understanding relativistic jets via multi-messenger ... · m. santander - understanding...

Marcos Santander (on behalf of Bindu Rani) University of Alabama

MMA SAG Session - HEAD 2019, Monterey, CA

Understanding Relativistic Jets Via Multi-Messenger Observations

M. Santander - Understanding Relativistic Jets Via Multi-Messenger Observations — MMA SAG session, HEAD 2019

AGN white papers

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Astro2020 Science White Paper

Multi-Physics of AGN Jets in theMulti-Messenger EraThematic Areas: X⇤ Multi-Messenger Astronomy and Astrophysics

Principal Author: Name: Bindu RaniInstitution: NASA Goddard Space Flight Center, Greenbelt, MD, USAEmail: [email protected]; Phone: +1 301.286.2531Lead authors: M. Petropoulou (Princeton University, USA), H. Zhang (Purdue University,USA), F. D’Ammando (INAF, Italy), and J. Finke (NRL, USA)Co-authors: M. Baring (Rice University, USA), M. Böttcher (North-West University, South Africa),S. Dimitrakoudis (University of Alberta, Canada), Z. Gan (CCA, USA), D. Giannios (Purdue University,USA), D. H. Hartmann (Clemson University, USA), T. P. Krichbaum (MPIfR, Germany), A. P. Marscher(Boston University, USA), A. Mastichiadis (University of Athens, Greece), K. Nalewajko (NicolausCopernicus Astronomical Center, Poland), R. Ojha (UMBC/NASA GSFC, USA), D. Paneque (MPP,Germany), C. Shrader (NASA GSFC, USA), L. Sironi (Columbia University, USA), A. Tchekhovskoy(Northwestern University, USA), D. J. Thompson (NASA GSFC, USA), N. Vlahakis (University ofAthens, Greece), T. M. Venters (NASA GSFC, USA)

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Figure 1: AGN jets, powered by accretion onto a central supermassive black hole, are the mostpowerful and long-lived particle accelerators in the Universe. Non-thermal processes operating injets are responsible for multi-messenger emissions, such as broadband electromagnetic radiationand high-energy neutrinos. Background spectral energy distribution is adapted from [116].

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Astro2020 White Paper

A Unique Messenger to Probe Active Galactic Nuclei: High-Energy NeutrinosAuthors: Sara Buson, University of Würzburg, University of Maryland Baltimore County ; Ke Fang, Stanford University; Azadeh Keivani, Columbia University; Thomas Maccarone, Texas Tech University; Kohta Murase, Pennsylvania State University; Maria Petropoulou, Princeton University; Marcos Santander* University of Alabama, Ignacio Taboada, Georgia Institute of Technology; Nathan Whitehorn, University of California - Los Angeles.

*Primary author: [email protected]; +1 (205) 348 4863

Credit: IceCube/NASA

Multi-messenger Astronomy and Astrophysics

Multi-Physics of AGN Jets in the Multi-Messenger Era

A Unique Messenger to Probe Active Galactic Nuclei: High-Energy Neutrinos

Principal author: B. Rani Lead authors: M. Petropoulou, H. Zhang, F.

D’Ammando, J. Finke. +18 co-authors

Principal author: M. Santander Co-authors: S. Buson, K. Fang, A. Keivani, T. Maccarone,

K. Murase, M. Petropoulou, I. Taboada, N. Whitehorn. +151 endorsers

https://arxiv.org/abs/1903.04504 https://arxiv.org/abs/1903.04447

https://arxiv.org/abs/1903.04504https://arxiv.org/abs/1903.04447



• What are the dissipation and particle acceleration processes? • Multi-physics sims: study particle acceleration under different conditions, deliver

temporal and spatial info on radiation and polarization. • High-res mm VLBI imaging and polarization from optical to X-ray / gamma.

• What are the high-energy radiation mechanisms? • Leptonic vs hadronic. Needs more than time-averaged SED predictions. • MM measurements: broadband, sensitive SED MWL coverage + HE neutrinos.

• Where and how do jets produce high-energy emission and neutrinos? • Large-scale MHD for jet launch and dissipation. Spectral and temporal obs. from

0.1 GeV to >TeV and high-res radio imaging. • Is gamma-ray emission related to jet structure?

• TeV Doppler factor crisis: Doppler factor derived from observations much lower than required to model the HE emission.

• VLBI (< 100 grav radii) studies of jets vs ligh-of-sight angle, comparison of models with observations.

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Key questions for AGN jets



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Table 1: Key scientific questions and future developments in instrumentation, theory & simulation.

TeV energies (see §2.2), which require high sensitivity and high spectral resolution. If the GeVAGN emission is produced by hadronic processes, then a bright component between the low- andhigh-energy humps of the SED is also expected (§2.2). MeV observations of AGN (in both quies-cent and flaring states) with future missions like All-sky Medium Energy Gamma-ray Observatory(AMEGO4, [82]), acting complementary to neutrino searches, could probe the HE hadronic com-ponent of AGN jets by detecting or setting upper limits to the predicted MeV emission. Sensitiveall-sky X-ray monitors, like ISS-TAO5 and STROBE-X6 [105], will be ideal for searches of EMcounterparts to HE neutrino events and for delivering uninterrupted X-ray light curves of brightAGN. HE polarimetric missions, like IXPE7 [126], AMEGO, and Advanced Energetic Pair Tele-scope (AdEPT [53]), will provide polarization observations of bright AGN at hard X-rays andMeV �-rays energies, which will disentangle the competing HE emission models (leptonic versushadronic). We advocate the support to future instruments with large effective areas, excellenttiming resolution, and wide fields of view that will be essential for advancing our understand-ing of jet physics in the next decade.

Current theoretical studies of AGN jets treat the fluid dynamics, particle acceleration, and ra-diation processes separately, due to the huge differences in physical scales. CPU-based codes,for example, have been successful in simulating the 3D dynamics of jets and particle accelera-tion, but without directly connecting the fluid dynamical scales with particle kinetic scales [e.g.,49, 70, 101, 114, 122, 133]. However, the jet dynamics and interactions with the surroundingmedium will determine the location of energy dissipation, which sets the physical conditionsfor particle acceleration and transport. These particles will radiate and interact with photons,which will in turn feedback onto the particle evolution. Current techniques for the radiativetransfer calculations, which involve Monte Carlo tracing and particle transport equation solvers,often neglect the acceleration processes as well as spatial particle advection and diffusion [e.g.,21, 24, 25, 34, 74, 97, 130]. The complexity of these problems calls for support to the develop-ment of multi-physics, multi-scale numerical simulations, and high-performance computing. Newdevelopments in GPU-based computing [62, 71, 108] will allow the simultaneous treatment ofmicro- and macro-physics in jet simulations. With the addition of radiative feedback physics (e.g.,absorption, scattering, hadronic interactions, and polarization) on top of the multi-scale jet simula-tions, we will be able to critically test our theories for AGN jet physics against the multi-messengerobservations in the next decade.4https://asd.gsfc.nasa.gov/amego/

5https://asd.gsfc.nasa.gov/isstao/

6https://gammaray.nsstc.nasa.gov/Strobe-X/

7https://ixpe.msfc.nasa.gov/

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Advocates for:

• Support to future instruments with large effective areas, excellent timing resolution, and wide fields of view that will be essential for advancing our understanding of jet physics in the next decade.

• Support to the development of multi-physics, multi-scale numerical simulations, and high-performance computing.



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• Detecting neutrinos from AGN would help in our understanding of particle acceleration and origin of cosmic rays.

• The evidence for a correlation between high-energy neutrinos and a blazar (TXS 0506+056) has left many open questions for the coming decade:

• What makes the 2014-15 “neutrino flare” of TXS 0506+056 special?

• Are there many neutrino emission sites?

• What is the best strategy for finding new sources, specially if no correlation with GeV-TeV gammas exists?



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• Advocate broadband, sensitive, wide-field coverage of a large number of AGN during the coming decade. Correlated studies with next-gen neutrino telescopes (specially IceCube-Gen2, KM3NeT, GVD)

• Low-energy (synchro peak of the SED) to monitor leptonic emission. High-energy observations from soft X-rays to TeV. Continuation of Swift and Fermi until new capabilities are identified. NuSTAR follow-ups and wide field X-ray instruments (e.g. TAP, STROBE-X WFM, TAO-ISS). AMEGO will be crucial at MeV + polarimetry (with IXPE + optical). VHE from CTA and wide field instruments (HAWC and others planned in the south).

Astro2020 White Paper: High-Energy Neutrinos from AGN

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a) b)

Fig. 1: a) Fermi-LAT ∞-ray sky map with the error region for the IceCube-170922A event overlaid [61]. b)Spectral energy distribution (SED) of TXS 0506+056 (red markers [61]) compared to the sensitivity of cur-rent (solid black, [62, 63, 64, 65, 66, 67]) and future (dashed gray, [68, 69, 70, 71, 72, 73]) EM instrumentsscaled for different exposures. Neutrino upper limits from the detection of IceCube-170922A [61] and thebest-fit neutrino spectrum from the 2014-2015 flare [74] are shown in blue compared to the seven-yearsensitivity curve for IceCube [75].

UHECR backgrounds hint at a common origin of these emissions [81]. The diffuse gamma-rayand neutrino backgrounds can also be explained simultaneously [82], [83, 60], which may be ex-plained by AGN embedded in galaxy clusters/groups or starburst galaxies [83, 60, 56]. Nonethe-less, the diffuse flux between 10°100 TeV cannot be solely explained by either pp scenarios forstar-forming galaxies or p∞ scenarios for AGN jets (including blazars and radio galaxies [e.g.,50, 84, 85, 86, 87, 88]; see [89] for a review). The contribution of ∞-ray blazars, in particular,to the diffuse neutrino flux has been constrained to the level of ª 10°30% by correlation andstacking analyses [90, 91, 58]. The dominant contribution to the diffuse neutrino flux in the 10-100 TeV range may come from sources that are either genuinely opaque to ∞-rays, such as AGNcores [45] or that are hidden to current ∞-ray detectors, such as MeV blazars [84].

The fact that ∞-ray-emitting AGN are not the dominant contributors to the bulk of the dif-fuse neutrino flux does not prevent them from being detectable point neutrino sources. Sev-eral studies claimed a connection between individual ∞-ray blazars and high-energy neutrinoevents, although with marginal correlation significances [92, 93, 94]. The first compelling ev-idence for the identification of an astrophysical high-energy neutrino source was provided in2017 by the detection of a high-energy neutrino event (IceCube-170922A) in coincidence witha strong EM flare of the ∞-ray blazar TXS 0506+056 (Fig.1a) [61]. In fact, blazar ∞-ray flares areideal periods for the detection of high-energy neutrinos due to the lower atmospheric neutrinobackground contamination and the higher neutrino production efficiency [e.g. 51, 23, 94, 95,58]. The detection of IceCube-170922A and the prompt dissemination of the neutrino sky posi-tion to the astronomical community triggered an extensive multi-messenger campaign to char-acterize the source emission [96, 97, 98, 99]. The rich multi-wavelength data set enabled for thefirst time detailed theoretical modeling that could explain the neutrino emission in coincidencewith the EM blazar flare [97, 99, 58, 100, 101].

A follow-up analysis of archival IceCube neutrino data also unveiled neutrino activity dur-ing a ª100-day window in 2014-15 [74]. Intriguingly, this detection was not accompanied by

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Astro2020 White Paper: High-Energy Neutrinos from AGN

flaring in ∞-rays as in the case of IceCube-170922A, although some debate exists about a poten-tial hardening in the blazar ∞-ray spectrum during the neutrino activity period [102, 103]. Thelack of sensitive multi-wavelength observations during this period is a significant hurdle in themulti-messenger modeling of the neutrino “flare” [58, 104, 105]. This is particularly true for thekeV to MeV band where no observations are available, but a high photon flux due to the cascadeof the hadronically-produced ∞-rays is theoretically expected [22, 106, 58].

So far, there is no convincing theoretical explanation for all multi-messenger observationsof TXS 0506+056, which has raised a number of important questions: What makes its 2014-15flare activity special? Is there more than one neutrino production sites in AGN? Can we findmore robust AGN-neutrino associations? What would be the best observing strategy, especiallyif GeV ∞-rays and TeV-PeV neutrinos are not produced at the same time? We outline next therequired observational capabilities to address these questions in the coming decade.

Multi-messenger Studies of AGN in the Next DecadeThe construction of next-generation neutrino telescopes coupled with an expansion of multi-wavelength follow-up efforts and the improvements in broad-band coverage and sensitivity ofnew EM observatories will provide a major boost in the identification and study of AGN as neu-trino emitters. We here outline a number of activities that will help solidify the AGN high-energyneutrino connection by detecting more sources beyond TXS 0506+056. Together with multi-wavelength follow-up campaigns [107], we will be able to probe the physics of neutrino and EMemission in AGN.

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Gradients indicate uncertainties in possible start/end of missions.

Fig. 2: Timeline of some of the instrumentsexpected to be involved in multi-messengerstudies of AGN in the coming decade (somenot yet funded or with unclear timelines).

Neutrino observatories: The primary back-grounds to the detection of astrophysical neutri-nos are muons and neutrinos produced by cosmicray interactions in the upper atmosphere. Thesehave a steeply-falling energy spectrum, with at-mospheric neutrinos becoming sub-dominant tothe observed astrophysical ones at ª 100 TeV. As aresult, the primary target energy range for detec-tion of neutrinos from AGN is in the 100 TeV–PeVrange, although clustering in time or space cansignificantly lower the energy threshold. The high-est possible neutrino flux from UHECR sourceshas been calculated assuming a calorimetric rela-tionship [81], which establishes that a gigaton orlarger scale instrument is needed to observe astro-physical neutrinos above 100 TeV.

IceCube is the largest operating neutrino in-strument in this energy range and the first to reacha gigaton mass. It uses the under-water/ice Cherenkov technique in the south polar ice capachieving an angular resolution of . 0.5o, and continuously observes the entire sky. IceCube’srealtime alert program notifies the astronomical community if a likely astrophysical neutrinosignal is identified to enable follow-up EM observations. This includes near-realtime publicalerts for single neutrinos events of likely astrophysical origin such as IceCube-170922A usingthe GRB Coordinate Network (GCN) [108]. Two underwater neutrino detectors are currently in

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