-rays (blue) andgamma rays (purple). Grey bands: gravitational wave and neutrino detectors. Dark colors: funded lifetime. Light colors: expected lifetime, where known,independent of funding decisions. Missions might last longer.


through their merger events, see sect. 11 and ref. [5]. SteadyGW sources related to spinning neutron stars and ultracom-pact X-ray binaries may also be detected and identified in thecoming decades, providing new diagnostics for these multi-wavelength sources that are particularly bright in the radioand X-ray bands, see sect. 5 and ref. [5].

To explain the science case for eXTP as an observatoryin a kaleidoscopic manner in this White Paper, material isdrawn from eleven White Papers that were written for theLOFT study by scientists from the community at large [9-19].LOFT was an ESA mission concept with two of the three pri-mary science goals of eXTP and larger versions of the LADand WFM. The outstanding capabilities of all four eXTP in-struments will enable to answer key questions that could notbe addressed for many years, due to a lack of facilities withsufficient sensitivity. These questions, across a broad swathof topics, will not be answered by any other planned futureX-ray facility. The eXTP science involves a wide range of ob-jects, from normal stars to supermassive black holes in othergalaxies. These objects have a wide range of fluxes. Thewide range in capabilities of the instrument package allowsone to deal with this range in fluxes: bright sources with theLAD, WFM, and PFA; faint sources mainly with the SFA.In this White Paper, the main questions are briefly discussedper category of objects, in order of approximate distance, thatcan be addressed with eXTP and that are not dealt with in theother eXTP White Papers on strong gravity, dense matter andstrong magnetism. Due to the breadth of the science, thispaper can only summarize those questions.

2 Flare stars

On the Sun, flares occur in close proximity to active regionsthat are characterized by boosted localized kG-strength mag-netic fields [20-23]. Loops from these regions extend into thesolar corona and, as the footpoints of these loops are jostledby solar convective motions, they are twisted and distorteduntil magnetic reconnection occurs. There is then a suddenrelease of energy, resulting in the acceleration of electronsand ions in these loops up to MeV energies, emitting non-thermal radio (gyrosynchrotron) and non-thermal hard X-ray emission (bremsstrahlung and Compton back scattering).These energetic particles both stream away from the Sun anddown the loops where they deposit substantial amounts of en-ergy to the lower solar atmosphere (the chromosphere), pro-ducing the observed intense hard X-ray emission at the loopfootpoints [23,24]. Flares are also observed on other late-typestars, with luminosities up to 1033 erg s−1 and durations fromless than an hour to several days or longer [25, 26]. The hightemperatures result in spectral peaks in the soft to hard X-rayregime, which make them particularly visible with wide-field

high duty cycle monitoring devices such as the WFM.eXTP can contribute greatly to our understanding of stellar

flares. Expanding our knowledge of stellar flaring is crucialto examining the influence of transient sources of ionizing ra-diation from a host star on exoplanet systems, important forhabitability concerns and space weather which other worldsmight experience (e.g., ref. [27]). Such studies extend thesolar-stellar connection in determining the extent to whichsolar models for conversion of magnetic energy into plasmaheating, particle acceleration and mass motions apply to themore energetic stellar flares. Extrapolating from lower ef-ficiency high-energy monitoring observatories, we estimatethat eXTP will detect at least 35 superflares per year fromstars exhibiting the extremes of magnetic activity. The antic-ipated source types include young stars, fully convective Mdwarfs, and tidally locked active binary systems. The wide-field monitoring capability of eXTP will also likely enablethe detection of flares from classes of stars hitherto not sys-tematically studied for their flaring, and will be important forexpanding our understanding of plasma physics processes innon-degenerate stellar environments. Four important ques-tions that eXTP can address are:• What are the properties of the non-thermal particles re-

sponsible for the initial flare energy input? The study of theflare distribution with energy and of their temporal behav-ior, e.g., spikes, oscillations, etc., will be enabled by eXTP’ssensitive hard X-ray capability provided by the LAD, far inexcess of previous missions.• What are the physical conditions of the thermal plasma

whose emission dominates the later stages of stellar flares?Through its soft X-ray capabilities, eXTP will enable us tostudy the variation of properties such as the temperature andelemental abundances as a function of time during flares.• What is the maximum energy that stellar flares can

reach? The wide-field, broad X-ray band and high-sensitivitycapabilities of eXTP will enable us to detect the rare “super-flares” that have been only sporadically detected by previousand current missions: better understanding of the prevalenceof such powerful events is important both for the physics ofstellar flares and for the implications of their effect on poten-tial planets orbiting these stars (see Figure 2).• What is the origin of flares from stars in nearby young

associations? eXTP sensitivity will allow one to detect flaresfrom stars in nearby young associations, such as the TW Hyagroup, at distances up to 60 pc. As discussed by ref. [23],young stars with accretion disks like TW Hya are expectedto have a second type of flares due to interactions betweenthe magnetic fields of the active regions and the disk. It is anopen question as to whether these flares have different proper-ties than the better known in situ stellar flares (especially dueto the very limited detections of these events in the X-rays).


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Figure 2 (a) Artist’s impression of the 2014 April 23 “superflare” of one of the stars in the wide M dwarf binary DG CVn (credit Scott Wiessinger of theNASA/GSFC Scientific Visualization Studio). (b) Simulated LAD spectrum of a 1000 s time segment of a flare with similar flux and spectrum to the second,slightly less powerful flare of DG CVn 3 h after the first outburst. Notice the well-exposed He-like Fe lines at 6.7 and 7.9 keV which would enable an accuratecoronal Fe abundance to be derived (e.g., ref. [28]).

3 Supernova remnants

Supernova remnants (SNRs), including pulsar wind nebu-lae (PWNe), are important Galactic producers of relativis-tic particles and emitters of diffuse X-ray synchrotron radia-tion [29, 30]. eXTP is expected to provide breakthroughs insome longstanding problems in the studies of shell SNRs andPWNe via fine measurement of the distribution of magneticfields and polarized emission.

The strong shocks driven by supernova explosions arecommonly regarded as the main factory of the cosmic rays(CRs) with energy up to 3 × 1015 eV (the so called “knee”),and accumulating evidence has also pointed to this notion[31, 32]. Despite the well established theory of diffusiveshock acceleration for CRs, many crucial problems in theacceleration process remain unsolved, such as the injectionmechanism of particles and energy, the efficiency of acceler-ation, the size of the acceleration region, and the upper limitof particle energy [33, 34]. Clues to solve the problems canbe provided by the non-thermal X-ray emission of the rela-tivistic electrons arising from the strong shocks of SNRs.

In particular, eXTP will be powerful in addressing the fol-lowing questions about SNRs and PWNe:•What effect does the orientation of injection with respect

to the direction of the magnetic field have on the shock accel-eration efficiency in SNRs? In the acceleration mechanisms,it remains unclear how the orientation of injection with re-spect to the magnetic lines affects the acceleration efficiency(“quasi-parallel scenario” or “quasi-perpendicular scenario”[35]). SN1006, a SNR with a bilateral-like non-thermal X-ray morphology, is an ideal example to study this problem.The most recent radio polarization observation favors the

case of quasi-parallel injection [36]. eXTP can resolve theX-ray structures in the SNR (30’ in diameter) and providespatially resolved physical information for them. The PFApolarimeter can measure the polarization degree and anglein the post-shock region of SN1006 with 15” angular res-olution (see Figure 3(a)). The measurement accuracy willbe about 2% for the polarization degree (σdop; assuming apolarization degree of 17%) and 3◦ for the position angle(σPA) after a 0.5 Ms observation of filaments 1 and 2. With a10 ks observation, the SFA can obtain spatially resolved spec-tra of the non-thermal emission of the shell (see Figure 3(b)for the simulated spectra of “filament 1” in the northeast ofthe SNR). The eXTP polarization measurement and spectralanalysis are thus expected to give a precise answer to a spe-cific physical problem.

•What is the intensity of the synchrotron radiation and theorientation of the magnetic field in the forward shock and, es-pecially, the reverse shock in SNRs? While CR accelerationhas been extensively studied at the forward shocks of SNRs,how the reverse shocks accelerate CRs became an intriguingquestion in recent years. Using a 1 Ms Chandra observa-tion towards Cas A, most of the non-thermal emission fromthe SNR was found in the reverse shock [37, 38]. eXTP willbe able to test this result and compare the polarization pa-rameters between the forward and reverse shocks. With anexposure time of 2 Ms, the PFA polarimeter can measure thepolarization degree and angle of 0.5′×1′ sub-structures in CasA (4.2-6 keV) with accuracies of σdop∼3% and σPA∼10◦ forthe forward shock (assuming a polarization degree of 10%),and σdop∼1.5% and σPA∼8◦ for the reverse shock (assuminga polarization degree of 5%). The spatially resolved polariza-tion measurements with eXTP will provide crucial informa-


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tion on the magnetic field at the reverse shock where CRs areeffectively accelerated.• What is the spatial distribution of the non-thermal X-

ray emitting components with respect to that of the ther-mally emitting regions in SNRs? Since the first detection inSN1006 [39], non-thermal X-ray emission has been detectedat the rims of several SNRs. This is believed to be due tosynchrotron radiation of the CR electrons accelerated by theSNR shocks (e.g., ref. [40]).

In addition to the non-thermal X-ray dominant SNRs (suchas SN1006 and RX J1713-3946) and a few young SNRs (suchas Cas A and Tycho) in which non-thermal and thermal X-rayemission can be spatially/spectroscopically resolved, thereare several SNRs in which potential non-thermal X-ray emis-sion can hardly be distinguished from the thermal emission.These SNRs include N132D, G32.4+0.1, G28.6-0.1, CTB37B, W28, G156.2+5.7, etc. [41]. Thanks to the large col-lecting area of eXTP in the hard X-ray band, short exposuresof SFA towards these SNRs can collect enough photons forspectral analysis to clarify whether the hard X-ray emissionis non-thermal. The enlarged sample of non-thermal X-rayemitting SNRs in various environments and at different ageswill allow one to study the mechanism of CR electron accel-eration and to constrain the leptonic emission, accelerationefficiency, magnetic field and other interstellar conditions.•Which are the mechanisms of amplification of the mag-

netic field at the SNR shocks? Several high-energy ob-servations of SNRs are consistent with a magnetic field at

the shock far exceeding the theoretically predicted shock-compressed strength. The joint capability of eXTP to mea-sure X-ray emission and polarization will allow one to testcurrent models of amplification of magnetic fields by SNRshocks. It is believed that such shocks propagate into a gen-erally turbulent interstellar medium [42]. The turbulence canbe pre-existing or caused by the shock itself via, e.g., a non-resonant streaming instability of cosmic-rays as first foundby ref. [43]. In the former case, the density inhomogeneitiestraversed by the shock front are expected to produce corru-gation of the shock surface; such a surface, as it propagatesfurther, triggers differential rotation of over-densities duringtheir shock-crossing generating vorticity, therefore amplify-ing the magnetic field via a small-scale dynamo process in thedownstream fluid [44-46]. The field growth is exponentiallyfast, as short as years or months (e.g., the rapid changes in hotspots brightness observed in the SNR RXJ1713.7-3496 and inCas A [47,48]), likely due to synchrotron electron cooling ina strong magnetic field.

eXTP’s polarimetric capability will be a valuable assethere in the sense that it will provide, for the first time, thepolarization in those hot spots, leading to estimates of the co-herence scale of the magnetic turbulence. This will be of theorder of the Field length (i.e., about 0.01 pc [49]) if the vor-tical mechanism dominates. Indeed, the growth time scale ofthe magnetic turbulence is approximately equal to the ratio ofthe Field length to the local shock speed. As described above,a 1 Ms exposure on Cas A would allow one to measure the po-


larization degree (angle) with an accuracy better than 1% (3◦)for the forward shock. The proposed analysis requires rela-tively high SNR shock speeds (not too high to have a growthtime at least of a few months) colliding with dense cloudsor propagating into a region with significant clumpiness. Byobserving the X-ray emission during the early phases of theshock/cloud collision, eXTP will allow one to constrain thegrowth time scale of the magnetic field.•What is the configuration of the magnetic field in PWNe

and what physical insight can we infer from that? PWNe,such as the Crab nebula, are high-energy nebulae composedof relativistic particles and magnetic fields created by the cen-tral pulsars. The energy input of pulsars, the particle accel-eration at the terminal shocks, the PWN structures (includingthe equatorial ring and bipolar jets) and other physical prop-erties (such as particle transport and magnetization factor) arenot fully understood yet (e.g., refs. [50, 51]). eXTP can per-form X-ray polarization measurements towards the Crab neb-ula with high spatial resolution, and obtain the detailed dis-tribution of magnetic field and polarization degree. The PFApolarimeter can measure the polarization degree and angleof the sub-structures (15′′) in the Crab nebula with a meanaccuracy of about 1.7% (σdop, assuming an polarization de-gree of 19%) and 2.6◦ (σPA), respectively (we considered anexposure time of 10 ks). The unprecedented X-ray polariza-tion imaging of the Crab nebula will play an important role inthe study of PWNe. Another PWN worth following up witheXTP is MSH 15-52 which has a size of 10′ and a total flux of∼ 5.5×10−11 erg s−1cm−2. eXTP mapping will provide polar-ization accuracies of σdop=2% and σPA=3.7◦ for 10 pointingswith a 300 ks exposure (assuming a photon index of −2 andpolarization degree of 15%).

4 Accreting white dwarf binaries

White dwarfs (WDs) accreting from a companion star in a bi-nary exhibit themselves as cataclysmic variables (CVs, e.g.,review by ref. [52]). CVs represent the most common endproducts of close binary evolution. These binaries displaya wide range of time scales in variability (from seconds-minutes up to months-years) that make them ideal laborato-ries for the study of accretion/ejection processes in the non-relativistic regime. Disk instabilities, the disk-jet connec-tion and magnetic accretion are common processes occurringin a wide variety of astrophysical objects. Accreting WDsprovide an excellent perspective of these phenomena. Fur-thermore, thermonuclear runaways giving rise to novae (e.g.,review by ref. [53]) have the potential to probe the accre-tion/ignition mechanisms, outflows and subsequent chemicalenrichment of the interstellar medium, as well as to address

the question of whether accretion dominates over outflow,possibly bringing the WD mass over the Chandrasekhar limityielding type Ia supernovae. This is a key aspect for under-standing Type Ia supernova progenitors. In addition, popu-lation studies of galactic X-ray sources have recently recog-nized a crucial role of accreting WD binaries in the origin ofthe Galactic Ridge X-ray Emission (GRXE), one of the greatmysteries in X-rays [54-58].

The X-ray domain is crucial to understand the physicalconditions close to the compact star (e.g., review by ref.[59]). Progress in many open questions is hampered by thelow quiescent luminosities (LX∼1029-33 erg s−1) and the un-predictability of large-scale variations that can reach lumi-nosities as low as ∼1032-1034 erg s−1 in dwarf novae (DNe)outbursts and as high as ∼1038 erg s−1 in nova explosions.The majority of persistent, steadily nuclear-burning accretingwhite dwarfs thought to exist in the Milky Way, its satellites,and M 31 have yet to be detected [60, 61]. These “supersoftX-ray sources” are candidate Type Ia supernova progenitors,and can exhibit oscillations on very short timescales (. 100 s)for as yet unknown reasons [62].

The unique combination in eXTP of large effective area,fair spectral resolution and excellent sensitivity of the LADand SFA instruments, as well as the wide field of the WFMinstrument, can uncover the wide range of variabilities to un-derstand better the underlying physical processes. The threemain questions that can be addressed are:•How does matter accrete onto white dwarfs and mix with

the CO/CNe substrate? Fast (seconds to minutes) X-ray vari-ability, both periodic and aperiodic, allow one to trace theaccretion flow close to the WD surface. In magnetic CVs,periodic coherent pulsations are a signature of magneticallyfunneled accretion and their spectral study allows one to in-fer the physical conditions of the post-shock region, the hotspot over the WD surface as well as complex absorption frompre-shock material. Furthermore, quasi-periodic oscillations(QPOs) at a mean period of a few seconds are expectedto arise from shock oscillations [63], but so far these wereonly detected in a handful of systems at optical wavelengths.The high shock temperatures (∼20-30 keV) imply that QPOsearches need to be extended into the hard X-rays.

In non-magnetic CVs, when accreting at high rates, a va-riety of low amplitude variability has been detected. DN os-cillations with a period of a few seconds were found in theoptical and soft X-rays, changing frequency during outburst.Slower (with a time scale of minutes) low-coherence QPOswere detected in a few systems in the hard X-rays during thedecline [64]. Their appearance is related to the transition ofthe boundary layer from optically thick to optically thin andthus have a great potential to infer details of boundary layerregimes. Their frequencies stay in ratio, like the low and high


frequency QPOs seen in low-mass X-ray binaries. Further-more, the presence of a break in the power spectra duringquiescence can allow one to infer disk truncation [65]. Thestriking similarities with X-ray binaries make CVs interestingfor a unified accretion scenario.

Peak temperatures reached during a nova explosion areconstrained by the chemical abundance pattern inferred fromthe ejecta and do not seem to exceed 4 × 108 K, so it is un-likely that the large metallicities reported from the ejecta canbe due to thermonuclear processes. Instead, mixing at thecore-envelope interface is a more likely explanation. The truemixing mechanism has remained elusive for decades. Mul-tidimensional models of the explosion suggest that mixingmay result from Kelvin-Helmholtz instabilities at the core-envelope interface (e.g., ref. [66]).

eXTP can address all this by studying selected samples ofmagnetic and non-magnetic systems, to detect for the firsttime low-amplitude fast aperiodic and periodic variabilitiesover the broad energy range offered by the LAD and SFAand to perform time-resolved spectroscopy.• How does mass ejection work in nova explosions? No-

vae are, after thermonuclear supernovae, the second most lu-minous X-ray sources among accreting WDs with emissionranging from the very soft to the hard X-rays. The soft X-rays probe the continuous nuclear burning on the WD sur-face, while the hard X-rays are believed to trace the earlyshocks inside the ejecta and between the ejecta and the cir-cumstellar material, as well as the onset of resumed accre-tion. The details of the mass outflow and the shaping andevolution of the ejected matter are still poorly known. A re-cent challenge is the unprecedented detection by Fermi LATof high-energy gamma rays (E > 100 MeV) in an increasingnumber of novae [67,68] indicating that these systems are thesite of particle acceleration. This was predicted for RS Oph[69], a symbiotic recurrent nova where the early strong shockbetween the nova ejecta and the red giant wind was traced inhard X-rays by RXTE [70]. In classical novae there is not ared giant wind to interact with the nova mass outflow, becausethe companion is a main sequence star; therefore, particle ac-celeration is more challenging to explain [71]. eXTP promptobservations of nova explosions would be crucial to betterunderstand such processes in both nova types (see Figure 4).

It is very important to observe over the broadest X-rayrange the spectral evolution of these explosions from the earlyhard X-ray onset to the later super-soft X-ray phase [70, 72].Furthermore X-ray variability and periodicities on many dif-ferent time scales, from days to less than a minute are still tobe understood. Short oscillations in the soft X-rays can origi-nate from H-burning instabilities [73] while those in the hardX-rays still remain to be explored.

Thanks to the WFM, eXTP will be able to catch the early

X-ray emission from bright novae to be followed-up by theSFA and LAD instruments to monitor over an unprecedentedenergy range the X-ray spectral evolution including the onsetof hard X-ray emission and of the super-soft X-ray phase aswell as to characterize for the first time fast variability overeXTP’s unique broad range. An example of the eXTP poten-tial is shown in Figure 4.

•What causes DN outburst diversity and what are the con-ditions for disk-jet launching? DN outbursts are believed tobe due to disk instabilities but there is still lack of knowl-edge of what changes occur at the inner boundary layer. Theboundary layer is optically thin during quiescence and be-comes optically thick during outburst [74]. Hard X-ray sup-pression during optical outbursts was believed to be a generalbehavior. Instead, the few DNe observed so far showed great

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Figure 4 Simulations of the spectral evolution of the symbiotic re-current nova RS Oph over its outburst, as seen by the LAD and SFA(a) and by the WFM (b). The spectra include a thermal plasma model(mekal) with solar abundances, evolving from kT=8.2 keV and a flux of3.2 × 10−9 erg s−1 cm−2 (0.5-20 keV) on day 3, down to kT=2.5 keV and aflux of 4.3×10−10 erg s−1 cm−2 (0.5-20 keV) on day 17 [70]. The high X-rayflux during the first 17 d would trigger the WFM in full resolution mode anda detailed study of the spectral evolution will be possible with the LAD andSFA.


diversity and not all of them show X-ray/optical anticorrela-tion [75]. The understanding of what fundamental parametersdivide the different regimes of the boundary layer is crucial.A further challenge is the detection of radio emission froma DN and from high mass accretion rate CVs [76-78] inter-preted as the presence of a radio jet. CVs were believed tobe unable to launch jets. It was recently shown that nova-likeCVs can have optically thin inefficient accretion disk bound-ary layers [79], allowing the possibility of retaining sufficientenergy to power jets. If this were the case, a radio/X-ray cor-relation, similar to that observed in neutron star and blackhole binaries [80], should be present in these systems.

eXTP can address these challenging issues by followinga few DNe with adequate multi-outburst coverage throughtime-resolved spectroscopy and fast timing in unprecedenteddetail.

Thus eXTP will be crucial to measure the spectral and tem-poral properties of the poorly known hard X-ray tails in thesesystems and to correlate them with the soft X-ray emission.When eXTP is expected to be operational, wide field areasurveys will have provided statistically significant samplesto allow detailed investigation in coordination with ground-based optical and radio facilities foreseen in the post-2020time frame (see Figure 1).

5 Binary evolution

The evolution of binary stars is one of the most importantand challenging problems in modern stellar astronomy. Bi-nary evolution—even when restricted merely to the evolu-tion of binaries with compact objects—touches on a range ofkey questions [81-84]: the formation of Type Ia supernovae,the formation of merging neutron stars and black holes, therate of production of millisecond pulsars, and the productionrate of a variety of classes of stars with abundance anoma-lies. The challenges in understanding the populations of bi-nary stars stem from the wide range of physics that goes intothem. In addition to normal stellar evolutionary processes,the following must be understood: the stability and efficiencyof mass transfer, the common envelope stage, the tides andangular momentum loss mechanisms that keep systems tight(i.e., magnetic braking and circumbinary disks), the spin-upof the accreting star, irradiation effects on the donor star, thedetails of the supernova explosions of (ultra)stripped stars,the kicks applied to neutron stars and black holes at birth, thespin-down due to the propeller effect, possible gravitationalwave emission mechanisms, and the initial parameter distri-butions of binary and multiple star systems.

Proper testing of theories of binary evolution must comefrom exploration of substantial samples of close binaries, and

good estimates of their system parameters. Because most X-ray binaries, especially with black hole primaries, are tran-sients, the ideal way to detect such objects is with all-skymonitoring. The WFM represents a major step forward in thecapabilities of all-sky monitors relative to present monitors.

eXTP will make essential contributions to several majoraspects of compact binary populations: the very faint X-ray transient problem [85-87]; the evolutionary link betweenlow-mass X-ray binaries and radio millisecond pulsars; theorbital period distribution of X-ray binaries, by finding theshort period systems selected against by shallower all skymonitors [88, 89]; the “mass gap” between black holes andneutron stars that has been tentatively observed in existingdata [90-92] and not observed in microlensing data [93]; un-derstanding the formation of millisecond pulsars in compactbinaries [94] as well as the details of their spin evolution [95].

eXTP will present a range of new opportunities to studybinary stellar evolution processes. These are:• What are the characteristics of the population of Very

Faint X-ray Binaries? The WFM will have the necessary sen-sitivity and large field of view to detect flares and outburstsfrom the Very Faint X-ray Binaries with luminosities below1036 erg s−1, allowing a census of these sources across a largefraction of the Galaxy. Furthermore, the WFM’s large field ofview will easily pick up the rare X-ray bursts (see sect. 6) thatvery faint X-ray transients exhibit if the accretor is a neutronstar [85]. By establishing a substantial sample of such ob-jects through the wide field coverage and good sensitivity ofthe WFM [14], it should be possible to engage optical follow-up on enough of them to uncover their nature, which is stillpoorly understood.• What is the evolutionary link between low-mass X-ray

binaries and radio millisecond pulsars? eXTP will providedetailed timing studies of known low-mass and ultracompactX-ray binaries (with an orbital period of less than ∼1 h, e.g.,refs. [96, 97]) which will help us to better understand theirevolutionary link to radio millisecond pulsars, such as the so-called redbacks and black widow systems [98].

A particular class of objects of interest for eXTP studiesis that of the transitional millisecond pulsars. These repre-sent the most direct evidence for an evolutionary link be-tween radio millisecond pulsars and low-mass X-ray bina-ries, since they are observed to transit between both states.Three, possibly four, are currently known [99-102]. Two ofthese have relatively low luminosities in their X-ray statesof 1033-34 erg s−1 while the third had an ordinary X-ray out-burst as an accretion-powered millisecond pulsar peaking at1036 erg s−1. There may be many more of these systems. De-tecting them in larger numbers, and mapping out the statisti-cal properties (e.g., pulse and orbital periods) of the resultingsample will enable a better understanding of the binary evo-


lution. Good candidates may be found among the so-calledredbacks, which are radio millisecond pulsars in binaries withan ablated companion star of mass around 0.1 M⊙. Observa-tions more sensitive than with XMM-Newton and Chandraare needed to study the common flaring behavior of thesesystems, for instance how the pulsar behaves on short timescales, to understand better the process behind switching be-tween X-ray and radio states. eXTP would make this obser-vational progress possible.

It is worth mentioning the puzzling fact that fully recycledradio millisecond pulsars are seen in systems with orbital pe-riods up to 200 d, but only 3 pulsar-containing low-mass X-ray binaries are known with orbital periods longer than 1 dand the neutron stars hosted are relatively slow pulsars (0.5-4.0 s). eXTP will be able to probe pulsations down to the msrange with an unprecedented sensitivity, hopefully allowingone to reveal the currently missing fast rotators.• What is the heaviest neutron star and the lightest black

hole? eXTP will allow one to determine whether the orbitalperiod distribution of black hole X-ray binaries follows thepredictions of binary evolution. At the present time, the se-vere selection biases against black hole X-ray binaries withorbital periods less than about 4 h (Figure 5(a)) lead to adeficit of what is likely to be the largest group of systems.Such systems are likely fainter, see Figure 5(b). Possibly,these contain the lighter black holes, especially if one in-cludes high-mass X-ray binaries (e.g., ref. [103]). By en-hancing the sample of objects for which masses can be esti-mated, combined with much more sensitive upcoming opticaltelescopes E-ELT and TMT to obtain dynamical mass esti-mates, eXTP will also help verify or refute the “mass gap”between neutron stars and black holes (Figure 6) which hasprofound implications for understanding how supernovae ac-tually explode.• How efficient is the process of neutron star spin up?

The process of spin-up of neutron stars in binaries (Figure 7)can be probed in two ways: by using LAD measurementsto expand the sample of neutron stars with good spin pe-riod measurements and by using the combination of WFMand LAD to discover and study the population of objectsthat are in the act of transitioning from low mass X-ray bi-naries into millisecond radio pulsars. The study of accret-ing neutron stars and the torques acting on them is impor-tant for probing disk-magnetosphere interactions [104-107]as well as investigating continuous GW emission from thespin of asymmetric neutron stars and the prospects of de-tecting these sources using second (Advanced LIGO andVirgo, KAGRA, LIGO-India) or third-generation (EinsteinTelescope in Europe, Cosmic Explorer in USA [108]) GWdetectors [95, 109]. Among the promising sources for dualobservations in both X-rays and gravitational waves is, for

example, Sco X-1, provided its neutron star spin rate is de-tected [110-112] which is where eXTP can help through thedetection of a weak pulsation. Recent calculations [113] sug-gest that transiently accreting millisecond pulsars are promis-ing as well.• How can eXTP help find sources for LISA? The very

tight X-ray binaries with orbital periods of less than 1 h,the ultracompact X-ray binaries, are ideal sources to be de-tected in GWs induced by the orbital motion by the upcom-ing space-borne GW detector LISA [116] and LISA-like mis-sions like TianQin [117] and TAIJI [118], missions with en-visaged launch dates in the first half of the 2030s. It is ex-pected that eXTP will discover a number of new X-ray bi-naries and may thus find additional ultracompact X-ray bina-ries (neutron star/white dwarf or double white dwarf binaries)that will provide important and guaranteed sources of GWsfor detection within the frequency band (∼mHz) of LISA andLISA-like missions.

(a)

(b)

Figure 5 (a) The orbital period distributions of dynamically confirmedblack hole X-ray binaries (red curve), and cataclysmic variables (greencurve, with the numbers multiplied by 0.1 to fit the two distributions wellon the same scale), taken from the Ritter catalog [114]. It is apparent thatthe distributions are very different from one another. Ref. [89] find a proba-bility of 0.0012 that these two distributions are drawn from the same parentdistribution. (b) Peak luminosity plotted versus orbital period for the blackhole X-ray transients seen with RXTE, with a linear fit. Reproduced bypermission of the AAS from ref. [88]. Similar relations are also seen forcataclysmic variables [14, 115] and accreting neutron stars [88].


Figure 6 Graphic showing neutron star and black hole mass measurements along the vertical axes (in units of solar mass) (from refs. [119-121]), includingthe 16 black holes and 2 probable neutron stars in GW150914, LVT151012, GW151226, GW170104, GW170608, GW170814, GW170817 [8, 122-127]. TheGW measurements are indicated with curved line pointing to the merger event. There is a clear gap between 2 and 5 solar masses. Credit: Frank Elavsky,LIGO-Virgo, Northwestern University.

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Figure 7 The spin distribution for the fastest known (ν > 100 Hz) accretingneutron stars (adapted from ref. [128]).

6 Thermonuclear X-ray bursts

While thermonuclear (type-I) X-ray bursts are well under-stood as a thermonuclear runaway in the upper freshly ac-creted layers of a neutron star in a low-mass X-ray bi-nary, many important questions are unanswered (e.g., refs.[13, 53, 129-131]). Some of those are fundamental, pertain-

ing to the structure of the neutron star and exotic nuclear pro-cesses, while others are related to interesting physical phe-nomena such as neutron star spins, convection and radiationtransport under strong-gravity circumstances, the geometryof accretion and magnetic field, and unusual stellar abun-dances. Since eXTP is particularly well suited to observeshort bright phenomena, it will be able to answer many ofthese questions.

About 20% of all bursts exhibit oscillations with a fre-quency very near that of the neutron stars spin (11-620 Hz).The oscillations during the burst rise can be explained by ahot spot expanding from the point of ignition [132-137], al-though this still requires further study. The oscillations dur-ing the tail have not yet been explained. They could be causedby non-uniform emission during the cooling phase (e.g., be-cause of different depths of fuel over the surface or coolingwakes [138]), but an interesting alternative involves globalsurface modes that may be excited by the motion of the de-flagration front across the surface. This was first suggestedby ref. [139], and further theoretical work was performed by(among others) refs. [140-144].

The primary nuclear processes in thermonuclear bursts arethe CNO cycle to burn hydrogen (β-limited “hot” CNO cy-cle above ∼8 × 107 K), the triple-α process to burn helium


(above several 108 K), the αp-process above 5×108 K to pro-duce elements like Ne, Na and Mg, and the rapid proton cap-ture process (rp-process, above 109 K) to burn hydrogen intoeven heavier elements (e.g., refs. [145-149]). In cases ofhydrogen-free helium accretion or where the β-limited CNOcycle burns all hydrogen to helium before ignition, bursts aredue to ignition in the helium layer. If the CNO burning is notβ-limited, it can be unstable leading to a temperature increasethat subsequently ignites helium. The ignition of helium maybe delayed in that regime [150, 151], resulting in pure hydro-gen flashes. If the CNO burning is β-limited and this burninghas no time to burn away the hydrogen, helium ignition oc-curs in the presence of hydrogen, a situation that leads to therp-process. Hundreds of proton-rich isotopes are producedduring thermonuclear burning, particularly by the rp-process,yielding rare isotopes, whose reaction rates have not yet beenquantified in laboratories [152].

In the past one and a half decades, two new kinds ofX-ray bursts have been discovered that are long and rare:“superbursts” [153-155] and “intermediate duration bursts”[156, 157] that ignite at 102 to 104 larger column depths thanordinary bursts. It has been proposed that superbursts are fu-eled by carbon [155,158], but it is unclear how the carbon cansurvive the rp-process or ignite [159-163] and whether super-bursts are sometimes not fueled by carbon [164]. In contrast,it seems clear that intermediate duration bursts are fueled byhelium on relatively cold neutron stars. The helium may ac-cumulate by slow and stable burning of hydrogen into heliumas described above, or by direct helium accretion at low ac-cretion rate (e.g., refs. [156, 157, 165, 166]). Due to theirlarger ignition depth, these long bursts can serve as probesof the thermal properties of the crust, one of the outstandingquestions of neutron star research. Superbursts have a prop-erty which is intriguing in light of this dependence: they ig-nite sooner than expected on the basis of the measured massaccretion rate and presumed crust properties [161]. It looksas though the thermal balance in the crust (determined by nu-clear reactions in the crust and neutrino cooling in the core)is different from expectations (see also ref. [162]). Bettermeasurements are needed to constrain more accurately therecurrence time and the time history of the accretion betweensuperbursts. Currently, 26 superbursts have been detected inthe last 20 years [167]. eXTP can double that in a few years.

eXTP will provide the opportunity to address the outstand-ing questions on X-ray bursts. A typical peak photon countrate with the LAD will be 105 s−1. Cases with ten times ashigh rates will also occur. These observations will provideunprecedented detailed insight at processes with sub-ms timescales. With the anticipated observation program, it is ex-pected that the LAD will detect at least several hundreds ofX-ray bursts, with about 5 times as much effective area (i.e.,

3.5 m2) and 5 times finer spectral resolution (i.e., 260 eV)than the previous burst workhorse RXTE-PCA [2] and thecurrently operational Astrosat-LAXPC [168] and about 50times as much effective area at 6 keV than the currently op-erational NICER [169]. The coded-mask imaging WFM,thanks to its 4 sr field of view (one third of the sky) will de-tect as many as ten thousand X-ray bursts in 3 years, whichis of the same order of magnitude as all bursts that were de-tected throughout the history of X-ray astronomy. It is clearthat eXTP will provide a rich and novel data set on X-raybursts. While the primary science motives for X-ray burst ob-servations with eXTP are dense matter physics and gravity inthe strong field regime, the secondary motives relevant to ob-servatory science concern their fundamental understanding,in connection with nucleosynthesis, hydrodynamics, flamespreading and accretion flows (see ref. [13]). As a benefit,the outcome of these studies will help reduce the risk of anysystematic errors in studies in the primary science goals.

eXTP will enable substantial advances in the understand-ing of thermonuclear X-ray bursts, on particularly the follow-ing questions:• What is the nature of burst oscillations? Oscillations in

burst tails can be studied at much better sensitivity than withprevious missions and at photon energies below 2 keV it willimprove on NICER [169] by having a four times larger effec-tive area. This will increase the percentage of bursts with de-tected oscillations, enable the detection of shorter oscillationtrains and faster frequency drifts, probe fainter amplitudesand make possible spectro-timing analyses. Thus, eXTP willenlarge our knowledge about burst oscillations including thepossible persistence of hot spots during the burst decay andthe potential roles of surface r and g modes (see ref. [170]).Simulations of measurements of burst oscillations are pro-vided in Figure 8, under different model assumptions (for de-tails, see ref. [171]). These simulations show that eXTP hasa diagnostic capability for the cause of burst oscillations inburst tails of a common type of X-ray bursts.• Is there a preferred ignition latitude on the neutron star

(as expected for fast spinning neutron stars with anisotropicaccretion); how do Coriolis, thermal and hydrodynamic ef-fects combine to control flame spread? A thermonuclear burstis expected to be ignited at one point on the neutron star. Thelatitude of that point may not be random, but influenced byCoriolis forces. A thermonuclear flame should subsequentlyspread over the stellar surface in a fashion that is determinedagain by rotation and by the thermal properties of the oceanin which this happens and the power of the thermonuclearflash [133, 135, 137]. Observational studies of the risingphases of X-ray bursts show diverse and interesting details(e.g., refs. [136, 172-174]). With the much larger area of theLAD, thermonuclear flame spreading can be studied at an

J. J. M. in ’t Zand, et al. Sci. China-Phys. Mech. Astron. February (2019) Vol. 62 No. 2 029506-16C

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Figure 8 LAD simulations of X-ray burst oscillations for a bright burster similar to 4U 1636-536 that is bursting faithfully for at least 40 years already.For details on the model, see ref. [171]. The simulations were carried out for a neutron star of M = 1.4 M⊙, R=10 km and ν=400 Hz, for an observer’sinclination angle of i = 70◦ (measured from the spin vector) and with a time resolution of 0.125 ms. (a) The smoothed light curves for the rise and decay of thebursts (with 11 ms time bins). The red, green and blue curves show simulated light curves for ignition co-latitudes (i.e., also measured from the spin vector)θs = 30◦, 85◦ and 150◦, respectively. The hot spot temperature is assumed to be Th = 3 keV and the temperature outside the hot spot ∆T = 1.5 keV lower.The flame spreading velocity goes as vflame ∝ 1/ cos θs, consistent with strong frictional coupling between different fuel layers [133]. The magenta and cyancurves are similar to the green curve (θs = 85◦) but with ∆T=2 keV and vflame ∝ 1/

√cos θs, respectively, the latter referring to weak frictional coupling. (b)

The simulated eXTP measurements of the fractional oscillation amplitudes for the same models as on the left panel. Note that the fractional amplitudes areshown on an expanded scale up to t = 1 s (burst rise). These simulations show that eXTP is capable of diagnosing ignition co-latitude and frictional coupling.They also indicate (see ref. [171]) that they can constrain the cause of the burst oscillations in burst tails.

unprecedented level of detail and in many more bursts. Forexample, simulations show (see Figure 8) that eXTP mea-surements would enable constraining the latitude of the burstignition.• What is the composition of the ashes of nuclear burn-

ing; how large is the gravitational redshift on the neutron starsurface? The SFA and LAD combined will be uniquely sensi-tive to details of the continuum spectrum that are induced byCompton scattering in the atmosphere and to absorption fea-tures from dredged-up nuclear ashes such as iron and nickelwhich may enable direct compositional studies (e.g., ref.[175]). Low-resolution evidence for such features has beenfound in RXTE data of bursts that generated strongly super-Eddington nuclear powers yielding strong photospheric ex-pansion due to radiation pressure [176, 177], as predicted byref. [178]. Surface absorption features will be gravitation-ally redshifted, which can provide additional constraints onthe compactness of neutron stars if rest wavelengths are iden-tified. In this respect, the SFA would provide unique data.Most burst data currently have no coverage of sub-2 keV pho-ton energies. Those that do and are not susceptible to detec-tor saturation, taken for instance with the grating spectrom-eters on Chandra and XMM-Newton, are of low statisticalquality due to effective areas less than 150 cm2. The SFAis anticipated to have pile-up fractions of a few percent fortypical burst peak fluxes of a few Crab units and has an ef-

fective area one order of magnitude larger, at a fair spectralresolution to detect absorption edges. This provides excellentopportunity for the study of narrow spectral features, partic-ularly absorption edges for systems at known distance withEddington-limited bursts from a slowly rotating neutron star,like IGR J17480-2446 in the globular cluster Terzan 5 (e.g.,refs. [179, 180]).• What characterizes the transition from stable to unsta-

ble nuclear burning? A long standing issue in X-ray burstresearch is the threshold between stable and unstable heliumburning [181-186]. Thermonuclear burning becomes stableonce the temperature becomes so high that the temperaturedependence of the energy generation rate levels off to that ofthe radiative cooling rate (which is proportional to T 4). Morethan 99% of all X-ray bursts are due to ignition of heliumthrough 3α burning and α-captures. It is predicted that theambient temperatures in the burning layer only become highenough (& 5 × 108 K) for stable burning when the mass ac-cretion rate is above the Eddington limit (e.g., ref. [187]).However, one finds evidence of stable burning (i.e., absenceof X-ray bursts and presence of mHz QPOs indicative ofmarginally stable burning) already at 10%-50% of Edding-ton (e.g., refs. [180, 182, 188-190]). A different perspectiveon this issue may be provided by eXTP, by following exten-sively the aftermath of superbursts for about a week. Thesuperburst is another means to increase the ambient temper-


atures. While the neutron star ocean cools down in the hoursto days following a superburst, models predict [191] that itis possible to follow the transition from stable to unstableburning on a convenient time scale, through sensitive mea-surements of low-amplitude (∼0.1%) mHz oscillations andthe return of normal, initially faint, bursting activity (see Fig-ure 9).•What burns in superbursts; what determines the thermo-

dynamic equilibrium of the crust and ocean; what determinesthe stability of nuclear burning; what is the nature of sur-face nuclear burning at low accretion rates? WFM observa-tions will measure more precisely the superburst recurrencetime and time profile (e.g., ref. [167]). This will constrainthe thermodynamic balance in the ocean and crust, and probein unprecedented detail the boundary between stable and un-stable thermonuclear burning [157]. WFM will accumulatelarge exposure times on all bursters (including yet unknownones) and thus detect rare classes of events, such as the mostpowerful intermediate-duration bursts at very low accretionrates.• How do bursts impact and influence the surrounding ac-

cretion flow; can bursts yield insights into the underlying ac-cretion physics? X-ray bursts can deliver a powerful impulseto the surrounding accretion disk (e.g., review by ref. [192]).The response of the disk to the bursts could include (1) an in-crease in the accretion rate due to Poynting-Robertson drag,(2) radiative driven outflows, (3) an increasing scale heightdue to X-ray heating or (4) a combination of all these effects[193]. Unique insights into the properties of accretion diskscould be obtained if this response can be measured for burstswith a wide range of durations. The response of the disk canbe measured by detecting the X-ray reflection features pro-

duced by the burst interacting with the accretion flow (e.g.,refs. [194,195]). The SFA and LAD will be able to detect re-flection features from bursts with integration times as short asa few seconds (e.g., ref. [196]). As the evolution of the burstis easily measured, changes in the reflection features can di-rectly map the changes in the accretion disk over the courseof the burst.

X-ray burst science is also a high priority for the nuclearscience community. X-ray bursts are unique nuclear phe-nomena, and identified as key problems for nuclear sciencein the US National Academies report NP2010 “An Assess-ment and Outlook for Nuclear Physics”, the 2015 US Nu-clear Science Long Range Plan, and the 2017 NuPECC Eu-ropean Long Range Plan. Major investments in a new gen-eration of radioactive beam accelerator facilities, in part mo-tivated by X-ray burst science, are now being made aroundthe world, including FRIB in the US, FAIR in Germany, andHIAF in China. New instruments such as the recoil sepa-rator SECAR at FRIB or storage rings at FAIR or Lanzhou(China) are being developed to enable nuclear reaction ratemeasurements for astrophysical explosions such as X-raybursts.

With a simultaneous advance in X-ray observational capa-bilities through eXTP there is a unique opportunity to com-bine accurate nuclear physics and precision light curve datato determine stellar system parameters, to identify differentnuclear burning regimes, to validate astrophysical 1D and 2Dmodels, and to constrain neutron star properties. In addition,the detection of spectral features from heavy elements on theneutron star surface would be of high importance for nuclearscience, as it would provide direct insight into the nuclearprocesses that power X-ray bursts. The Joint Institute for

Figure 9 Simulation of the ordinary bursting behavior (blue curve) around a superburst (red curve) in a hydrogen-rich low-mass X-ray binary (from ref.[191]). The superburst quenches ordinary bursts for one day after which a regime of marginal burning commences, characterized by mHz QPOs. The lowerpanel shows possible WFM and LAD measurements of such a superburst, if it is at 8 kpc, on-axis, placed at 45◦ from the Galactic center and if the accretionrate is 10% of the Eddington limit.


Nuclear Astrophysics (JINA-CEE) is now fostering the closeconnections between observational and nuclear science com-munities that are needed for nuclear physics based model pre-dictions to guide the interpretation of observations, and fornew observations to motivate nuclear experiments.

7 High-mass X-ray binaries

Massive supergiant, hypergiant, and Wolf-Rayet stars(M &10 M⊙) have the densest, fastest, and most structuredwinds. The radiatively accelerated outflows trigger star for-mation and drive the chemical enrichment and evolution ofGalaxies [197]. The amount of mass lost through these windshas a large impact on the evolution of the star. The winds cangive rise to an extremely variable X-ray flux when accretedonto an orbiting compact object in a high-mass X-ray binary(HMXB). The understanding of the relation between the X-ray variability and stellar wind properties has been limited sofar by the lack of simultaneous large collecting area and goodspectral and timing resolution.

In the past decades, observational evidence has been grow-ing that winds of massive stars are populated by dense“clumps”. The presence of these structures affect the massloss rates derived from the optical spectroscopy of stellarwind features, thus leading to uncertainties in our understand-ing of their evolutionary paths [198]. HMXBs were long con-sidered an interesting possibility to probe clumpiness [199].As X-rays released by accretion trace the mass inflow rateto the compact object, an HMXB provides a natural in-situprobe of the physical properties of the massive star wind in-cluding its clumpiness [200-202]. The so-called “SupergiantFast X-ray Transients” (SFXTs) form the most convincingevidence for the presence of large clumps. In X-rays, the im-print of clumps is two-fold: (1) clumps passing through theline of sight to the compact object cause (partial) obscura-tion of the X-ray source and display photo-electric absorp-tion and photo-ionization; (2) clumps lead to temporarily in-creased accretion and X-ray flares. A number of hours-longflares displayed by the SFXTs could be convincingly associ-ated with the accretion of dense clumps [203].

The compact objects in HMXBs are usually young neu-tron stars (typical age 106-7 yr) that are known to possessstrong magnetic fields (&1012 G). These channel the accret-ing material from distances as large as ∼103-104 km down tothe surface of the compact object, yielding X-ray pulsations,and change the spin of the compact object. Large positiveand negative accretion torques have been measured in bothwind and disk-fed HMXBs on time scales between years anddecades. These torques result from the yet unknown cou-pling between the neutron star magnetic field and the accret-

ing material [204-206]. Extending the time scale of long-term monitoring of the behavior of pulsars in HMXBs is es-sential to make progress in understanding this phenomenon.Previous large field-of-view instruments provided the oppor-tunity to monitor many X-ray pulsars in HMXBs, symbioticX-ray binaries (SyXBs) in which the donor star is a red gi-ant, and in intermediate systems between HMXBs and low-mass X-ray binaries. Particularly striking were the findingsfor GX 1+4 and 4U 1626-67: after about 15 yr (for GX 1+4)and 20 yr (for 4U 1626-67) of spin-up, both systems showeda torque reversal, which made them switch to a spin-downphase [207, 208]. Other systems, such as Cen X-3, Vela X-1, 4U 1907+09 and Her X-1, often showed torque reversals,sometimes superimposed on a longer term trend of eitherspin-down or spin-up [207, 208].

At low mass inflow rates, the accretion might be inhibitedcompletely through the onset of so-called centrifugal and/ormagnetic barrier, which could drive the rapid variability ob-served in the SFXTs and other HMXBs [205]. The observedvariability is thus often used to probe the strength of the dipo-lar component of the neutron star magnetic field. This alsoshapes the geometry of the emission region in X-ray pulsars,which, in turn, influences their observed properties. Most no-tably, for luminosities above the critical value ∼1037 erg s−1,the local accretion rate at the neutron star polar areas mightexceed the Eddington limit, leading to the formation of anextended accretion columns [209]. The transitional luminos-ity setting the threshold between sub- and super-critical ac-cretion is directly related to the compactness of the neutronstar and its magnetic field strength, and can thus be used toconstrain these parameters. The presence of an accretion col-umn is expected to alter dramatically the intrinsic beaming ofthe pulsar X-ray emission, as well as its spectral energy dis-tribution and polarization properties [209]. Observing thesechanges can thus be used to obtain a direct measurement ofthe critical luminosity from the observations [210].

The propagation of X-ray photons emerging through thestrongly magnetized plasma within the neutron star emittingregion also affects the observed spectra. The electron scatter-ing cross section in strong magnetic field is largely enhancedaround the electrons gyro-frequency, leading to the so-calledcyclotron resonance scattering features (CRSFs) often de-tected in the X-ray spectra of young accreting pulsars (fora recent review see ref. [201]). The scattering cross sectionstrongly depends on the angle between the direction of thephoton propagation and the magnetic field orientation. Thisimplies that the polarization degree and angle, the X-ray con-tinuum spectrum, and the CRSF parameters depend on pulsephase. Modeling these changes allows us to reconstruct theconfiguration of the magnetic field and its orientation withrespect to the observer and the neutron star.


The mass accretion rate for black holes in HMXBs tendsto be less variable compared to the NS case. However, mostblack holes in HMXB show some hard-to-soft state variabil-ity, similarly to what is observed in BH low-mass X-ray bi-naries. The most studied and observed BH HMXBs are CygX-1 [211, 212] and LMC X-3 [213, 214]. These sourcesproved particularly useful to “X-ray” the stellar wind of theirdonor stars, complementing similar studies carried out onNS HMXBs and discussed above. For example, the Galac-tic black hole Cyg X-1 shows a periodic modulation of theabsorption column, which is due to the absorption of X-raysfrom the black hole in the massive stellar wind of its donorstar, HDE 226868. As shown by ref. [215], the large scatterin individual measurements of the absorption column to theblack hole can be used to study the size distribution of clumpsin the stellar wind, which is of great interest for understand-ing the physics of the winds of massive stars [216].

The instruments on-board eXTP will dramatically open upperspectives for research in all above mentioned fields. Inparticular the following questions can be addressed:• What are the physical properties of massive star wind

structures and how do these affect the mass loss rates fromthese objects? With current instruments, integration times ofseveral hundreds to thousands of seconds are needed to get arough estimate of clump properties and only an average pic-ture of the clump accretion process can be obtained. Simi-lar observations performed with the LAD and SFA on-boardeXTP will dramatically improve our present understanding ofclumpy wind accretion and winds in massive stars in general,by studying (spectral) variability on time scales as short as afew to tens of seconds Figure 10. This will permit a detailedinvestigation of the dynamics of the clump accretion processand obtain more reliable estimates of the clump mass, radius,density, velocity, and photo-ionization state. In turn, it willimprove our understanding on the mass loss rates from mas-sive stars. As structured winds are not spherically symmet-ric, they are also predicted to emit polarized X-ray radiation.Measurements with the PFA can yield additional informationto constrain the stellar mass loss rate [217]. The improvedcapabilities of all eXTP instruments compared to the currentfacilities will permit to conduct also similar studies on theSyXBs [218], thus exploring the structure and composition ofthe still poorly known winds of red giants stars (which mightbe accelerated through the absorption of the stellar radiationby dust grains rather than heavy ions). Finally, it is impor-tant to understand the consequences of inflated envelopes ofmassive giant stars [219] on the observational properties ofHMXBs in which the donor stars are filling, or close to fill-ing, their Roche lobes. The ability of the eXTP instruments(especially the WFM) to detect and monitor transient sourcesin a very broad range of mass accretion rates will allow us to

study the spectral and temporal behavior of accreting neutronstars with strong magnetic fields. Follow-up observations ex-ploiting the high sensitivity of the SFA will allow us to ob-serve the so-called propeller effect in dozens of X-ray pul-sars. For the sources with measured cyclotron energies it willmake possible to determine the configuration of the neutronstar magnetic field. Monitoring observations of long-periodpulsars at low mass accretion rates will constrain the physi-cal mechanisms regulating the interaction of plasma in a lowionization state with the neutron star strong magnetic field[220].• What are the geometry and physical conditions within

the emission region of X-ray pulsars, and their relation tothe observed pulse profiles, spectra, and polarization in theX-ray band? Pulse phase-resolved spectroscopy of accretingpulsars has been one of the main tools to diagnose their emis-sion region properties since decades. eXTP will bring suchinvestigations to an entirely new level. The large effectivearea of the SFA and the LAD will provide spectra and pulseprofiles of unprecedented statistical quality even within com-paratively short integration times. It will be possible for thefirst time to study the spectral and pulse profile evolution onvery short timescales comparable with the spin period of theneutron star. The energy resolution of the LAD in a broadenergy band will allow us to perform detailed studies of thecomplex CRSF shapes observed in some sources [221], andinvestigate dependence of the CRSF parameters on the pulsephase, which is essential to constrain the line formation mod-els and the emission region geometry.

In addition, eXTP will provide for the first time highlycomplementary simultaneous X-ray polarization data. Theexpected strong polarization and high X-ray fluxes of the ac-creting pulsars make these sources a prime target for X-raypolarimetry studies. The polarization degree and angle carryinformation on the geometry of their emission region, mag-netic field configuration, and orientation of the pulsar with re-spect to the observer [222]. A major change of the observedpolarization signatures is expected with the transition fromthe sub- to super-critical accretion, i.e. at the onset of ex-tended accretion columns. eXTP observations can be usedto detect such transitions and the corresponding changes inthe polarization properties of X-ray pulsars, as illustrated inFigure 11.• What are the mechanisms triggering torque reversals in

HMXBs and what leads to orbital and super-orbital modula-tion of the X-ray emission from these systems? For systemswith well known distances, simultaneous measurements offluxes and spin-rates can provide constraints on the magneticfield [223]. If the pulsar magnetic field is also known fromcyclotron line measurements, then the relationship betweenthe spin rate and the luminosity can be used to constrain the


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Figure 10 (a) Sketch of the accretion by the upper neutron star of a clump ejected by the lower supergiant star (credits: ESA). (b) Changes in the flux andspectral parameters during the flare recorded by XMM-Newton, from the SFXT IGR J18410-0535 [203]. Violet boxes represent the measurements obtainedwith XMM-Newton, while black points represent values obtained from the simulated LAD +SFA spectra with exposure times as short as 100 s. Compared toXMM-Newton, the dynamic process of the clump accretion can be studied in much more details and fast spectral changes can be revealed to an unprecedentedaccuracy (we remark also that the pile-up and dead-time free data provided by the LAD and SFA greatly reduce the uncertainties affecting the XMM-Newton,data obtained so far from HMXBs during bright flares). (c) Shown, for comparison, is an example of a XMM-Newton, spectrum extracted in a 1 ks-longinterval (source flux 3×10−10 erg s−1 cm−2) during the decay from the flare shown on the left and the corresponding simulated SFA and LAD spectra. The Fe-Kline at 6.5 keV, used to probe the clump material ionized by the high X-ray flux, is barely visible in XMM-Newton, but very prominently detected in both theSFA and LAD spectra.

neutron star parameters, probing not only accretion physicsbut also fundamental physics in these highly magnetized ac-creting systems. The WFM is a perfectly suited instrumentfor these studies in that it is a wide field monitor capable ofsimultaneously measuring the source flux and accurately de-termining its spin frequency. Its lower energy threshold willmake it the most sensitive monitor for these types of systemsthat has ever flown. Measurements from the WFM of pulsedflux, unpulsed flux, frequency and frequency derivatives of

accreting X-ray pulsars will contribute to the solution of thequestions mentioned above, and it will also trigger target ofopportunity observations with the LAD/SFA at the onset oftorque reversals. Early detections of these state changes arecritical to initiate early observations by the LAD/SFA in or-der to understand the associated spectral/timing variation Asa byproduct of the monitoring observations, it will be possi-ble to carry out detailed spectroscopic analyses as a functionof orbital, superorbital and spin phase. The orbital depen-


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Figure 11 The combination of the LAD large effective area and energyresolution will allow us to monitor the luminosity-related changes of theobserved CRSF energy in transient X-ray pulsars with unprecedented accu-racy. (a) A simulation of thirty 10 ks LAD observations of the X-ray pulsarV 0332+53, where a transition from super- to sub-critical accretion has beenrecently detected through a luminosity-dependent change of the observedCRSF energy [210]. eXTP will allow us to investigate the same effect witha much larger accuracy and to investigate changes in pulse profile shape andpolarization signatures associated with similar transitions. (b) The expo-sure required to measure the polarization of a given strength at a 3σ signifi-cance level for the typical super-critical and sub-critical fluxes observed fromV 0332+53. The polarization signal strength predicted by ref. [222] in thetwo cases are also shown. The eXTP PFA will allow us to detect and com-pare the polarized emission in both the sub- and super-critical regime usingrelatively short exposure times, performing also studies of the dependenceof the polarization properties as a function of the pulse phase.

dence, for instance, can be used to map long-lived structuressurrounding the neutron stars in these systems, like accretionwakes.

Last but not least, it is worth mentioning that manyHMXBs are transient and thus the discovery of new sourcesor new outbursts from previously known objects in thisclass will greatly benefit from the capabilities of the WFM.Figure 12 shows that a large fraction of all these sources canbe efficiently monitored during each single pointing of theWFM, let alone in sequential different pointings.

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Figure 12 Comparison of WFM’s field of view with the 2-20 keV map,in Galactic coordinates, of the X-ray sky as observed with MAXI (courtesyof T. Mihara, RIKEN, JAXA, and the MAXI team). The WFM will havethe largest field of view and monitor simultaneously a large fraction of allcurrently known HMXBs and low-mass X-ray binaries.

8 Radio-quiet active galactic nuclei

The standard model of active galactic nuclei (AGN) predictsa supermassive black hole (SMBH) surrounded by a geomet-rically thin, optically thick accretion disk. Like the case ofX-ray binaries, the seed photons from the disk are Comptonscattered into the X-ray band in a hot corona, the main differ-ence being that the accretion disk in AGN emits at optical/UVinstead of X-ray temperatures. The UV and optical seed pho-tons from the disk are Compton scattered into the X-ray bandin a hot corona above the accretion disk which produces thehard power-law spectrum extending to energies determinedby the electron temperature in the hot corona, with a cut-offenergy ranging from 100 to 200 keV [224-228]. This primaryX-ray radiation in turn illuminates the inner disk and is partlyreflected towards the observer’s line of sight [229], producinga Fe-K line that will be heavily distorted, with a pronouncedred wing—a clear signature of strong field gravity effects onthe emitted radiation. The phenomenology of AGN is quitesimilar to stellar black holes accreting from companion starsin X-ray binaries, except for a difference in time scales and adifference in typical temperatures of spectral thermal com-ponents from the accretion disk that scales with the blackhole mass because the size of the innermost stable circularorbit scales with mass. The inner structure in AGN is sur-rounded by rapidly rotating clouds of gas—the Broad LineRegion (BLR). A toroidal absorber is located at a distanceof a parsec from the black hole and within the Narrow LineRegion (NLR) clouds. These regions contribute with addi-tional features to the primary continuum: (1) a narrow Fe-Kline at 6.4 keV produced in the BLR, in external regions ofthe accretion disc or in the molecular torus; (2) a reflectionhump above 10 keV, due to the reprocessing of the continuumfrom the optically-thick medium (such as the torus and/or theaccretion disk); and (3) the absorption and variable features


produced in the ionized and cold material around the inner re-gion (up to the NLR distance scales). The typical broadbandspectrum as may be observed by eXTP is shown in Figure 13.

Thanks to the large effective area and the CCD-classspectral resolution of the LAD and SFA, coupled with thelarge sky coverage of the WFM, eXTP will make a deci-sive step forward in our understanding of the inner struc-ture of AGN. The unprecedented SFA spectral sensitivitywill allow one to observe hundreds of AGN down to aflux of 10−13 erg s−1 cm−2 in the local universe with a verygood accuracy (i.e., a signal-to-noise ratio in excess of 100).Thanks to the LAD effective area, the brightest AGN (3 ×10−11 erg s−1 cm−2) can be characterized with extreme ac-curacy. Moreover, the WFM will be capable of continuousX-ray monitoring of large numbers (&100) of AGN on timescales of a few days to weeks. The WFM will thus providean essential synergy with those observatories that will, in par-allel to eXTP, be opening up AGN time domain science onthose time scales at other wavebands, for example with CTA(TeV energy domain), Meerkat/ASKAP/SKA (radio wave-lengths) and LSST (optical band, see Figure 1). Here, thefocus is on three key aspects of AGN physics (related to theaccretion properties and variability), which will be specifi-cally addressed by the unique capabilities of eXTP:

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Figure 13 (a) eXTP (SFA in black, LAD in red) spectrum. (b) Residu-als as obtained by a 100 ks integration of a bright 1 mCrab AGN composedof: the continuum, three components of ionized absorbers, the cold reflec-tion and narrow Fe line Kα, Fe Kβ and Ni Kα, the ionized lines FeXXVand FeXXVI and the blurred ionized reflection component (black hole spina = 0.7). (c) The model complexity we used for the simulation is similarto the case of the AGN SWIFT J2127.4+5654 [227,230] and where separatespectral components are indicated with different colors (in blue the primarycut-off power-law, in cyan the blurred reflection component originated in theinnermost regions nearby the central BH and in magenta the contributionof the a cold reflection component originated in a medium distant from thecentral BH.

• What are the emitting materials around the central su-permassive black hole? The origin of the narrow Fe emissionline observed in AGN is not completely clear, while this isa fundamental piece of information about the SMBH centralengine and its environment. The width of the line is less thanfew thousand km/s (in terms of full width at half maximum,[231]) and suggests an origin within the BLR or the puta-tive molecular torus (at pc scale) or within the BLR closer tothe BH. A powerful method to study the origin of the narrowFe line is to measure the time lag between the variation ofthe continuum and the line [232]. The reverberation mappingmethodology is used with the optical and UV broad emis-sion lines with great success [233] and, recently, in X-rays toinvestigate the origin of the broad Fe line [234, 235]. How-ever, the uncertainties on the Fe line flux (about 10%) and thelow sampling frequency of the X-ray continuum, prevent usapplying this method to a large sample of AGN, especiallyon the daily time scale expected for a BLR origin [236-238].eXTP will allow one, for the first time, to perform X-ray re-verberation of the narrow core and broad component (see theeXTP white paper on strong gravity [4]) of the Fe line with asingle mission.

The very large field of view of the WFM is a powerfultool to monitor the continuum intensity of virtually all thesources in the sky. To demonstrate the unique capabilitiesof the WFM we used the 1-year exposure sky map shownin Figure 14 as a good approximation of the adopted point-ing strategy. The WFM can produce continuous light curveswith daily 3σ detections for bright sources (i.e., brighter than∼ 1 × 10−10 erg s−1 cm−2 in the 7-50 keV band, i.e., abovethe Fe K edge). Weaker objects (∼5 × 10−11 erg s−1 cm−2)will be detectable in weekly exposures. These time scales areperfectly suited for the narrow Fe line reverberation analy-sis, since the expected time scales are from days to weeksto years [239], depending on the location and geometryof the material and the intrinsic variability of the sources(therefore, on the black hole mass and the accretion rate).At the same time, the large area available with the SFAand LAD will allow one to perform well-sampled (weeklyto monthly) monitoring of the Fe-K line and Compton re-flection component with short targeted observations. TheCCD-class energy resolution will easily allow one to disen-tangle the broad Fe line component from the narrow core.In fact, in only 1 ks a narrow Fe line of equivalent width100 eV can be recovered with an uncertainty of 1%-2% forbright sources (∼1 × 10−10 erg s−1 cm−2 with ∼103 SFA plusLAD counts in the line). The same uncertainties can be re-covered for weaker sources (∼ 5× 10−11 erg s−1 cm−2) with5 ks exposures. Thanks to the broad bandpass of the LAD,eXTP will be able to recover the reflection continuum with10% accuracy in a 1 ks exposure for the brightest sources


Figure 14 Number of AGN per 4π sr as a function of the position in the sky that are bright enough to be accessible to the WFM for studies of X-ray variability.For these sources, a light curve with at least 10 time bins in a 1-year period can be built, achieving in each bin a signal-to-noise ratio of at least 3. The lateralbar reports the total number of variable AGN observed on the sky, assuming an example of a 1-yr pointing plan. Only regions of the sky covered with morethan 10 cm2 of net detector area have been considered.

(∼1 × 10−10 erg s−1 cm−2). This measurement, together withthe WFM daily monitoring of the continuum above 7 keV(even with a low signal-to-noise ratio), will be a instrumen-tal in identifying the origin of the narrow distant reflectioncomponents in AGN.•What are the absorbing regions in the environment of su-

permassive black holes, from kpc to sub-pc scale? There isstrong evidence of at least three absorption components onvery different scales: on scales of hundreds of parsecs, on theparsec scale, and within the dust sublimation radius, on sub-parsec scale (e.g., refs. [225, 238, 240]). The most effectiveway to estimate the distance of the absorber is by means ofthe analysis of the variability of its column density. In par-ticular, rapid (from a few hours to a few days) NH variabilityhas been observed in most bright AGN in the local Universe[241-247], suggesting that obscuration in X-rays could due,at least in part, to BLR clouds or in the the inner regions ofinfrared-emitting dusty tori. These observations with XMM-Newton and NuSTAR have achieved a precision for the cover-ing factor of ∼5%-10%. Detailed simulations show that sucha measurement (assuming the same observed fluxes) will beobtained with eXTP with significantly higher precision (1%-2%), on even shorter time scales (few ks instead of ∼100 ks).This would allow, for the first time, an extensive study of X-ray eclipses, which are frequently seen in local bright AGN(e.g., ref. [246]). Following the time evolution of the column

density and of the covering factor over timescales of a few kswill provide unprecedented information on the structure, dis-tance and kinematics of the obscuring clouds. In particular,close to the lower end of the BH mass range in AGN, witha single 100 ks observation it will be possible to fully char-acterize the circumnuclear absorbing medium by samplingeclipsing events from multiple clouds (e.g., ref. [248]). whichis now precluded by the short timescales involved. Moreover,the broad-band energy range offered by eXTP will allow oneto explore in detail also those clouds with larger column den-sities in Compton thick AGN, whose variability mostly af-fects the spectrum at high energies.• How does the central engine produce the observed vari-

ability? Our knowledge of the X-ray variability in AGN hasadvanced substantially in the last 15 years, thanks mainlyto monitoring campaigns with RXTE and day-long XMM-Newton observations of a few X-ray bright AGN [249-253].AGN show red noise in their power spectral density func-tions (PSD) with a power law that decreases at high frequen-cies as a power law with index −2. Below some character-istic frequency the PSDs flatten and the break frequenciesscale approximately inversely with the BH mass, from BHX-ray binaries to AGN [254]. These characteristic frequen-cies seem linked to both the BH mass and the properties ofthe accretion flow. Although all the relevant accretion disktime scales depend on the mass of the central object, only a


few of them may depend (indirectly) on the accretion rate aswell. So far, our knowledge is based on the power-spectralanalysis of a few objects. A substantial improvement of theanalysis of X-ray variability in AGN would be possible byavoiding the problems introduced by large statistical noiseand irregular time sampling. The WFM plays a crucial rolein our understanding of the X-ray variability in AGN. TheWFM will survey large areas of the sky, allowing one to studylong-term X-ray variability of bright/nearby AGN. Using theAGN number counts observed by previous missions, we com-puted the number density of sources that will be accessible toeXTP in each region of the sky (assuming an average AGNspectrum and Galactic absorption). Using the typical AGNflux distribution, we derived the number of sources that arebright enough to build a light curve with a worst resolutionof 1 month at signal-to-noise ratio of 3. Figure 14 shows thenumber density map of AGN that fulfill these criteria. Ref.[255] demonstrated that it is possible to retrieve the intrin-sic variability (within a factor of 2) even when using sparselight curves with low signal-to-noise ratio, such as the onesexpected here for most of the faint AGN population in theWFM sample. Depending on the exact monitoring pattern,much smaller uncertainties are expected for the brightest 20sources. The WFM, even in its first year of operations, isexpected to return light curves with at least 10 data pointsper year for several hundreds of variable AGN, allowing bothindividual (for the brightest) and ensemble studies of AGNvariability on long (monthly) time scales. One will thereforebe able to determine accurately the intrinsic variability am-plitude (on long time scales) for hundreds of AGN and studythe variability-luminosity relation with an unprecedented ac-curacy. This relation will then be compared with similar re-lations for AGN at higher redshifts (see, e.g., that derivedfrom the Chandra Deep Field-South survey [256]), obtained,however, with much higher statistical uncertainties, due tothe limited sample size, that prevent us to probe effectivelythe luminosity, redshift and timescale dependence of the in-trinsic AGN variability (e.g., ref. [257]).

9 Radio-loud active galactic nuclei

eXTP is planned in the same time frame as other observato-ries that are opening up a window to short time scale variabil-ity for AGN, such as the Cherenkov Telescope Array (CTA)at TeV energies and the Square Kilometer Array (SKA) andAtacama Large Millimeter Array (ALMA) in the radio and(sub-)mm bands, respectively (see Figure 1). This offers aunique opportunity to interpret short time scale variabilityand connect accretion and jet physics in radio-loud AGN.

eXTP will be able to follow up a large number of AGN.

We estimate this to be a few thousand with SFA and a fewhundred simultaneously with the LAD. These numbers arederived from the 30-50 keV log N − log S reported in ref.[258], after applying a flux rescaling to match the LAD andSFA energy bands. This extrapolation is well justified by thefact that a hard X-ray survey preferentially selects Flat Spec-trum Radio Quasars and low energy peaked BL Lacs due tothe hardness of their X-ray spectra, which are generally welldescribed by a single power law in this entire energy range. Inaddition, the WFM can provide monitoring of bright AGN ontime scales of days to weeks thanks to its large sky coverage.

Among the radio-loud AGN, blazars are certainly themost promising in the era of the time domain astronomy.They are among the most powerful persistent [259] cosmicsources, able to release (apparent) luminosities in excess of1048 erg s−1 over the entire EM spectrum, from the radio tothe very high energy γ-ray band. This intense non-thermalemission is produced within a relativistic (bulk Lorentz factorΓ = 10-20) jet pointing toward the Earth. Relativistic effectslead to the beaming of the radiation within a narrow cone ofsemi aperture ∼1/Γ rad with strong apparent amplification ofthe luminosity and shortening of the variability time scales inthe cases in which (as in blazars) the jet is closely aligned tothe line of sight.

Blazars come in two flavors: those that show optical broademission lines (typically observed in quasars that are calledFlat Spectrum Radio Quasars or FSRQs) and those that ingeneral display rather weak or even absent emission lines,collectively grouped in the BL Lac object class. The lat-ter generally show the most extreme variability, with varia-tions as fast as a few minutes, and the most energetic photons(above 100 GeV).

The conventional scenario (e.g., ref. [260]) foresees asingle portion of the jet dominating the overall emission,at least during high activity states. However, this simpleidea has recently been challenged by the observation of veryfast (minutes) variability, which requires the existence ofvery compact emission sites. The most extreme exampleis PKS 2155-304, which experienced two exceptional flaresin 2006—with a recorded luminosity of the order of severaltimes 1047 erg s−1—on top of which events with rise times of∼100 s have been detected. Even assuming that these strongevents originated in very compact emission regions, the phys-ical conditions should be rather extreme and Lorentz factorsof Γ∼50-100 seem unavoidable [261]. Physical mechanismspossibly triggering the formation of these compact emissionregions include magnetic reconnection events [262] and rela-tivistic turbulence [263, 264].

Alternatively, narrow beams of electrons can attain ultra-relativistic energies (Γ∼106) in the black hole vicinity throughprocesses involving the black hole magnetosphere [265,266].


In this case, it is expected that the emission shows up only inthe high energy γ-ray region, giving no signals at lower fre-quencies (thus resembling the case of the so-called “orphan”flares; different from the simultaneous X-ray and TeV flaringobserved from PKS 2155−304 in 2006).

eXTP can address the following questions with regards toradio-loud AGN:• What causes the fast γ-ray flares? Until now, ultra-fast

variability has been recorded only in the γ-ray band, espe-cially at TeV energies, at which the Cherenkov arrays arecharacterized by gigantic collection areas, required to probesuch short time scales. Current instruments do not provide ananalogous sensitivity in the key X-ray band, at which the lowenergy component of TeV emitting BL Lacs (the so-calledHBLs) peaks and thus it is not possible to study potentialultra-fast variability events at these frequencies. Moreover,such events appear to be quite rare, with a duty cycle of lessthan 1%. Therefore, monitoring of the sky is required to catchthese events.

The LAD, thanks to its timing capability, will allow oneto detect the X-ray counterpart of the very fast variabil-ity observed at TeV energies for 2-10 keV fluxes above2 × 10−11 erg s−1 cm−2, shedding light on the nature of theX-TeV connection in HBLs [267]. Moreover, eXTP polari-metric measurements of HBLs will constrain the nature ofthe electron population(s) responsible for the X-ray emis-sion, providing us with the magnetic field orientation and itschange during the flaring activity as compared with the aver-age emission. In these blazars, the polarimeter energy bandwill cover the region of the synchrotron peak, where flux andpolarization degree demand sub-daily integration times. Ifone assumes a 2-10 keV flux of 2 × 10−10 erg s−1 cm−2, onecan measure a minimum detectable polarization of ∼ 10% in5 ks with the PFA. This value is in the range of polarizationmeasurements for synchrotron emission as derived from opti-cal measurements (between 3% and 30%). A highly orderedmagnetic field would imply higher polarization degrees. Inthis respect, eXTP will be able to probe the evolution of themagnetic field lines and their role in powering fast variationsas detected by the LAD (flare rise over 100 s as detected inthe TeV range).• What accelerates highly energetic electrons? A fur-

ther potential of eXTP in the study of HBLs is measuringthe change in spectral curvature of their synchrotron spectrawhich will enable a direct study of the mechanism of acceler-ation of highly energetic electrons. These studies will benefitfrom the simultaneous observations of the LAD and SFA thatprovide the broad band coverage (0.5-30 keV) of the syn-chrotron emission.• Where is the high-energy dissipation region? The SFA

sensitivity is very promising for the study of FSRQs with a

0.5-10 keV flux & 2 × 10−13 erg s−1 cm−2. For these objects,eXTP will probe the rise of the inverse Compton (IC) emis-sion whereas new generation radio and (sub-)mm instrumentswill provide excellent timing capabilities to study short time-scale correlations with the synchrotron spectral domain. Thenature of the photon seeds for the IC emission responsiblefor the high-energy emission is still strongly debated. Tightconstraints are expected to come from the investigation oftime scales of variability and polarimetry. In this respect, theSFA will be crucial in a multi-frequency context providing:(1) temporal investigation on second time scales; (2) spectraltrend investigation on minute time scales. With CTA, thismay provide a multi-wavelength perspective to understandbetter the rapid (unexpected) TeV emission recently detectedin some FSRQs. The WFM will provide not only an excel-lent trigger for both CTA and (sub-)mm observatories but alsolong-term monitoring of a large sample of blazars.

Moreover, polarimetric measurements performed in the2-10 keV band, in comparison with polarization at lowerfrequencies, represent a powerful tool to solve the long-standing problem of the origin of the seed photons andtherefore constrain the high-energy dissipation region for thebrightest objects and flares on daily time scale. The aver-age X-ray fluxes of FSRQs are actually much lower thanthose of high synchrotron-peak blazars (in the range 10−12-10−11 erg cm−2 s−1) and small amplitude variations have beendetected during the flaring activity. This implies much longerintegration times to achieve MDP values of the order of a few%, important to disentangle among different leptonic models(external Compton versus synchrotron self Compton, SSC)by modeling the X-ray emission. In particular, in the caseof FSRQs with flux ∼2 × 10−11 erg s−1 cm−2, a minimum de-tectable polarization of 7% will be obtainable in 100 ks.• What is the disk-jet connection in misaligned AGN?

WFM long term monitoring (monthly time scale) of radiogalaxies hosting efficient accretion disks (mainly Fanaroff-Riley II sources, hereafter FRIIs; Figure 15) will provide anoptimal tool to investigate the disk-jet connection by trigger-ing both radio and LAD X-ray follow-ups. eXTP will inves-tigate the disk-jet connection in a similar manner as in X-raybinaries, that exhibit the same behavior [268].

Recent detections of radio galaxies hosting inefficient ac-cretion flows (Fanaroff-Riley I, hereafter FRI) at high andvery high energies (e.g., IC 310 [271]) are providing a fur-ther sample of radio-loud sources well suited for follow-up observations with eXTP. Combining SFA and LAD withCTA simultaneous observations will be crucial in constrain-ing the TeV emitting region, likely connected with the mag-netosphere surrounding the central engine, as derived by therapid variability observed by MAGIC [271]. FRI behave likehigh synchrotron-peak blazars (with lower luminosities) and


Figure 15 Chandra 0.3-7 keV image of the FRII radio galaxy 3C 111 with 3.6 cm radio contours overlayed [269]. In the inset the multi-wavelength lightcurve from mm to γ-rays is shown (from ref. [270]). The simultaneity of the flare is impressive: the core luminosity was increasing from millimeter to X-rayfrequencies exactly when the greatest flux of γ-ray photons occurred. This is a clear indication of the cospatiality of the event. The outburst of photons isdirectly connected to the ejection of a new radio knot (the time of the ejection is indicated by the black arrow).

therefore simultaneous observations in X-rays and at TeVenergies are the key to interpret their spectral energy dis-tribution in terms of SSC rather than hadronic models (notruled out so far). Given the typical fluxes of TeV FRI (afew ×10−12 erg s−1 cm−2), SFA (combined with LAD for thebrightest flares) observations will allow one to detect the X-ray counterpart of the fast TeV variations, providing tightconstraints on the location of the TeV emitting region andthe particle acceleration process [272].

• How important is reflection in spectra of radio-loudnarrow-line Seyfert 1 galaxies (NLSy1s)? NLSy1s are of-ten considered as a third class of AGN with relativistic jets[273, 274]. LAD observations will be very promising notonly for studying the physical mechanisms responsible forthe higher energy emission (in γ-rays or at TeV energies) andthen the blazar-like behavior, but also for determining the roleof the thermal component. The combination of SFA and LADfollow-up of the high energy flares from NLSy1 detected inγ-rays or at higher energy γ-rays will allow one to followthe X-ray spectral variations of these sources in the broad en-ergy range (0.5-30 keV) down to a 10−12 erg s−1 cm−2, unveil-ing the interplay between the Comptonized radiation from thecorona and the non-thermal emission from the jet. The LADobservations extending up to 30 keV will play an importantrole in the detection and characterization of the Fe-K line, andtest with high accuracy the reflection model applied to the X-

ray spectra of NLSy1. This is to investigate the possible linkbetween accretion and jet properties.

10 Tidal disruption events

Observational evidence is mounting to support a scenariowhere most galactic nuclei host SMBHs. Gas inflow causesa small (∼1%) fraction of SMBHs to accrete continuously formillions of years and shine as AGN. Most are expected tobe quiescent, accreting if at all at a very sub-Eddington rate[275]. Observationally, it is therefore hard to assess the pres-ence and mass of most SMBHs beyond the local universe.Occasionally, a sudden increase of the accretion rate may oc-cur if a large mass of gas, for instance a star, falls into the tidalsphere of influence of the black hole and finds itself torn apartand accreted. One calls these events ’tidal disruption events’(TDEs, see ref. [276]). TDEs can result in a sudden increaseof EM emission. They can reach the luminosity of a quasarbut they are rare (∼10−4 yr−1 per galaxy; in particular types ofgalaxies, it may be ∼10−3 yr−1 per galaxy [277,278]) and lastseveral months or years. TDEs are a multi wavelength phe-nomenon, that has so far been detected in radio, optical, UV,soft and hard X-rays. They display both thermal and non-thermal emission from relativistic and non-relativistic matter(for a recent review on observations, see ref. [279]).


The detection and study of these flares can deliver otherimportant astrophysical information beyond probing the pres-ence of a SMBH. First, TDEs can allow one to discover in-termediate mass black holes, as their peak bolometric lumi-nosity is expected to be inversely proportional to the blackhole mass. Second, TDEs are signposts of supermassive bi-nary black holes, as their light curves look characteristicallydifferent in the presence of a second black hole, which acts asa perturber on the stellar stream [280]. Recently, a disruptionby a SMBH binary candidate was claimed [281]. Thirdly,as also mentioned below, TDEs give us the opportunity tostudy accretion and jet formation in transient (∼month to yeartime scale) accretion episodes around SMBHs in contrast toAGN. Fourthly, dormant SMBHs represent the majority ofthe SMBHs in our universe. X-ray flares from TDEs provideus with the opportunity to probe the mass and spin distribu-tion of previously dormant SMBHs at the centers of normalgalaxies as well as the geometry of the debris flow of the ac-cretion flare through X-ray line reverberation [282]. Finally,in X-rays TDEs represent a new probe of strong gravity, forinstance tracing precession effects in the Kerr metric (see refs.[283, 284]).

The first TDEs were discovered in ROSAT surveys of theX-ray sky (e.g., refs. [285, 286]), see ref. [287] for a re-view. Later, GALEX allowed for the selection of TDEs atUV frequencies [288, 289]. Many of the most recent TDEcandidates are found in optical transient surveys [290-292].An alternative method to select TDE candidates is to lookfor optical spectra with extreme high-ionization and Balmerlines [293, 294]. This class of TDEs is called “thermal”, andthe spectral energy distribution is believed to be associatedwith phenomena that lead to mass accretion onto the blackhole.

Currently, the theoretical picture proposed to interpret theobservational properties of TDEs is as follows. After stel-lar disruption, part of the stellar material is accreted ontothe black hole, causing a luminous flare of radiation. If thestar is completely disrupted, its debris is accreted at a de-creasing rate according to M ∝ t−5/3 [276, 295]. Therefore,TDEs allow one to study the formation of a transient accre-tion disk and its continuous transition through different accre-tion states. The super-Eddington phase—which occurs onlyfor SMBH masses M < 107 M⊙—is theoretically uncertain,but it may be associated with a powerful radiatively drivenwind [296] that thermally emits 1041-1043 erg s−1 mainly atoptical frequencies [297, 298]. The disk luminosity (1044-1046 erg s−1) peaks instead in the far-UV/soft X-rays [298].As an alternative scenario to thermal emission from an accre-tion disk, the optical emission has been suggested to comefrom shocks as the stellar debris self-crosses or from repro-cessed X-ray light (e.g., ref. [299]).

A few years ago, the Swift Burst Alert Telescope (BAT)triggered on two TDE candidates in the hard X-ray band[291,300]. A multi-frequency follow-up from radio to γ-raysrevealed a new class of non-thermal TDEs. It is widely be-lieved that emission from a relativistic jet (bulk Lorentz factorΓ ≈ 2 [301, 302]) is responsible for the hard X-ray spectrum(with a negative power-law photon index between −1.6 and−1.8) and the increasing radio activity [303] detected a fewdays after the trigger. More recently, ref. [304] reported an-other candidate relativistic TDE (Swift J1112.2-8238). Ref.[305] found nine TDE candidates in BAT data after searchingthe data from 53000 galaxies out to 100 Mpc. Finally, refs.[306,307] discovered the first radio outflow associated with athermal TDE, suggesting that jets may be a common featureof TDEs.

The best studied of the relativistic events is SwiftJ1644+57 (Sw J1644 in short [303, 308]). The two main fea-tures that support the claim that Sw J1644 is a TDE are (1)the X-ray light curve behavior, that follows LX ∝ M(t−τ)−5/3

after a few days (τ ≈ 3 d) from the trigger [309], and (2) theradio localization of the event within 150 pc from the centerof a known quiescent galactic nucleus [310].

If one uses the Sw J1644 radio and X-ray light curves to es-timate prospects for any future planned missions, one obtainsthat non-thermal TDEs can be discovered at a higher ratethanks to triggers provided by radio surveys and to follow-ups at higher energies. The wide field-of-view survey ofSKA will effectively detect TDEs in the very early phase, al-lowing follow-up X-ray observations in the very early phase[311, 312]. In synergy with SKA in survey mode, the SFAcan in principle successfully repoint to each radio candidatein the sky accessible at any moment to eXTP (i.e., 50% of thetime), and measure its light curve decay index and spectralproperties. The X-ray counterpart of radio-triggered eventscan be followed up with the LAD too (see Figure 6 in ref.[309]), thus extending the energy coverage up to 30 keVwith a better characterization of the non-thermal process andof the jet energy budget. The X-ray polarimeter will def-initely play the major role. For events followed up withina few weeks from the beginning, a fraction of ∼10% of theradio-triggered events will have a 2-10 keV flux greater than10−10 erg s−1 cm−2. For these events, it is possible to derivea minimum detectable polarization of ∼10% over a 5 ks timescale. This will allow one to test the variation of magneticfield over a time scale of one hour and derive the time neededfor the transient jet to build up a large scale coherent magneticfield. Moreover, if eXTP were able to follow-up TDEs within1 d, with the trigger provided by LSST, SKA, the EinsteinProbe or WFM and if the source is bright enough in 2-10 keV(> 10−10 erg s−1 cm−2), it will be possible to investigate alsopolarization angle variations aimed at assessing the role of jet


precession in the early phase of the event. In this case, oneforesees a set of daily (50 ks) observations [313, 314] spreadover 10 d in order to appreciate angle variations within a fewdegrees.

eXTP is therefore very well suited for the X-ray follow-upof jetted TDEs in the SKA era. The expected rate is betweena few to several tens per year (depending on the value of Γ).The attractive prospect is that of building for the first timea sample of well studied TDEs associated with non-thermalemission from jets. This will allow one to study disk-jetformation and their connection, in a way complementary toother more persistent sources such as AGN and X-ray bina-ries. In addition, jetted TDEs can uniquely probe the pres-ence of SMBHs in quiescent galaxies well beyond the localuniverse (redshift range 1.5-2).

Moreover, the SFA energy band and sensitivity will enablethe follow-up of thermal TDEs, that are expected to be trig-gered in a large number with LSST. These will be studiedwith eXTP in a tight connection with the X-ray binary tran-sients. The combination of the soft telescopes, the broad en-ergy coverage and the polarimetric capabilities makes eXTPa great X-ray mission to study all the variety of tidal disrup-tion events and to provide the unique opportunity to answerthe following questions:• What is the rate of non-thermal TDEs? The SFA sensi-

tivity will allow one to follow up and identify any radio trig-gered TDEs. The WFM can allow the serendipitous detectionof non-thermal TDEs in X-rays at a higher rate (between oneand a few tens per year) than any current missions. The com-bination of the two will provide a unique tool to establish therate of non-thermal TDEs and study the mechanisms power-ing the hard X-ray emission.• Do TDEs exhibit similar spectral states as X-ray binary

transients? The SFA energy band and sensitivity will enablethe follow-up thermal TDEs that are expected to be triggeredin large numbers with LSST. Several thousands per year areexpected to be triggered up to z∼1 [297]. The X-ray follow-up with SFA will make it possible to study the evolution ofthe thermal TDEs and possibly see the transition from softto hard spectra as expected at later stages due to the coronalComptonized component [315].•What are the masses and spins of the previously dormant

SMBHs at the centers of normal galaxies? AGN cover onlya small portion of the SMBHs in our universe. The distribu-tion of the masses and the spins of these SMBHs is impor-tant to our understanding of the cosmological growth of massand evolution of the spin of the majority SMBHs. At presentand in the near future, we are only able to study the proper-ties of these dormant SMBHs (also some intermediate-massblack holes in dwarf galaxies) through these flaring events.Among them, X-ray flaring TDEs allow us to probe the massand spin with X-ray line reverberation (e.g., ref. [282]). For

future X-ray bright relativistic TDEs or thermal TDEs, it ispossible to detect spectral features with 1 ks SFA and LADexposures.

11 Gamma-ray bursts and supernovae

Gamma-ray bursts (GRBs, for reviews, see refs. [316-318])are short (∼10 s), bright (up to 10−5 erg s−1 in 10 keV-1 MeV),energetic (with fluences up to more than 10−4 erg cm−2) andfrequent (about one per day). Observational efforts since the1990s have provided, among others, (1) the accurate local-ization and characterization of multi-wavelength afterglows[319, 320], (2) the establishment of cosmological distancesand supernova-like radiated energies [321], (3) the identifica-tion of two classes of GRBs, long and short, with a divisionat 2 s [322], and (4) the connection of long GRBs with TypeIb/c supernovae [323]. The most important open questionsin our current understanding of GRBs involve: the physicalorigin of sub-classes of GRBs (i.e., short, X-ray rich, sub-energetic and ultra-long GRBs), the physics and geometry ofthe prompt emission, some unexpected early afterglow phe-nomenology (e.g., plateaus and flares), the connection be-tween GRBs and particular kinds of supernovae, the natureof the “central engine” and the use of GRBs as cosmologicalprobes.

Short GRBs were long suspected to be connected to bi-nary neutron-star merger events [322, 324]. The detec-tion of short GRBs associated with old stellar populations,their offsets from their host galaxies and the identificationof a candidate kilonova explosion supported this associa-tion (e.g., refs. [325-327]). The suspicion received unprece-dented support through the coincident detection of the shortGRB170817A [7] and the binary merger gravitational-waveevent GW170817 [8, 328]. The masses of the two binarycomponents measured through the GWs are more consis-tent with a set of dynamically measured neutron star massesthan with that of black holes. The tidal disruption of neu-tron stars (see Figure 16 for stills from a simulation) is ex-pected to yield EM radiation because of the delayed accre-tion of part of the expelled matter onto the merger product,the strong radio-active decay of r-process elements producedduring the merger, the shocks resulting from an expandingcocoon and/or jets.

The prospect of GW detections with Advanced LIGO andVirgo, KAGRA and LIGO-India in the 2020s have been cal-culated to be rather promising (tens to hundreds per year) fordistances of up to ∼300-500 Mpc [330,331]. So far, very few(4-5, see, e.g., refs. [332, 333]) supernova-less GRBs (in-cluding the so-called long-short GRBs and short GRBs) havebeen localized within this distance regime. This may improvewith the softer response by the WFM and its larger field of


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view. It was thought that, due to the beaming effect in spe-cial relativity, only relativistic jets from the mergers of twoneutron stars or of a black hole and a neutron star beamedtoward the Earth can be detected as short GRBs. The detec-tion of the off-axis GRB170817A [7], without any noticeablesignal of a relativistic jet in our line of sight, suggests betterprospects for the detection of off-axis X-ray emission, possi-bly produced as a result of the shock propagation or by themildly relativistic cocoon (see, e.g., refs. [334, 335]). Thisemission has a much larger opening angle, and despite themuch lower luminosity, it might still be observable by eXTPwithin the LIGO-Virgo detection horizon. Moreover, inde-pendently from short GRBs, some models predict detectableand nearly-isotropic X-ray emission powered by the mergerremnant [336-342]. With its unprecedented large field ofview, WFM may help to confirm or exclude these modelsby possibly detecting EM signals from the remnant on theneeded time scale (from ∼ 1 s to longer). GRB170817Awould have been detectable with the WFM with a signal-to-noise ratio during the peak of up to about 10 if its spectrumwould be on the 1σ soft side of the error margins of the spec-tral shape parameters measured with Fermi-GBM [7].

The nature of the second supernova in tight binaries, whichare responsible for producing the binary neutron star progen-itors of short GRBs and GW sources, is an ultra-stripped su-pernova [343]. These supernovae produce relatively faint andrapidly decaying light curves [344, 345]. A prime candidate

for such a supernova event is SN 2005ek [346]. Further-more, as a result of severe mass transfer in the X-ray phasejust prior to these supernovae (via so-called Case BB Roche-lobe overflow), such systems are also likely to be observablewith eXTP as pulsating ultra-luminous X-ray sources (seesect. 12.1).

Another interesting class of soft GRBs may arrive from su-pernova shock breakouts. Wide-field soft X-ray surveys arepredicted to detect each year hundreds of supernovae in theact of exploding through the supernova shock breakout phe-nomenology [347, 348]. Supernova shock breakout eventsoccur in X-rays at the very beginning of the supernova ex-plosions, allowing for follow-up at other wavelengths to startextremely early on in the supernova, and possibly sheddinglight on the nature of the progenitor of some types of super-novae. To date, only one supernova shock breakout has beenunambiguously detected, in Swift observations of NGC 2770[349]. It has a fast-rise, exponential decay light curve witha rise time of 72 s, and an exponential decay time scale of129 s, and a peak X-ray luminosity of 6.1 × 1043 ergs s−1.

An interesting target for eXTP are the recently discoveredultra-long GRBs [350,351]. These events are soft gamma-raybursts lasting hours (instead of seconds). Only a few eventsare known, and most of their properties remain to be studied.We know however that these events present a possible faintthermal component, and that they may be linked to progeni-tors very similar to population III stars (i.e., extremely mas-


sive, metal poor stars [352]). These objects are not brighterthan normal long GRBs (albeit the total energy budget is farlarger), but last for so long that it is possible to re-point eXTPand still to observe the very bright prompt phase. This bright-ness, combined with the unprecedented large collecting areaof the LAD, would allow spectral studies impossible with anyother mission to date, studying in detail the evolution of thethermal component and its still debated nature [353].

Thanks to its unique combination of field of view, broadbandpass, spectral resolution, large effective area, and polar-ization capabilities, the instruments on board eXTP, particu-larly the WFM (see Figure 17), can address questions of fun-damental importance to understanding the above phenomena:• What are X-ray Flashes? eXTP can enable a substan-

tial increase (with respect to past and current missions) of thedetection rate of X-Ray Flashes (XRFs), events commonlyinterpreted as a softer sub-class of GRBs (but see also ref.[354]) which likely constitutes the bulk of the GRB pop-ulation but still lacking a good body of observational data[355-357]. With respect to HETE-2 WXM (2-25 keV), themost efficient XRF detector flown so far, WFM will increasethe XRF detection rate 7-fold, with an expected rate of 30-40XRF per year (e.g., ref. [358]). See also Figure 17.• What are the main radiative mechanisms at the basis of

GRB prompt emission? Measurement of the GRB spectraand their evolution down to 2 keV are an indispensable toolfor testing models of GRB prompt emission and will be stud-ied with unprecedented detail thanks to the WFM’s uniquecapabilities (see Figure 18(a)).• What is the nature of GRB progenitors? Transient ab-

sorption edges and emission lines in the prompt and afterglowX-ray spectra of long GRBs are expected in several scenar-ios for their origin. Firm detections or deep limits on suchfeatures would be essential for the understanding of the prop-erties of the circum burst matter (CBM) and hence the natureof GRB progenitors. In addition, they could enable us to de-termine the GRB redshift directly from X-ray observations.Up to now, only a few marginal detections were provided byBeppoSAX, XMM-Newton and Chandra, while no signifi-cant evidence for line emission was found in Swift afterglowdata. As illustrated in Figures 18 and 19(b), eXTP, thanksto the good energy resolution of both WFM and SFA, com-bined with a large field of view and a large area, respectively,can greatly contribute to solve this issue and possibly open anew discovery space in the understanding of the GRB phe-nomenon.• Can supernovae be discovered promptly? Supernova ex-

plosions can be detected at their onsets thanks to the shockbreakout effect. According to recent supernova rate estimates[347], about one to two supernova breakouts per year shouldbe detectable with the WFM out to 20 Mpc. If one takes the

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Figure 19 As an example, we considered the redshifted Fe-K line that wasmeasured by BeppoSAX in the X-ray afterglow spectrum of GRB000210[369]. (a) A simulated SFA spectrum as obtained considering the same af-terglow intensity and line equivalent width as detected by BeppoSAX 12 hafter the onset of the GRB. An exposure time of 50 ks is assumed (averageflux of 8× 10−13 erg s−1 cm−2). The detection of the iron line with the eXTPinstrument would be outstanding. (b) The same simulation for Swift/XRT.No line is detected. This confirms the low sensitivity of XRT to similar fea-tures. These unique and unprecedented results would provide an importantstep forward in the investigation of the circumburst material composition anddistribution, with important consequences for shedding light on the nature ofGRB progenitors and emission physics.

case of NGC 2770 as a template, the optimum integrationtime to detect such events out to 20 Mpc is 240 s (the WFMsensitivity for the detection of impulsive events within thistime interval is ∼30 mCrab).• What is the structure and evolution of GRB jets? The

contribution to GRB science by the LAD and especially thePFA may provide breakthrough information on the jet struc-ture and on the nature of the afterglow flares and plateaus[362-365], as different models make different predictionson the evolution of the polarization during these phases[366, 367]. However, this will strongly depend on space-craft re-pointing capabilities and is likely limited to the 20%brightest events. For instance, by using the mean parame-ters of the X-ray afterglow light curves quoted by ref. [368](i.e., a flux normalization at the beginning of the plateau of6 × 10−10 erg s−1 cm−2 and 3.96, −0.16 and 1.59 for the de-

cay indices in the three phases—steep, shallow, normal—respectively), we computed the average flux for an exposureof 100 ks starting at different epochs after the trigger. We findthat sensitive polarimetric studies with the PFA (i.e., averagefluxes above 1 mCrab) can be performed for a typical X-rayafterglow only if it is observed no later than a few minutesafter the GRB trigger. However, when considering X-ray af-terglows with a bright plateau phase (i.e., 10 times brighter atthe start of the plateau), an average flux of few mCrab can bereached for a 100 ks exposure even starting the observationsafter 6 h.• What is the end product of a neutron star merger? The

end product of a merger of two neutron stars can either bea black hole or a heavy neutron star that may be hypermas-sive (temporarily supported by for example fast spins) withmagnetic fields of order 1015 G (thus a magnetar, e.g., ref.[338]). This probes a very interesting question: what is thethreshold mass above which a neutron star cannot exist (seealso sect. 5)? This is intimately related to the neutron starequation of state. A possible observational means to distin-guish between both kind of compact objects is, apart fromthe GW signal [5], the detection of a pulsed EM signal afterthe merger event, for instance in the so-called plateau phaseof GRB afterglows that is suggested to be due to rapidly ro-tating magnetar (e.g. refs. [370, 371]). eXTP would bewell equipped to hunt for such a signal, provided the LADis pointed quick enough and the prompt signal is very lumi-nous (roughly in the neighborhood of its Eddington limit fora distance of 40 Mpc and a signal duration of 104 s, or more).

Finally we note that, as a public community service, eXTPwill include a capability to localize short transients automati-cally onboard using data from the WFM. These positions, ora subset of them, will be disseminated via a VHF networkand the internet in a similar fashion as for the SVOM mission[1, 372].

12 Objects of uncertain nature

12.1 Ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) have observed fluxesindicating (assuming isotropic radiation) LX > 1039 erg s−1,and often > 1040 erg−1. As these luminosities exceed theEddington limit (at which released radiation drives away in-flowing mass) for typical stellar black hole masses, they indi-cate the presence of either much larger-mass black holes, ofsuper-Eddington accretion rates, and/or beaming of radiationinto limited angles. The recent discovery of X-ray pulsationsby XMM-Newton in three ULXs [373-376] proves that someULXs contain neutron stars, and the sinusoidal pulse shape


indicates low beaming, so these systems clearly accrete atsuper-Eddington rates (likely from high-mass donors, e.g.,refs. [377, 378]). These systems are now being intensivelystudied, to try to understand the magnetic fields of their neu-tron stars, their long-term spin behavior, and the nature of theaccretion flows.

eXTP can provide a substantial contribution to ULX sci-ence, by finding and timing ULX pulsars. eXTP’s SFA hasan effective area roughly 5.5 times that of XMM-Newton’s pncamera at 6 keV (the relevant comparison, since ULX pulsa-tions are stronger at high energies), and an angular resolu-tion of 1′, enabling high count rates for relatively faint ob-jects. As pulsation detection is generally a function of countrate and pulse fraction, the higher SFA effective area meansthat eXTP should be able to detect pulsations over twice asfar away (e.g., ∼10 Mpc vs. 5 Mpc for 1040 erg/s ULXswith 20% pulsed fractions), and thus within an effective vol-ume 8-10 times larger. A systematic eXTP program to sur-vey bright ULXs within 10 Mpc (identified by, for example,eROSITA on Spectrum-X-Gamma, along with Chandra andXMM-Newton surveys), and the brightest ULXs beyond that,may discover ∼30 new ULX pulsars, allowing significant sta-tistical conclusions about the ULX population. Repeated tim-ing of these ULXs will measure their spin-up rates. eXTP’shigh effective area will also allow studies of QPOs [379], timelags [380], and eclipse searches (e.g., ref. [381]), opening upsubstantial discovery space for these extreme objects.

12.2 Fast radio bursts

In recent years, there has been much excitement about theenigmatic Fast Radio Bursts (FRBs [382]). These are bright,millisecond duration, radio bursts with large dispersion mea-sures pointing at extra-galactic distances and large luminosi-ties. A few tens have been detected since 2001 and estimatesabout their all-sky rate range into the thousands per day [383].Radio telescopes are being adapted to detect many more andaiming to distribute alerts in close to real time [384]. The ori-gin of these bursts is undetermined and the models range fromneutron-star collapse to black holes, neutron star or whitedwarf mergers and non-cataclysmic pulsar events (for a re-view, see ref. [385]). There is one FRB source which hasrepeated tens of times [386], while all other observed FRBshave not been observed to repeat (although it is possible thatrepeats have been missed due to instrument sensitivity limi-tations and survey strategies, [387]). The case of the repeatergives rise to a models where the energy is from magneticreconnection events from a young magnetar [388] or giantpulses from a rapidly rotating neutron star [389]. However,there is no consensus yet [385].

To date FRBs have no counterparts outside the radio band.

However, there are models predicting FRBs associated withsome GRBs and, hence, a faint, broadband afterglow maybe expected (e.g., ref. [390]). Coordinated searches ofFRBs in the repeating source revealed no detection in X-rays and gamma-rays with 0.5-10 keV flux limits of order10−7 erg s−1 cm−2 [391]. This limit can be vastly improvedwith LAD and SFA to values of 10−10 to 10−11 erg s−1 cm−2.Because of the WFM’s large field of view, the possibility ofa FRB being detected inside that field of view is enhanced.Detection of the prompt few millisecond duration emissionwill be difficult though given the flux limit of approximately10−6.5 erg s−1 cm−2. If there is a longer duration X-ray coun-terpart, chances will become better.

Conversely, eXTP could instead provide candidates for tar-geted searches for FRBs. Many of the progenitor theories re-quire a young neutron star that is rapidly rotating and/or has ahigh magnetic field [385]. Neutron stars of this type are pro-posed to be the central engines of some GRBs and superlumi-nous supernovae (see sect. 11). Additionally, the host galaxyof the repeating FRB is consistent with the host galaxies oflong GRBs and superluminous supernovae [392]. However,for long GRBs and superluminous supernovae progenitors, itwill take a few decades to clear the surrounding medium toallow the radio emission to escape [392]. The short GRB pro-genitors do not have this issue and bursts may escape muchsooner. eXTP will be able to identifying the plateau phase(the signature of the formation of a magnetar [336]) followingGRBs and these sources can be targeted with rapid responseand late-time FRB searches [393].

We thank the external referees for their advise prior to submission and A.Patruno for a revision of Figure 7. This work was supported by the Royal So-ciety, ERC Starting (Grant No. 639217), the European Union Horizon 2020Research and Innovation Programme under the Marie Sklodowska-CurieGlobal Fellowship (Grant No. 703916), the National Natural Science Foun-dation of China (Grant Nos. 11233001, 11773014, 11633007, 11403074,11333005, 11503008, and 11590781), the National Basic Research Programof China (Grant No. 2015CB857100), NASA (Grant No. NNX13AD28A),an ARC Future Fellowship (Grant No. FT120100363), the NationalScience Foundation (Grant No. PHY-1430152), the Spanish MINECO(Grant No. AYA2016-76012-C3-1-P), the ICCUB (Unidad de Excelencia’Marıa de Maeztu’) (Grant No. MDM-2014-0369), EU’s Horizon Pro-gramme through a Marie Sklodowska-Curie Fellowship (Grant No. 702638),the Polish National Science Center (Grant Nos. 2015/17/B/ST9/03422,2015/18/M/ST9/00541, 2013/10/M/ST9/00729, and 2015/18/A/ST9/00746), theStrategic Priority Research Program of the Chinese Academy of Sciences(Grant No. XDA15020100), and the NWO Veni Fellowship (Grant No.639.041.647).

Author contributions This paper is an initiative of eXTP’s Science Work-ing Group 4 on Observatory Science, whose members are representatives ofthe astronomical community at large with a scientific interest in pursuing thesuccessful implementation of eXTP. The paper was primarily written by S.Drake (stellar flares), Y. Chen and P. Zhou (supernova remnants), M. Her-nanz, D. de Martino (accreting white dwarfs), T. Maccarone (binary evolu-tion), J. in ’t Zand (thermonuclear X-ray bursts), E. Bozzo (high-mass X-raybinaries), A. de Rosa (radio-quiet active galactic nuclei), I. Donnarumma


(radio-loud active galactic nuclei), E. Rossi (tidal disruption events) and L.Amati (gamma-ray bursts), with major contributions by C. Heinke, G. Sala,A. Rowlinson, H. Schatz, T. Tauris, J. Wilms, W. Yu and X. Wu. The con-tributions were edited by J. in ’t Zand, E. Bozzo, J. Qu and X. Li. Other coauthors provided input to refine the paper.

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