an italian proposal for mirax science case, configuration ... · inaf istituto di astrofisica...

INAF

ISTITUTO DI ASTROFISICA SPAZIALE E FISICA COSMICA Roma - Bologna

An Italian proposal for MIRAX

Science case, configuration and expected performances

Prepared by L. Amati, M. Feroci, F. Frontera, C. Labanti

Contributions by G. Ghirlanda, G. Ghisellini, R. Salvaterra, ………….

Reference MIRAX-IASF-SCI-001v6

Issue 1 Revision 6

Date 23 Dic 2010

Science case, concept and expected

performances

Doc: MIRAX-IASF-SCI-001v6 Issue:1, Revision: 6 Date: 23 Dec 2010

Pag.1/43

1


performances


Pag.2/43

2

TABLE OF CONTENTS

1. INTRODUCTION......................................................................................................................................................... 4

1.1. CONTEXT .................................................................................................................................................... 4 1.2. SCOPE OF THIS DOCUMENT ............................................................................................................................ 6

2. SCIENCE CASE ......................................................................................................................................................... 8

2.1. SCIENCE OBJECTIVES SUMMARY ................................................................................................................... 8 2.2. GAMMA-RAY BURSTS ................................................................................................................................... 9 2.3. X-RAY ALL-SKY MONITORING........................................................................................................................ 19

3. SCIENCE REQUIREMENTS..................................................................................................................................... 27

4. MAIN FEATURES OF THE INSTRUMENTATION PROPOSED.............................................................................. 29

4.1. INSTRUMENT SUMMARY ............................................................................................................................... 29 4.2. THE X-RAY ALL-SKY MONITOR (XRM)............................................................................................................ 30 4.3. THE SOFT GAMMA-RAY SPECTROMETER (SGS) ............................................................................................... 31

5. EXPECTED PERFORMANCE.................................................................................................................................. 33

5.1. GAMMA-RAY BURSTS ................................................................................................................................. 33 5.2. COMPARISON TO OTHER GRB EXPERIMENTS .................................................................................................. 35 5.3. ALL-SKY MONITORING ................................................................................................................................. 38

6. REFERENCES.......................................................................................................................................................... 41


performances


Pag.3/43

3


performances


Pag.4/43

4

1. INTRODUCTION

1.1. CONTEXT MIRAX is the first Brazilian astrophysical space mission (INPE; P.I Prof. J. Braga), scheduled for a fly in a nearly equatorial low Earth (~600 km) orbit on board the LATTES satellite in the 2015 – 2016 timeline. In addition to MIRAX, LATTES will have on board also the geophysical mission EQUARS, exploiting the multi-payload platform (PMM) developed by INPE. The mass and power budget are ~100 kg and ~90 W, respectively; the data link will be through the X band. Presently, MIRAX consists only of a small CZT coded mask camera (Hard X-ray Imager, HXI) of limited area, FOV and spectral capabilities, with the primary scientific objective of monitoring the Galactic center in the 10-200 keV energy band. Spacecraft parameters and MIRAX payload main features

Mass ~500 kg (PMM total), ~120 kg (MIRAX payload) Power ~240 W (total), ~100 W (MIRAX payload) Orbit Near equatorial (15o) , circular, ~600 km, 4 years Telemetry Telemetry: X-band, ~2-3 Mbps TBD Launch 2015; launcher to be selected Instrument parameters

Hard X-ray Imager (HXI) – INPE-CFA-UCSD

Broad band GRB/sky Monitor (INAF/INFN/Univ.)

Energy range 5-200 keV 1 keV – > 10 MeV Angular Resolution 5 arcmin 4 arcmin in 1-50 keV Detector type 2mm-thick CZT array Silicon drift chamber/NaI+CsI Spectral resolution < 3 kev @ 60 keV

(FWHM) ~1 keV @ 60 keV, 200 eV @ 6 keV

Localization <1 arcmin (10 σ) < ~ 1 arcmin (10 σ) in 2-50 keV Field-of-view 70º x 70º FWHM 3.5 sr Sensitivity ~ 2 mCrab (1 day, 5 σ) <5 mCrab (1 day, 5 σ) in 2-50

keV Table 1: Main features of the Lattes satellite as of June 2010 Following contacts and meetings with Prof. Joao Braga from the INPE (Brazil) and PI of the MIRAX mission, of an Italian collaboration including INAF/IASF Bologna, INAF/IASF Rome, University of Ferrara, INFN Trieste, and ICRAnet, an payload proposal has been submitted and discussed with the INPE, in a meeting held in June 2010. The instrumentation proposed is expected to give a high scientific return in the field of the GRB astrophysics and in sky monitoring of celestial sources. It exploits: a) the R&D activities supported in the last years by ASI, INAF and INFN for an innovative Silicon Drift Detector; b) the experience acquired from the PI-ship of the BeppoSAX PDS and GRBM instruments; c) the experience acquired from the PI-ship of the Super-AGILE experiment; d) the excellent competence acquired in the data analysis and interpretation of the GRB and celestial source observations.


performances


Pag.5/43

5

Figure 1.1: A schematic view of the Lattes satellite and key orientations Table 2: The Italian MIRAX collaboration as of September 2010

IASF Roma

Marco Feroci**, Andrea Argan, Riccardo Campana**, Imma Donnarumma, YuriEvangelista**, Ettore Del Monte**, Sergio Di Cosimo, Giuseppe Di Persio,Francesco Lazzarotto, Marcello Mastropietro, Ennio Morelli, Fabio Muleri,Enrico Costa, Luigi Pacciani**, Massimo Rapisarda, Alda Rubini, Paolo Soffitta

INAF

IASF Bologna

Lorenzo Amati, Claudio Labanti, Martino Marisaldi, Fabio Fuschino, MauroOrlandini, Alessandro Traci

OAB, Merate

Gabriele Ghisellini, Giancarlo Ghirlanda

SNS, Pisa Elena Pian

Sez. Trieste Andrea Vacchi, Gianluigi Zampa, Nicola Zampa, Alexander Rashevski, ValterBonvicini

Sez. Bologna

Giuseppe Baldazzi

INFN

LNGS Francesco Vissani, Giulia Pagliaro

Ferrara Filippo Frontera*, **, Alessandro Drago**, Cristiano Guidorzi*, Ruben Farinelli*,Lev Titarchuk

Como Ruben Salvaterra* L’Aquila Francesco Villante** Pavia Piero Malcovati, Luca Picolli, Marco Grassi

University

Pisa Ignazio Bombaci** SNS, Pisa Mario Vietri* ICRANET Pescara Remo Ruffini and his team * = INAF associate, ** = INFN associate


performances


Pag.6/43

6

.

1.2. SCOPE OF THIS DOCUMENT This document describes the scientific case, the scientific requirements, the concept design and the expected performances of the GRB and X-ray all-sky monitor proposed by the Italian collaboration for the inclusion in the payload of MIRAX. More technical details on the payolad design and requirements are reported in [DA 03]. The instrumentation proposed is the result of activities supported partially by the ASI under the contract ASI-INAF for the support of the high energy astrophysics studies [DA 04] and partially by other ASI contracts for the support of R&D.


performances


Pag.7/43

7


performances


Pag.8/43

8

2. SCIENCE CASE

2.1. SCIENCE OBJECTIVES SUMMARY The MIRAX payload proposed by the Italian collaboration has two main scientific objectives:

a) measuring photon spectrum and timing of the prompt emission of gamma ray bursts (GRBs) over a broad energy band, from ~1 keV to 10 MeV, combined with arcmin location accuracy;

b) monitoring celestial X-ray sources in 2 – 50 keV with a broad FOV, a few arcmin source location accuracy and a few mCrab daily sensitivity.

Concerning GRBs, MIRAX will achieve scientific goals of fundamental importance and not fulfilled by GRB experiments presently flying (e.g., Swift, Konus/WIND, Fermi/GBM, MAXI) and future approved missions (SVOM). These can be summarized as follows:

- detection and study of transient X-ray absorption column / features for tens of medium/bright GRBs per year. These measurements are of paramount importance for the understanding of the properties of the Circum-Burst Matter (CBM) and hence the nature of GRB progenitors (still a fundamental open issue in the field). In addition, as demonstrated by us with BeppoSAX (Amati et al. 2000, Science, 290, 953), the detection of transient features can allow the determination of the GRB redshift to be compared, when it is the case, with that determined from the optical/NIR lines;

- to perform an unbiased measurement of the GRB time resolved spectral shape and its evolution down to about 1 keV in photon energy which is crucial for testing models of GRB prompt emission (still to be settled despite the considerable amount of observations). For the strongest GRBs, time resolved spectra with very short integration time (ms time scale, TBV).

- to study the erratic time variability down to sub-millisecond time scale. This is crucial to establish the intrinsic Magneto-Hydrodynamic (MHD) time scales of the GRB source

- to provide a substantial increase (with respect to the past and current missions) in the detection rate of X-Ray Flashes (XRF), a sub-class of soft / ultra-soft events which could constitute the bulk of the GRB population and still have to be explored satisfactorily.

- to extend the GRB detection up to very high redshift (z > 8) GRBs, which is of fundamental importance for the study of evolutionary effects, the tracing of star formation rate, ISM evolution, and possible unveiling of population III stars;

- to perform an accurate determination of spectral peak energy, which is a fundamental quantity for the test and study of spectrum-energy correlations and the possible use of GRBs as cosmological probes;

- to achieve a good detection efficiency for short/hard GRBs, whose difference with long GRB in terms of progenitors and emission physics is a central issue in the field;

- to provide fast and accurate location of the detected GRBs to allow their prompt multi-wavelength follow-up with ground and space telescopes, thus leading to the identification of the optical counterparts and/or host galaxies and to estimate of the redshift, a fundamental measurement for the scientific goals listed above, comparing it with that determined from X-ray absorption lines (see above).

Concerning all sky-monitoring, the scientific objectives include:

- detection and localization within a few arcimn of Soft Gamma Repeaters (SGR) and many other classes of galactic X-Ray Transients (XRT), like, e.g., galactic low and high mass X-ray binaries in


performances


Pag.9/43

9

outburst, cataclismic variables, accreting ms pulsars, etc., for spectral and timing studies and to trigger follow-up observations by ground and space observatories (included SRG itself), a fundamental service for the world-wide community, given the expected demise of RXTE/ASM in the next few years;

- to perform an all-sky survey in the 1 – 50 keV complementary, e.g., to that by eROSITA at lower energies.

In the following we provide more details on the science case driving our payload configuration proposed.

2.2. GAMMA-RAY BURSTS Discovered in the late 60s by military satellites and revealed to the scientific community in 1973, Gamma-Ray Bursts are one of the most intriguing "mysteries" for modern science (e.g., Mèszàros 2006, Gehrels et al. 2009). Indeed, despite they are outstanding (fluences up to more than 10-4 erg/cm2 released in a few tens of s) and very frequent (about 0.8/day as measured by LEO satellites) phenomena, their origin and the physics at the basis of their complex emission remained mostly obscure for more than 20 years. And even today, despite the huge observational efforts of the last decades, which provided a good characterization of the bursts temporal and spectral properties, the accurate localization and consequent discovery of their multi--wavelength afterglow emission, the determination of their cosmological distance scale and the evidence of a connection with peculiar type Ib/c SNe, several relevant open issues have still to be addressed, both from the observational and theoretical points of view. 2.2.1. GRB prompt X-ray emission: physics One of the most relevant open issues in GRB science is the understanding of the physics at the basis of the complex “prompt” emission. Indeed, while the basic properties of the afterglow emission (fading law of the flux and power-law spectral shape) can be satisfactorily explained in the framework of the "standard" fireball plus external shock scenario, the physics underlying the complex light curves and the fast spectral evolution of the "prompt" emission (i.e., the burst itself) is still far to be understood. Presently, there is a "forest" of models invoking different kinds of fireball (kinetic energy dominated, Poynting flux driven), of shocks (internal, external) and of emission mechanisms (synchrotron and/or Inverse Compton originated in the shocks, direct or Comptonized thermal emission from the fireball photosphere, and mixtures of these), which is very difficult to discriminate. Under this respect, the measurements of the low energy (<20keV) light curves and spectra are of key importance. A simple analysis of the morphology of the light curve as a function of energy shows how much information is contained in the X-ray emission of GRBs (Figure 2.1). For instance, the broadening and increasing time lag of the pulses observed in X-rays with respect to gamma-rays (Figure 2.1) is a crucial test for the emission models. Strong constraints on the models can be put by the low energy spectra (Figure 2.2). For instance, it was found that for several GRBs the slope of the X-ray spectrum during the early emission was inconsistent with the predictions of optically thin synchrotron shock models (Preece et al. 2000, Frontera et al. 2000). Another significant example is the inconsistency of the BeppoSAX/WFC spectra (2-28 keV) with the low energy spectrum predicted by models in which the emission of GRBs is dominated by Comptonized thermal emission of the photosphere (Ghirlanda et al. 2007). It is worthwhile to notice that this evidence could not be found with BATSE spectra alone, which have a low energy threshold at 25 keV (Figure 2.2). Another intriguing case is the detection by BeppoSAX WFCs plus GRBM of a


performances


Pag.10/43

10

transient emission feature in the prompt emission of GRB990712 (Frontera et al. 2001). The feature was interpreted as evidence of blackbody emission from the fireball photosphere (Figure 2.2). The bulk of the study of the prompt X-ray emission of GRBs was performed by BeppoSAX and HETE-2, thanks to their energy band extending down to ~2 keV, leaving several open issues and evidences to be further investigated. Unfortunately Swift/BAT, Konus-Wind, GLAST/GBM, Suzaku/WAM, INTEGRAL/IBIS cannot perform this kind of studies due to their passband, that has a low energy threshold higher than 15 keV, but GBM (8 keV). Only in a few cases, like in the case of GRB 060126, thanks to the trigger by a precursor, or in the case of the very long duration GRB 060218, Swift could point its X-ray telescope (0.3-10 keV) to the GRB before the bulk of its prompt emission. Thus, the capability of measuring spectral and timing properties of prompt emission of GRBs down to ~1 keV should be a basic requirement for next generation of GRB experiments.

Figure 2.1 Relevance of the study of GRB prompt emission in the soft X-ray energy band (Frontera et al. 2000). Left: BeppoSAX light curves of 3 GRBs in 40-700 keV (bottom panels) and 2-28 keV (top panels) (Frontera et al. 2000). Right: Swift/XRT + Swift/BAT + Konus-WIND light curves of GRB060126, a rare case in which Swift was triggered by a precursor and could point XRT to the GRB before the bulk of its prompt emission. The amount of information added by the soft energy band is apparent.


performances


Pag.11/43

11

Figure 2.2 Relevance of the study of GRB prompt emission in the soft X-ray energy band. Left: BeppoSAX 2-700 keV time resolved spectra of GRB 970111 (Frontera et al. 2000). The soft energy band allows to detect a clear deviation from the predictions of synchrotron shock models(dashed lines). Middle: a transient thermal component in the spectrum of GRB 990712 (Frontera et al., 2000). Right: extrapolation to X-rays of the best fit models (Band function, cut-off power law and BB + power-law) of the BATSE spectrum of GRB 080329 (Ghirlanda et al. 2007). The “quasi thermal” model can be ruled out based on low energy (2-28 keV) WFC data. 2.2.2. GRB prompt X-ray emission: circum-burst environment and progenitors The nature of the progenitors of long (>2 s) celestial Gamma ray Bursts (GRBs) is still an open issue. Long GRBs are most probably connected to the collapse of massive fast rotating stars (hypernova model, e.g., Paczynski 1998). In the most extensively discussed scenario, the collapsar model, a black hole forms due to the initial failing of the supernova explosion and the GRB originates from a jet emerging along the rotation axes. A strong prevision of the collapsar model is that the revitalized Supernova explodes simultaneously to the GRB, since they are both powered by the energy extracted from the accretion disk around the black hole. In other models Supernova and GRB can be temporally separated, with the Supernova preceding the GRB by a time delay which can widely vary. For instance in the supranova model (Vietri & Stella 1998) the GRB is associated with the delayed collapse of a rotationally stabilized neutron star originated by a previous supernova explosion. An alternative model is based on the collapse of the neutron star, formed at the moment of a (partially successful) SN explosion, into a more tightly bounded compact star which can be a quark star or a hybrid hadron-quark star (Berezhiani et al. 2003). The energy released during the burning of hadrons into quarks powers the inner engine of the GRB and it can also help the Supernova to explode completely (Drago et al. 2008). The time delay between the first (partially successful) supernova explosion and the GRB is regulated by the slow-down of the rapidly rotating compact star and/or by the mass accretion on the neutron star and it can vary from minutes to days or years, depending on the rotational frequency of the neutron star and on its initial mass. All these models predict that the pre-burst environment is characterized by a high density gas, due to strong winds from the massive progenitor in the case of a hypernova or a substantial enrichment of heavy elements by a previous supernova explosion (SN) in the case of the supranova model (e.g., Weth et al. 2000) or in the case of the quark-deconfinement model. Actually the discovery of GRB030329, found to be connected with the energetic supernova SN2003dh (e.g., Hijorth et al. 2003, Stanek


performances


Pag.12/43

12

et al. 2003), would points toward a hypernova model, even if the simultaneity of the two events has not been fully proved. It is however still not yet clear whether all GRBs are connected with energetic supernovae, being in many cases no evidence of a supernova spectral shape in the optical emission of GRB afterglows. In addition there are even cases in which energetic supernovae have no simultaneous GRB events associated with them, as in the case of SN2002ap (e.g. Wang et al. 2003). The presence of a post-supernova environment can be tested from the study of the GRB at low energy X-rays. The X--ray spectrum of the burst should show a low-energy cut-off and/or absorption features due to the interaction of the burst photons with circumburst material. In both cases, due to the progressive photoionization of the neutral gas by the GRB photons (e.g., Boettcher et al. 1999) or to the electron temperature increase of an already almost photoionized medium, both the absorption features and the low-energy cut-offs are expected to be transient. In fact, with the Wide Field Cameras (WFCs, 2-28 keV, Jager et al. 1997) and the GRBM (40-700 keV, Frontera et al. 1997) aboard the BeppoSAX satellite, variable absorption was detected in the X--ray spectrum of the prompt emission of a few GRBs (GRB000528, Frontera et al. 2004; GRB010222, in ‘t Zand et al. 2001; GRB010214, Guidorzi et al. 2003). Also evidence of transient absorption features (at 3.8±0.3 keV from GRB990705, Amati et al. 2000; at 6.9±0.5 keV, Frontera et al. 2003) soon after the onset of the GRB was reported (see Figures 2.3 – 2.4). These features were present in the first part (GRB rise) of the burst and faded away soon after. Interpreted by Amati et al. (2000) as a cosmologically redshifted K edge due to neutral Fe around the GRB location, the redshift of GRB990705 could be derived (0.86±0.17) and later confirmed from the optical redshift (zopt = 0.84) of the associated host galaxy (Le Floc’h et al. 2002). With this assumption, the Iron relative abundance with respect to the solar one was derived, finding Fe/Fesun = 75±19, which is typical of a supernova environment. An alternative explanation of the transient absorption feature from GRB990705 was given by Lazzati et al. (2001), who assumed that the feature is an absorption line due to resonant scattering of GRB photons on H-like Iron (transition 1s-2p, Erest = 6.927 keV). Also in this case the redshift derived is consistent with that of the host galaxy and the line width is interpreted as due to the outflow velocity dispersion (up to ~0.1c) of the material, which should have a Fe abundance 10 times higher than that of the solar environment. Thus in both scenarios, the observed feature points to the presence of an iron-rich circumburst environment. Also in the case of GRB011211, the absorption feature points to an Iron-rich environment outflowing at very high speeds from the GRB site (Frontera et al. 2003). The study of variable absorption could provide strong support to the ionization process of the circumburst environment and its composition as a consequence of the huge radiation flux produced in a GRB event. Notice that the SWIFT mission (Gehrels et al. 2004), in spite of the enormous effort to promptly follow-on (slew time ~ 1 min) in the 0.2-10 keV band the GRBs detected with the GRB localization telescope BAT (10-150 keV), is still unable to study the early phases of the 1-300 keV prompt emission , when the absorption features are expected (in the case of GRB990705, the absorption feature was visible only in the first 13 s) and a significant absorption cut-off is expected. This will be also true for the foreseen Chinese mission SVOM and, e.g., it is true fro the GBM aboard the Fermi satellite. As discussed above, in rare cases of trigger on a precursor or of very long events, Swift can point its XRT (0.3-10 keV) to the burst location before the end of the prompt emission. In several cases, like, e.g., GRB 050904 and GRB 060218 (Campana et al. 2007a and 2008), Swift/XRT could measure a column density significantly in excess to the galactic one and decreasing with time (Figure 2.4), thus further supporting the need of detectors capable of measuring the X-ray emission since the early phases of the GRB with excellent spectroscopic capabilities (energy resolution of a few hundreds of eV).


performances


Pag.13/43

13

Figure 2.3 Transient absorption features in the X-ray energy band: absorption edges detected by BeppoSAX/WFC in the first 8 s of GRB 990705 (left) and in the first 200 s of GRB011211 (right) (Amati et al., 2000; Frontera et al. 2004).

Figure 2.4 Transient absorption features in the X-ray energy band: variable absorption column NH measured by BeppoSAX/WFC in the prompt emission of GRB 000528 (left, Frontera et a. 2004) and by Swift/XRT in the late prompt / early afterglow emission of GRB 050904 (right, from Campana et al. 2007a). The early study of the spectral evolution of the burst phase is also mandatory if GRBs have to be used to investigate the condition of the interstellar medium in high resdhift objects. Most of the opacity evolution, which is principally due to photoionization of gas-phase ions and of dust grain evaporation, takes place in the early phase of the event (e.g. Lazzati & Perna 2003). 2.1.3 X-Ray Flashes (XRFs) Since ~2000, a new class of fast transients (X-Ray Flashes, XRFs) has been discovered and studied, first by BeppoSAX (e.g., Heise et al. 2001) and then by HETE-2 (Sakamoto et al. 2005, Pelangeon et al. 2008). XRFs


performances


Pag.14/43

14

show similar durations of long GRBs, are isotropically distributed in the sky and show similar rate of occurrence. Their main property is however that most of the prompt emission occurs in the X-ray band (2-20 keV), with negligible emission at higher energy (Figure 2.5). Likely this class is strictly related to that of GRBs extending it (Figure 2.5). However, their origin, in particular the absence of gamma-ray emission, is still debated. They could be normal GRBs with high cosmological redshift (>5), in which the gamma-ray radiation is down-shifted to the X-ray range (however, the few XRFs wth known redshift are characterized by z < 2) , or an extension of the GRB phenomenon. In the fireball model for GRBs, XRF could be characterized by a lower bulk Lorenz factor, due to a high baryon load of the fireball (“dirty fireball”). Actually also a "clean fireball" might produce XRFs, because higher Lorentz factor flow would cause internal shocks at larger radii where magnetic field density (or equivalently internal energy) is smaller (Mochkovitch et al. 2004). A recent analysis based on the complete HETE-2 spectral catalog (Pelangeon et al. 2008) shows that most XRFs lie at low / moderate redshift and that, important, they are likely predominant in the GRB population and closely linked to the ‘classical’ GRBs. Thus, the study of XRFs, which requires detectors with a passband down to soft X-rays, is very important for the full understanding of the GRB physics, true rate and thus progenitors. The SWIFT mission, given the hard energy band of the BAT GRB detector (15-150 keV), is not giving a relevant contribution to the study of the XRFs. A remarkable exception is the very soft and long GRB 060218, which lasted thousands of seconds and thus could be measured by the Swift X-ray telescope, which can point a GRB position typically within 60-100 s. In addition to some peculiar properties linked to the SN progenitor, Swift could measure the very low peak energy Ep of this event, showing that it lies on the Ep,i-Eiso correlation holding for long normal GRBs (Figure 2.6), a further evidence of a link between XRFs and GRBs.

Figure 2.5 X-ray flashes: typical light curve (left, from Amati et al. 2004) and distribution of spectral peak energy (right, from Kippen et al. 2001).


performances


Pag.15/43

15

Figure 2.6 Left. The continuous lines (red = based on GRBs with known redshift; black = based on GRBs with known redhshift and estimated pseudo-redshift) show the unbiased Ep distribution, showing the predominance of the XRFs (low Ep) population as inferred from HETE-2 data by Pelangeon et al. (2008). Right. XRFs extend the Ep,i-Eiso correlation holding for long normal GRBs. 2.1.4 High-redshift GRBs Our observational window on the Universe extends up to z=8.2, the redshift of GRB 090423 (e.g., Salvaterra et al. 2009), the most distant object discovered so far, and then recovers at z=1000, the epoch of primordial fluctuations measured by BOOMERANG and WMAP. The formation of the first objects, stars, and protogalaxies, should have taken place at epochs corresponding to z=10-30, certainly beyond z=7. These first gravitationally bound proto-systems are the result of the evolution of the primordial fluctuations observed at z=1000, this evolution depending on cosmological models and dark-matter properties. The big observational gap in between these epochs is then particularly serious. Discovering and studying the first "light" from primordial gravitationally bounded object in the Universe at z=10-30 is thus a primary goal of Cosmology and Astrophysics. Far Infrared and X-rays are the only two windows in which these studies can be attempted, with the X-ray revealing the most energetic part of the phenomenon. According to the theory, the very first stars (population III) formed about 180 million years after the Big Bang. They were very massive (> 100 solar masses) objects so that in a few millions years collapsed and exploded, likely producing a GRB (Woosley & Heger 2006). The explosion enriched with metals their environs for the subsequent generation of stars, and led to the formation of primordial black holes, seeds of the huge black holes now found at the center of nearly all galaxies. The only mean to detect the early populations of stars in the foreseeable future is through their explosive end, leading to a GRB (for instance, Meszaros & Rees 2010 predict that the death of a pop III star should produce a GRB with a spectral peak at ~50 keV). Indeed it is expected that a substantial fraction of GRB lies at redshift greater than 7 (e.g. Bromm & Loeb 2002) so we should have already detected some of them, but we miss the redshift information, accessible only through X-ray (or in the FIR and below) Thus, the detection and a few arcmin localization of high redshift GRBs, needed to allow optical follow-up and hence redshift estimate, is of fundamental importance not only for GRB physics and progenitors, but also for the study of the star formation rate evolution, of pop III stars and, more in general, of the properties of the universe at the ionization epoch. Several recent studies (e.g., Salvaterra et al. 2008 - 2009) show that lowering

short

sub-en.

XRFs


performances


Pag.16/43

16

the threshold of the GRB detector energy band can increase substantially the sensitivity to high-z GRB (Figure 2.7).

Figure 2.7 Left. Redshift distribution of GRBs.. Right. Fraction of the sky to be observed in order to detect 1 GRB per yr at z ≥ 10 as function of the photon flux limit and detector energy band: the lower the threshold, the higher the efficiency to high z GRBs (from Salvaterra et al. 2008). 2.1.4 GRB physics and cosmology with spectrum-energy correlations Because of their brightness (isotropic-equivalent radiated energies up to more than 1054 erg) and distance distribution (z up to 8.2, <z>~2.3) (see Fig. 2.7), GRBs are promising candidates to constrain cosmological parameters if, similarly to Supernovae (SN), they (or a subclass of them) can be proven to be used as standard candles. GRBs have a redshift distribution skewed towards higher redshifts with respect to SN (z upt to ~1.7). Therefore, GRBs provide information complementing that derived from SN only on early epochs of the Universe, when dark energy was supposedly starting to counterbalance the gravitational pull of dark matter. This requires that the energy or the luminosity is precisely estimated from observable quantities. Correlations linking either energy or luminosity to observable quantities have been presented in the literature. It was shown (Frail et al. 2001) that by accounting for the GRB jet opening angle the true collimation corrected energies Eγ clusters around ~ 1051 erg but they are still too dispersed for precision cosmology (Bloom et al. 2003). Even by considering the (rest frame) peak spectral energy of the EFE spectrum, Ep, which was discovered to be strongly correlated with Eiso (Amati et al. 2002, 2006), GRBs are not standard candle due to the significant dispersion of the Ep-Eiso correlation (Fig. 2.8).


performances


Pag.17/43

17

Figure 2.8 Left: typical photon and energy spectrum of a GRB. Right: the Ep,I – Eiso correlation (from Amati et al. 2009)

Figure 2.9 Left: Comparison between the Ep,i – Eiso (pink shaded region) and Ep,i – Eγ (orange shaded region) correlations . Right: confidence contours in the ΩM – ΩΛ plane as determined with the Ep,i – Eγ correlation compared with those obtained with SN Ia (see text for more details). Figures from Ghirlanda et al. (2009). However, it was found evidence that this correlation tightens by substituting Eiso with the collimation corrected energy Eγ (Ghirlanda et al. 2004a). The lower scatter of this correlation (Figure 2.9) prompted the investigation of the use of GRBs as standard candles (Ghirlanda et al. 2004b, Dai et al. 2004). The results obtained with the first sample of 19 GRBs (Ghirlanda et al. 2004b) and with the more recent sample of 29 bursts (Figure 2.9) are promising (though not competitive with the constraints obtained with other cosmological probes due to the still limited number of bursts). Indeed, GRBs should be considered as complementary cosmological tools (for the estimate of the cosmological parameters) due to their much higher redshift coverage and for the advantage that their detection is free from extinction problems. The main drawback of the use of the Ep-Eγ correlation in cosmology comes from the need to estimate the jet opening angle. This parameter is derived, within the standard afterglow theory, from the measure of the break time tb of the burst optical light curve. Therefore, in order to populate the Ep-Eγ correlation, in addition to the measure of the prompt emission spectrum and of the redshift of GRBs, also a long lasting (up to several days after the trigger) follow up campaign of the optical afterglow is required. Furthermore, the observational body brought by SWIFT casts some doubts about the interpretation of the observed breaks in terms of jet outflows


performances


Pag.18/43

18

(e.g., Campana et al. 2007b). Based on this, in 2008 Amati et al. reported the possibility of using the simple Ep – Eiso correlation to extract information on cosmological parameters. Despite its larger scatter, this correlation has the advantages of being based on 2 observables only, thus reducing the impact of systematic effects and significantly increasing the number of GRBs that can be used for the analysis (~115 against ~25, as of mid 2010) with respect to the Ep-Eγ correlation. As shown in Figure 2.10, the Ep – Eiso correlation can indeed already provide useful constraints on cosmological parameters. In addition, increasing the sample improves the accuracy, thus showing that the method is still not dominated by systematics. One of the greatest expectations of the use of GRBs for cosmology is the possibility to study the nature of the Dark Energy. Indeed, GRBs would extend the SNIa redshift range out to epochs before the reacceleration (see, e.g., Firmani et al. 2006 for the combination of GRB and SNIa to contrain the Dark Energy equation of state). Most of the issues for the use of GRBs as standard candles are related to the still limited number of bursts used to this purpose and to the lack of a clear theoretical interpretation for the observed empirical correlations. One major problem is that there are not enough bursts at low redshifts to calibrate these correlations. This introduces a circularity problem when the same correlations are used to constrain the cosmological parameters (Ghisellini et al. 2005). A solution to this problem is to collect a relative large number of bursts (~ a dozen) within a small redshift bin ∆z/z~0.1 (Ghirlanda et al. 2006). The small number of bursts also prevent to study possible systematics effects which might be present in these correlations (e.g. evolution of bursts properties with redshift). Thus, a significant increase of the number of GRBs with measured redshift and well determined spectral parameters is a fundamental goal for next generation GRB experiments. This requires a broad field of view, a spectral characterization of the prompt emission over a broad energy band (from a few keV to a few MeV), joined with arcmin source location accuracy allowing prompt optical follow-up and thus favouring optical spectroscopy and redshift detemirnation. It is important to note that, as discussed by, e.g., Zhang & Meszaros (2002), Amati (2006), Amati et al. (2007), in addition to cosmology, the Ep,i – Eiso and other spectrum-energy correlations can provide insights on the physics of the prompt emission of GRBs and on the identification and understanding of different sub-classes (short/long, sub-energetic, local GRB associated with SN, XRFs) of GRBs. Time resolved spectral analysis of sub-samples of BATSE, BeppoSAX and Fermi GRBs (e.g., Liang et al. 2004, Ghirlanda et al. 2010, Frontera et al. 2010) have shown that, for several events, the Ep,i – luminosity relation is conserved within single GRBs. This evidence is important because it further supports a physical origin of the Ep,i – Eiso correlation and gives further clues to the emission mechanism(s) responsible for the prompt emission. This kind of studies, in addition to the above requirements, demands an effective area of at least 500 cm2 up to 5 – 10 MeV, in order to collect enough photons to perform high time-resolution (10 ms or less) spectroscopy for the brightest GRBs.


performances


Pag.19/43

19

Figure 2.10 Estimating cosmological parameters with the Ep,i – Eiso correlation. Left: results with the sample of 70 GRB of Amati et al. (2008) and with the present improved sample of 117 GRBs. Right: results with the present improved sample of 117 GRBs compared with those from other probes.

2.3. X-RAY ALL-SKY MONITORING The most sensitive observatories have in general a narrow field of view (~1º or less) and are designed to perform studies of individual sources. However also a wide field instrumentation is needed. One of its goal is to trigger Target of Opportunity observations with the narrow field observatories, to catch the most interesting states of the sources, often unpredictable, with very high sensitivities. Also, deep observations of persistent but variable sources are requested to can be set in the context of the history or evolution of the source (or class of sources). Indeed, often wide field telescopes have been part of the payload of many satellite X-ray missions (e.g., RXTE, BeppoSAX, INTEGRAL, Swift, AGILE, Suzaku) and are desired to be also in future missions. Indeed, the most recent ASTRONET report (“The ASTRONET Roadmap: A Strategic Plan for European Astronomy”) identified the All Sky Monitoring as one of two specific gaps in the strategic planning for the future European space missions. Independently of the service function, long-term, continuous monitoring of celestial X-ray sources allows the study of source/class properties not easily or inaccessible to specific, short observations, although more sensitive. Examples include the discovery of new Galactic transient sources, the discovery and long-term evolution of orbital, super-orbital and spin periodicities, period derivatives and quasi-periodicities (QPOs), the intensity and spectral state changes in BHC, the multi-frequency correlation between timing, spectral and intensity parameters, the discovery and monitoring of bursting behaviour (bursters, SGRs, AXPs, ...), complete all sky surveys. In addition to Galactic sources, All Sky Monitoring is also important for AGN studies, as shown, e.g., by the recent Swift/BAT survey (Ajello et al. 2009). The real need is for a simultaneous monitoring of the largest fraction of the sky. In fact, it is worth stressing the fundamental difference between “sampling” and


performances


Pag.20/43

20

“continuous monitoring” of the sky. The former is much less demanding in terms of instrumental resources, both for hardware and mission operation. However, when the target sources are transient and variable on short timescales, the difference between short and sparse observations and continuous monitoring may be crucial. We show an example in Figure 2.11, where data of Vela X-1 from Swift/BAT (a “sampling instrument”) and AGILE/SuperAGILE (a “continuous monitoring” instrument) are compared. In this case, more sensitive but shorter observations 1 day apart provided a completely different view of the source status and missed the >2 Crab flare. Figure 2.11 Flux data from Vela X-1, as observed by Swift/BAT(black) and AGILE/SuperAGILE (red), highlighting the difference between continuous monitoring and sparse sampling. 2.2.1. Monitoring of timing parameters A wide field of view allows to monitor simultaneously a large number of variable, periodic, quasi-periodic and aperiodic sources (Fig. 2.12). This monitoring allows to derive different properties of these sources, among which their timing parameters. In Figure 2.13 we show an example from the SuperAGILE data, showing the Vela field, where the X-ray binary pulsars Vela X-1, Cen X-3 and GX 301-2 are observed simultaneously, both in imaging (left), flux (center) and pulse shape (right). It is worth noticing that a number of sources in this class have long spin and orbital periods (hundreds of seconds and tens of days, respectively) making them practically inaccessible to the narrow-field observatories. The study of the impact of the variation of the geometry and intensity of the accretion flow along the binary orbit on the accretion torque and its manifestation on the pulse shape requires a continuous, long-term monitoring and it is a science topic within the reach of only the wide-field experiments. With reference to the above SuperAGILE observations, GX 301-2 is an eccentric binary with ∼690 s spin and ∼42-day orbital period.


performances


Pag.21/43

21

Figure 2.12 Example of simultaneous monitoring of large fields with SuperAGILE: detection and localization with orthogonal one-dimension images, light curves and phase analysis of sources in the Vela region.

Figure 2.13 Example of long-term monitoring of the timing parameters of the X-ray binary GX 301-2 with long spin (∼690 s) and orbital (∼42 days) periods (Evangelista et al., ApJ 2010). Left: history of the spin period, displaying the secularly variable accretion torque as monitored by the 70’s till today. The shaded region, zoomed in the panel on the bottom, shows the SuperAGILE contribution. Right: the 42-day orbital flux variation (top panel) and the orbital variation of the pulsed fraction. 2.2.2. Monitoring of spectral parameters Galactic compact objects display large spectral variability. Of particular interest are the spectral transitions of the black hole candidates, that are often associated with significant variations in the aperiodic and quasi-periodic timing properties as derived from the power spectra. The state transitions are unpredictable and the only way to pinpoint the source with much more sensitive (imaging and/or spectral and/or timing) narrow field telescopes, is to monitor the source frequently or continuously. In Figure 2.14 we show an example of the


performances


Pag.22/43

22

state transitions of the BHC GX 339-4, as observed through the wide field experiments RXTE/ASM (soft X-rays) and BATSE (hard X-rays) and followed-up in the radio band. The typical variations in the energy spectra (panel on the right) are outstanding and reflect the large variations in the geometry of the emitting region and the connected physical processes.

Figure 2.14 An example of spectral state transition of a black hole candidate, here GX 339-4 (Fender et al. ApJ 1999). The panel on the left shows the effect of the transition on the count rate in a soft X-ray (RXTE/ASM) and a hard X-ray (BATSE) experiment. Correspondingly, the Low-Hard state of the source exhibit strong radio emission (top panel).The different spectral states are best seen in the energy spectra, panel on the right. 2.2.3. X-Ray Transients Among the most important results obtained by X-ray monitors such as the Wide Field Cameras onboard the X-ray satellite BeppoSAX and the All Sky Monitor onboard Rossi-XTE, there is the discovery that the X-ray sky is extremely variable. In particular, most of the X-ray binaries are transient systems, which spend part of their time in a quiescent state, with X-ray luminosity sometimes below the detection threshold of typical X-ray satellites, and occasionally the show X-ray outbursts during which the X-ray luminosity increases by several orders of magnitude (often up to the Eddington limit) and then gradually decreases when the source comes back to the quiescence state. The Fig. 2.15 shows a synthetic view of the variability of the X-ray sky, as recently collected by Soderberg et al. (2009). It is therefore of fundamental importance for the scientific community the existence of a sensitive X-ray monitor that can be used to spot the start of an X-ray outburst in order to collect the primary data and alert observations with appropriate instruments, since the most important and peculiar behaviour of these sources are usually observed when they are at the maximum of their luminosity.


performances


Pag.23/43

23

Figure 2.15 An interesting representation of the variable X-ray sky collected by Soderberg et al. 2009: the panel on the left groups the different classes of objects by their typical variability time scale vs the peak luminosity they reach during an outburst. The panel on the right shows the expected detection rate of each class of objects. It is emblematic in this sense the example given by the sub-class of sources known as Accreting Millisecond Pulsars (X-ray binaries containing a neutron star that has been spun-up by accretion of matter and angular momentum to very short - millisecond - spin periods). All the known sources of this sub-class are transient systems, which spend most of their time in quiescence and only very rarely show X-ray outbursts; only during these bright phases they show coherent X-ray pulsations. In these cases is therefore fundamental to catch the start of an outburst in order to determine the timing properties of the source (such as spin frequency and its derivative, orbital period, and even the orbital period derivative) which can give important information on the accretion torques onto the neutron star, the nature of the companion star, the evolution of the binary system, and even on the neutron star equation of state. Up to this moment this job has been performed by the All Sky Monitor onboard RXTE. However, RXTE is at the end of its operational lifetime. An X-ray Monitor with broad FOV and arcminute source location accuracy is therefore necessary to have the possibility to improve the spectacular results obtained up to now. The near future will also see the full operation of large field of view, monitoring instruments in other wavelengths, from the already operating Fermi in the gamma-rays, to CTA in the high-energy gamma-rays, to LSST in the optical and LOFAR in the radio. The variable sky will then be studied for the first time across the entire electromagnetic spectrum with unprecedented sensitivity and completeness. 2.2.4. Soft Gamma Ray Repeaters and Anomalous X-ray Pulsars Among the X-ray transients, the Soft Gamma Repeaters and Anomalous X-ray Pulsars (SGRs and AXPs, respectively, see Hurley 2000, Woods 2003 and Mereghetti 2009 for reviews) deserve a special note, being the objects most likely hosting the strongest magnetic field in the universe. They are X/gamma-ray transient sources that unpredictably undergo periods of intense bursting activity, separated by relatively long intervals (years, decades) of quiescence. To date, the SGR class includes six sources (SGR0525-66, SGR1627-41, SGR1806-20, SGR1900+14, SGR0501+4516 and SGR1833-0832) plus one candidate, SGR1801-23 (only


performances


Pag.24/43

24

two bursts detected) with position not well known (area of 80 arcmin2) to investigate its possible optical/radio counterpart. All confirmed SGRs, on the basis of their early determined positions, appeared to be located within young supernova remnants (SNRs) of ages ≤104 yr. However, from more precise locations, in most cases this association has been questioned and in some cases attributed to random chance (Gaensler et al. 2001). All SGRs appear to be in our galaxy, except SGR0525-66 which is in the Large Magellanic Cloud. Typically, bursts from SGRs have short durations(~0.1 s), recurrence times of seconds to years, energies of ~1041 ergs, assuming a distance D = 10 kpc. Their hard X--ray spectra (>25 keV) are consistent with an Optically Thin Thermal Bremsstrahlung (OTTB) with temperatures of 20--40 keV. During quiescence, persistent X-ray emission (< 10 keV) has been observed from three of them (SGR0526-66, SGR1806--20, SGR1900+14) with luminosities of 1035-1036 erg/s and power-law spectral shapes. In the case of SGR1900+14, an additional blackbody component (kTbb~ 0.5 keV) is requested (Woods et al. 2003). From the last three sources, during quiescence, also X-ray pulsations with periods in the range from 5 to 8 s and spin-down rates of 10-11 - 10-10 s/s have been detected. In the case of SGR1900+14 and SGR1806-20, evidence of X--ray lines has been reported: an emission line at ~6.5 keV from the former source (Strohmayer & Ibrahim 2000), while an absorption--like feature at ~5 keV from the latter source (Ibrahim et al. 2002). Very rarely, ``giant'' hard X- /soft gamma-ray flares have been observed. They show durations of hundreds of seconds, pulsations during most part of the event but the initial spike, and peak luminosities even greater than 1044 D210 erg/s. Giant flares have been observed from SGR0525-66 and SGR1900+14. On the basis of their locations and their spectral and temporal properties, in the absence of companion stars, SGRs are thought to be young (< 104 yrs) isolated neutron stars (NS) with ultrastrong magnetic fields (Bdipole>1014 gauss), or ``magnetars''. The magnetar model (Thompson & Duncan 1995) considers a young neutron star with a very strong magnetic field (1014 - 1015 G), whose decay powers the quiescent X-ray emission through heating of stellar interior, while the low-level seismic activity and the persistent magnetospheric currents (Thompson et al. 2002) periodically cause big crustquakes which trigger short bursts and large flares. In the magnetar scenario, the absorption feature from SGR1806-20 can be interpreted as ion-cyclotron resonance in the huge magnetic field of the NS. SGRs share some properties (pulse period distribution, spin-down rate, lack of a companion star, quiescent X-ray luminosity) with those of a peculiar class of neutron stars, the so--called anomalous X-ray pulsars (AXPs, see, e.g., Mereghetti 1999 for a review). Additional evidence for a link between the two classes has been provided by the detection of outbursting activity also from the AXPs 1E~2259+586 (e.g., Kaspi et al. 2003) and 1E1048.1-5937 (Gavriil et al. 2002). As above discussed, thus far all the burst observations of SGRs have been performed with hard X-ray detectors and low energy resolution (scintillator detectors). An X-ray detector (1-50 keV) with better spectroscopic performance, can perform an unprecedented study of the X-ray spectra and time variability of the transient X/gamma-ray emission from these sources, with the determination of the broad band continuum spectrum, possibly discovering X-ray absorption/emission features during the burst emission. In fact the consistency of the hard X-ray spectra of the burst with OTTB is far being the expected emission model from these sources


performances


Pag.25/43

25

Figure 2.16 Left: 1 – 120 keV spectrum of a short pulse from SGR1900+14, showing how simultaneous measurements in the soft and hard X-ray energy bands allow to discriminate different models. Right: relation between the two Black body components characterizing the same source. 2.2.1. All-sky survey in 2-50 keV All sky surveys in the soft X-rays dates back to the ROSAT (<2 keV) and HEAO-A1 times. The future plans include e-Rosita, planned for launch in 2012, that will perform the deepest sky survey below 10 keV. In the hard X-rays, Swift/BAT obtained a very good sky coverage, reaching 15-50 keV limiting fluxes as low as ∼10-11 erg cm-2 s-1 (0.4 mCrab, Figure 2.17), and detecting ∼1250 sources. An all sky monitor experiment as the one proposed for MIRAX operating all-sky in the energy range 2-50 keV will bridge Swift and e-Rosita. Figure 2.17 Limiting flux obtained by Swift/BAT over the full sky (color code units in c.g.s.), as derived by the 54-month Palermo Swift/BAT catalogue (Cusumano et al. 2010).


performances


Pag.26/43

26


performances


Pag.27/43

27

3. SCIENCE REQUIREMENTS

In order to achieve the scientific goals discussed and summarized in the previous section, the instrumentation required should show the following features. For Gamma-Ray Bursts • Energy passband from ~1 keV up to ~10 MeV. The lower threshold is fundamental for the study of transient X-ray absorption features and the substantial increase in the detection and study of XRFs and high-z GRBs. The wide band is of key importance for the identification estimate of fundamental spectral parameters like the low energy photon index and the peak energy Ep (GRB physics and cosmology) and to increase the detection efficiency for short GRB. • Energy resolution <~500 eV for photon energies < 10 keV for the study of the emission/absorption features from GRBs and the measurement of the peak energy of XRFs;; an energy resolution < 15% for photon energies > 50 keV is enough for accurate measurement of the spectral peak energy. • Source location accuracy of a few arcmin in order to trigger and allow follow-up observations of the detected transients by other telescopes, which is essential for fulfilling the scientific objectives for GRBs, for the all-sky monitor functionality and to perform a sensitive X-ray all-sky survey. • Field of View (FOV) of at least 2 sr combined with a sensitivity of ~ 500 mCrab (5σ, 1s)in the 3-50 keV energy band, in order to allow detection and localization of ~150 GRBs and XRFs per year (including ~2-4 events at z > 6), the sensitive spectroscopy of ~2/3 of them and the use of the instrument as an all-sky monitor for galactic transients (SGRs and other XRTs). • An average effective area of ~1000 cm2 up to 200 keV, ~500 cm2 up tp ~10 MeV within a FOV of 2 sr, to allow sensitive broad band spectroscopy of GRBs (for GRB physics and cosmology studies) and to increase the overall trigger efficiency, especially for short (and spectrally hard) GRBs. • On board data handling electronics allowing GRB trigger, discrimination of false triggers and fast source position reconstruction, in order to allow prompt alert distribution and follow-up observations. For all-sky monitoring • Energy passband from ~1 keV up to ~50 keV: the extension to low energy is of fundamental importance in order to catch most of the photons from the sources and in order to optimize the joint analysis with lower energy detectors; the extension to the hard X-ray optimazes the detection efficiency and spectral sensitivity to the hardest X-ray transients and SGRs; • Source location accuracy of a few arcmin, in order to allow the identification and analysis of different sources presented in the FOV, especially crowded in the Galactic plane fields and to allow sensitive follow-up of transient sources with telescopes at all wavelengths;


performances


Pag.28/43

28

• Field of view > 3 sr, in order to observe simultaneously a large fraction of the sky, thus increasing the exposure / elapsed time ratio for each source in the sky, optimizing the probability of catching fast X-ray transients and allowing a nearly continuous sampling of sources light curves • Timing resolution of ~10 µs and energy resolution of ~500 eV,, in order to allow accurate timing and spectral analysis, qhich is of paramount importance fot the understanding of the nature and physics of both persistent variable sources and the different classes of X-ray and soft gamma-ray transients. • Effective area of ~500 cm2 from a few keV up to a few tens of keV, in order to acquire light curve and spectral with good enough statistical quality for sensitive spectroscopic and timing analysis


performances


Pag.29/43

29

4. MAIN FEATURES OF THE INSTRUMENTATION PROPOSED

4.1. INSTRUMENT SUMMARY The Italian payload we propose for MIRAX is composed of two independent instruments, aimed at two main observational objectives: a) detection, localization and wide-band (about 1 keV – 10 MeV) spectral and timing measurements of Gamma Ray Bursts (GRBs); b) All Sky Monitoring, i.e., long-term monitoring of celestial sources and discovery of new ones. Particular care will be devoted to the monitoring of the Galactic plane. The instrumentation is then be composed of a thee pairs of coded-mask X-ray imagers (X-Ray Monitor, XRM) and two modules of 4 scintillator spectrometer units (Soft Gamma-ray Spectrometer, SGS). The instrumental approach to achieve the science goals is to use separate detectors for the low and high part of the energy band. The low energy (about 1 - 50 keV) is best covered with Silicon detectors, whereas the high energy portion of the spectrum (15-10000 keV) is covered with inorganic scintillators in phoswich configuration. Given that the LATTES satellite hosts along with MIRAX the payload EQUARS devoted to the observation of the Earth, it will move like the International Space Station, with the MIRAX zenith continuously drifting and covering 360 deg in each orbit. This zenith-drift properties allows to observe almost the entire sky in an orbit if the field of view of the MIRAX telescopes is as large as possible in the direction orthogonal to the direction of motion of the spacecraft, while in the other direction it can be smaller, being covered through the satellite motion. We have achieved this goal by adopting the configuration illustrated in Figures 4.1. The XRM consists of a 3 pairs of Silicon Drift detectors (initially developed for the ALICE experiment for the CERN/LHC accelerator) surmounted by mono-dimensional coded masks (see main features in Tables 1 and 3). The SGS consists of two modules of 4 phoswich (= PHOSsphor sandWICH) units each. The detector units are made of NaI(Tl) and CsI(Na) scintillators viewed from a single photomultiplier (PMT). The photon energy lost in each scintillator can be established from the pulse shape analysis of the signals provided by the PMT. The same technique was successfully developed for the PDS instrument aboard BeppoSAX, one of the most sensitive instruments launched thus far. The phoswich configuration is the best technique to derive unbiased spectra in the hard X-/gamma-ray energy band. Specific features of XRM and SGS are described below.

Figure 4.1: An hypothesis of allocation of the XRM (in orange color) and SGS (in magenta color) experiments onboard the MIRAX payload and Lattes satellite.


performances


Pag.30/43

30

4.2. THE X-RAY ALL-SKY MONITOR (XRM) For the ASM experiment we envisage a set of three pairs of coded-mask X-ray cameras, each one with asymmetric 2D position capability. The fine imaging coding direction, with angular resolution in the range of ~5 arcminutes, will be oriented at 90º in each pair of cameras, in order to guarantee an overall 2D arcmin-localization capability by intersecting the information gathered from the two units. Both X-ray cameras will have also the capability to coarsely identify the second coordinate of sources, with an angular resolution in the range of a few degrees. This will help in improving the signal-to-noise ratio for the individual detection and to decrease the confusion limit. Each X-ray camera will be composed of a matrix of Silicon Drift Detectors, each one approximately 40-50 cm2, to form an X-ray sensitive plane with a total geometric area of about ~6-700 cm2 per unit. The field of view will be limited by a wide field collimator, to an opening of approximately ~4 steradian (TBV) covered by a coded mask, with a code reflecting the 2D asymmetric position capability of the detectors. The mask will be at a distance of approximately ~12-15 cm from the detector and it will have a size ~1.5 times that of the detector plane. This will result in a fully coded field of view (FCFOV) of ~0.7-0.8 steradians and a partially coded field of view (PCFOV) of ~3 steradians, for each of the cameras. The X-ray cameras will cover the nominal energy range 2-60 keV. In Fig. 4.2 we show a pictorial view of the X-ray cameras and coded mask.

Figure 4.2: Pictorial view of a XRM unit. With the aim of covering the largest portion of the sky, a preliminary configuration of the ASM units is shown in Fig. 4.1, with the 3 pairs of detector misaligned by 60º to each other in the direction orthogonal to the spacecraft motion. In Fig. 4 we show a cross section of the same sketch, where the field of view of the individual units is shown. In this configuration the total FCFOV approaches 2.5 steradian at any time, while the PCFOV extends the field of view to approximately 6 steradian, with the central unit adding partially coded area to some of the directions covered in full coding by the other units. With this configuration, considering the


performances


Pag.31/43

31

spacecraft zenith-pointing configuration, each direction will cross the field of view at a “speed” of 4º/minute. A single source at the zenith will then have a “transit time” across the FCFOV of approximately 700 s.

4.3. THE SOFT GAMMA-RAY SPECTROMETER (SGS) The SGS will be in charge of providing detection trigger and spectral information for GRBs in the nominal energy range 15 keV – 10 MeV. It will be composed by 8 units each one being an independent detector in which the active part is made by scintillating material with PMT readout and electronics. This will allow for an efficient read-out and improve the particle background rejection. The baseline design includes two set of 4 units each, passively collimated to reduce the background and slightly misaligned (~20°) in order to optimize the response to the same field of view as the X-ray imagers. The individual detectors will be made of NaI and CsI scintillating crystals, optically coupled in phoswich configuration and read-out by a single photomultiplier, using the pulse shape information to discriminate the signal. This technique, exploits the different time constant of two optically coupled scintillators, allowing to discriminate between photons releasing their full energy in the top scintillator and those releasing energy in both detectors. It was successfully adopted for the PDS instrument onboard BeppoSAX, providing excellent and still unique results, and will allow a reduction of the BKG by a factor of ~10 up to ~200 keV with respect to the use of a single crystal device (e.g., BATSE, Fermi/GBM). The dimension of each crystal will be: geometrical area 14x14 cm2 (12x12 cm2 active area) and thickness 4 cm (1 scm NaI + 3 cm CsI). Each of the 2 collimators will be made of Pb, 0.3 cm thick, with a total height of 10 cm (4 cm from the bottom to the top of the crystals plus 6 cm above them).

Figure 4.3: Sketch of a detection unit (left) and 1 of the 2 assemblies (right) of the the Soft Gamma-ray Spectrometer (SGS) Tables 4.1 and 4.2 summarize the expected main characteristics of the XRM and SGS instruments. Based on these features, we evaluated the expected performances of our proposed payload both for the GRB and ASM science, which are summarized in the following Figures and their captions, also compared to those of present and future experiments. In these simulations we took into account the expected orbit and inclination of the


performances


Pag.32/43

32

LATTES satellite, we adopted the standard level and spectrum of the diffuse cosmic X-ray background and we based on previous experiments (BeppoSAX, superAGILE) for the evaluation of the “intrinsic” background. .

Table 4.1: Main characteristics of the MIRAX/XRM

Expected Value Energy Range 2-50 keV Energy Resolution From 250 to 500 eV FWHM Time Resolution ∼10 µs Effective Area ∼550 cm2 in FCFoV (spectroscopy) Angular Resolution 5 arcminutes Point Source LocationAccuracy

<1 arcminute

Field of View ∼2.5 steradians FCFoV ∼3.5 steradians PCFoV

Sensitivity (5-σ, 1 detector, imaging, for spectroscopy 1.4 better)

700 mCrab or ∼2 ph/cm2/s in 1s,FCFOV 90 mCrab or ∼0.3 ph/cm2/s in 60s,FCFOV <26 mCrab/orbit in >Half Sky <10 mCrab/elapsed day over All Sky ∼ 3 mCrab/50 ks

Source Transit Time in FoV 700 s /orbit Telemetry (orbit average) 3800 kbits/s

Table 5.1: Main characteristics of the MIRAX/SGS

Expected Value Energy Range 15 - 10000 keV Energy Resolution 15% @ 60 keV , 8% #662 keV Time Resolution ~ 1 µs Effective Area (on axis) ∼1000 cm2 up to 300 keV,

~ 700 cm2 up to 1000 keV Field of View ∼2.5 steradians Sensitivity (5-σ) ~ 1 Crab in 1s Telemetry (orbit average) 40 kbits/s


performances


Pag.33/43

33

5. EXPECTED PERFORMANCE

5.1. GAMMA-RAY BURSTS In Fig. 5.1 (left panel) we show the effective area of each of the two MIRAX Italian instruments XRM and SGS, for any given sky direction; in Figure 5.1 (right panel) we plot the expected sensitivity to GRBs expressed, following Band (2003), in terms of minimum detectable 1s photon peak flux in 1 – 1000 keV as a function of spectral peak energy. As an example of simulation of the MIRAX/XRM+SGS capabilities, Figures 5.2 and 5.3 (left panel) show the transient absorption edge at 3.8 keV detected with BeppoSAX/WFC from GRB990705 (Amati et al. 2000, Science, 290, 953) as it would be detected with MIRAX. The much better capability of MIRAX is outstanding: the same line observed with WFC at a significance level of about 5 sigma is expected to be detected with MIRAX at a significance level of 50 sigma. This means that with MIRAX we expect to detect transient absorption lines about one order of magnitude weaker (see Fig. 5.3 right panel).

Figure 5.1 Left: Effective area as a function of energy of the XRM (2 modules) and for the SGS instrument (2 modules). Right: GRB trigger sensitivity of MIRAX expressed in terms of minimum detectable 1s photon peak flux in the 1 – 1000 keV energy band as a function of the spectral peak energy Ep (see Band 2003). Left: XRM vs. SGS.


performances


Pag.34/43

34

Figure 5.2: Sensitivity of MIRAX to absorption edges. Left panel: the spectrum of the first 13 s of GRB 990705 characterized by a transient absorption edge at 3.8 keV (see Figure 2.3) ad would be measured by the XRM. The residuals with respect to a Band spectral function with no edge are shown. The edge would be detected with a significance of more than 40 σ. Righ panel: the same but shown in terms of deconvolved spectrum

Figure 5.3 – Left panel: Sensitivity of MIRAX to absorption edges. Left: the same simulation of Figure 5.2 but including the SGS. The detection significance of the edge is improved to more than 50 σ. Right panel: significance of the detection of an absorption edge as a function of GRB fluence by assuming the same spectrum, energy, optical depth and duration of the absorption edge detected in GRB 990705. The different significance obtained by using the XRM alone or XRM + SGS is shown.


performances


Pag.35/43

35

5.2. COMPARISON TO OTHER GRB EXPERIMENTS A summary of the comparison between the expected features and performances of MIRAX with those of the current most relevant GRB experiments is presented in Figures 5.4, 5.5 and Table 5.1. As can be seen from Fig. 5.4, comparing top panels and bottom left panel, the effective area below ~15 keV down to ~1 keV is unprecedented and is joined with an extension of the energy band up to ~10 MeV with an effective area even better than that of the Fermi/GBM experiment. The bottom right panel of Fig. 5.4 shows the GRB trigger sensitivity of MIRAX expressed in terms of minimum detectable 1s photon peak flux in the 1 – 1000 keV energy band as a function of the spectral peak energy Ep (see Band 2003). As can be seen, the combination of the XRM and SGS instruments allows a much better sensitivity for soft GRBs (XRFs, high redshift GRBs) with respect to other GRB detectors, while maintaining a comparable sensitivity for normal GRBs. Note that the capability of determining Ep from the spectrum depends on the instrument passband. That of MIRAX (1-10000 keV) is much broader than BATSE (20-2000 keV) and BAT (15-150 keV). Figure 5.5 (adapted from Salvaterra et al. 2008), shows the expected probability of MIRAX of detecting a very high redshift (z > 10) GRB/year as a function of detectors FOV and peak flux sensitivity, compared to that of other present and possible future GRB experiments. As can be seen, the low energy threshold of the instrument is crucial in increasing the detection probability of such events. Based on the models and assumptions of Salvaterra et al. (2008, 2009) combined with the MIRAX capabilities summarized in Tables 5.1 and 5.2, we predict the following hig-z GRBs detection rates: 4-12 GRB/year at z > 6, 1-3 GRB/year at z > 8, 0.3-0.5 GRB/year at z > 10. All these features, together with the broad field of view, the arcmin location accuracy, the energy resolution of ~200 eV at 6 keV and the timing resolution down to a few µs, will allow the proposed MIRAX instrumentation to achieve the unique and fundamental scientific goals described above.


performances


Pag.36/43

36

Figure 5.4 – Top-left panel: Highest and lowest (depending on GRB direction) Swift/BAT effective area as a function of photon energy. Top-right panel: Fermi/GBM effective area of a single detector as function of energy. The total effective area along a given direction is about 3 times higher (about 300 cm2). Bottom–left panel: MIRAX effective area as a function of energy. The extension of the MIRAX effective area to 1 keV is unique, while that at high energies (>150 keV) is three times that achieved with Fermi/GBM. Bottom-right panel (adapted from a figure by Band 2003): expected MIRAX flux sensitivity compared with that of BATSE and Swift/BAT.


performances


Pag.37/43

37

Figure 5.5 Probability of detecting a GRB/year at z > 8 as a function of detector’s FOV and 1 s peak flux sensitivity (updated from Salvaterra et al. 2008). As can be seen, the FOV – sensitivity combination of MIRAX is the most favourable w/r to present and possible future GRB experiments.

Mission Experiment Energy band (keV)

Field ofview (sr)

GRB Location accuracy

Max. eff.area (cm2)

Energy resolution (keV)

#GRB /year

Nominal operation /launch

CGRO BATSE 25 - 2000 9 > 3° 2000 15@100 keV 300 1991 - 2000 BeppoSAX WFC 2 – 28 0.26 3’– 5’ 180 1.5@6 keV 15 1996 - 2002 GRBM 40 - 700 6 > 10° 700 20@100 keV 200 HETE-2 WXM 2 - 25 0.8 - 70 1.8@6 keV 15 2000 - 2006 FREGATE 7 - 400 1.74 - 40 15@100 keV 70 Swift BAT 15 - 150 1.4 3’ 2600 3@60 keV 120 2004 - WIND Konus 15 - 10000 4π - 250 15@100 keV 250 Fermi GBM 8 - 30000 9 > 3° 300 15@100 keV 250 2008 - INTEGRAL ISGRI 20 - 200 0.1 1.5 1300 3@60keV 10 2003 - MAXI (1) GSC 2 - 30 1.5ox160o 0.1o 5350 [email protected] keV 5 2009- SSC 0.5 - 12 1.5ox90o 0.1o 200 [email protected] keV 2 MIRAX XRM 1 - 50 3.5 1’ 550 0.2@3 keV 200 2015 - SGS 20 - 5000 3.5 - 1100 10@100 keV 200

Table 5.1 Main features of the MIRAX proposed payload compared with those of past and present GRB experiments. (1) MAXI is aboard the ISS and thus its zenith continuously drifts as in the case of MIRAX. However in the case of MAXI the FOV in the drift direction has a very small FOV (1.5 deg vs. 45 deg of MIRAX), thus it has a very small chance of observing the entire prompt emission of a GRB. In addition it cannot distinguish between a GRB and another type of transient event (like X-ray bursts).


performances


Pag.38/43

38

5.3. ALL-SKY MONITORING In Fig. 5.4, it is shown the MIRAX payload capability of surveying the almost entire sky per orbit. The MIRAX capability of performing key importance studies of X-ray bursts emitted by Low Mass X-ray Binaries is shown in Figures 5.5 and 5.6, where results obtained with RXTE/PCA are compared with those expected with MIRAX

Figure 5.6: The sky accessible to the ASM experiment after every orbit (90 minutes). Thick lines represent the center of the field of view of the three subsystems

Figure 5.7: In Low Mass X-ray Binaries with known distance, a time-resolved spectral modelling of PRE (Photospheric Radius Expansion) type I X-ray bursts can provide information about the Mass and the Radius


performances


Pag.39/43

39

of the Neutron Star (by measuring the “touchdown flux” and the BB normalization). Left: the case of 4U 1608-52 as studied by Gruver et al. (ApJ 2010) with RXTE/PCA.Right: expected results with the MIRAX/XRM

Figure 5.8: Spectral evolution of a flare of the microquasar GRS 1915+105. Left:measurements by RXTE/PCA. Right: the spectrum as would be measured by the MIRAX/XRM in 500 s. As can be seen form the table, the accuracy in the estimate of spectral parameters obtainable in 500 s is the same as that oibtained by the PCA in 3500 s.


performances


Pag.40/43

40


performances


Pag.41/43

41

6. REFERENCES

Amati L., Frontera F., Vietri M. et al., Science, 290, 955 (2000) Amati L., Frontera F., Tavani M. et al, A&A, 390, 81 (2002) Amati L., Frontera F., J.J.M. in’t Zand et al, A&A, 426, 415 (2004) Amati L., MNRAS, 372, 233 (2006) Amati L., Della Valle M., Frontera F., et al., A&A, 463, 913 (2007) Amati L., Guidorzi C., Frontera F. et al., MNRAS, 391, 577 (2008) Amati L., Frontera F., Guidorzi C., A&A, 508, 173 (2009) Ajello M., Costamante L., Sambruna R.M. et al., ApJ, 699, 603 Astronet, “A Science Vision for European Astronomy”, http://www.eso.org/public/outreach/press-rel/pr-2007/Astronet_ScienceVision.pdf (2007) Band D., ApJ, 588, 945 Berezhiani Z., Bombaci I., Drago A. et al., ApJ, 586, 1250 (2003) Böttcher M., Crider A.W., Dermer C. D., Liang E. P., A&A, 343, 111 (1999) Bromm V., Loeb A., ApJ, 575, 11 (2002) Campana S., Lazzati D., Ripamonti E. et al., ApJ, 654, L17 (2007a) Campana S., Guidorzi C., Tagliaferri G. et al., A&A, 472, 395 (2007b) Campana S., Panagia N., Lazzati D. et al., ApJ, 683, L9 (2008) Cusumano G., La Parola V., Segreto A., A&A, 510, 48 (2010) Dai Z.G., Liang E.W., Xu D., ApJ, 612, L101 (2004) Drago A., Pagliara G., Schaffner-Bielich J., JPhG, 35, 014052 (2008) Evangelista Y., Feroci M., Costa E., ApJ, 708, 1663 (2010) Fender R., Corbel S., Tzioumis T., 519, L165 (1999) Firmani C., Avila-Reese V., Ghisellini G., Ghirlanda G., MRAS, 372, L28 (2006) Frail D.A., Kulkarni S.R., Sari R. et al., ApJ, 562, L55 (2001) Frontera F., Costa E., Dal Fiume D. et al., A&AS, 122, 357 (1997) Frontera F., Amati L., Costa E. et al., ApJS, 127, 59 (2000) Frontera F., Amati L., Vietri M. et al., ApJ, 550, L47 (2001) Frontera F., ChJAS, 3, 439 (2003) Frontera F., Amati L., Lazzati D., ApJ, 614, 301 (2004) Frontera F., Amati L., et al., ApJ, submitted (2010) Gaensler B.M., Slane P.O., Gotthelf E.V., Vasisht G., ApJ, 559, 963 (2001) Gavriil F.P., Kaspi V.M., Woods P. M., Nature, 419, 142 (2002) Gehrels N., Chincarini G., Giommi P. et al., ApJ, 611, 1005 (2004) Gehrels N., Ramirez-Ruiz E., Fox D.B., ARA&A, 47, 567 (2009) Ghirlanda G., Ghisellini G., Lazzati D., ApJ, 6161, 331 (2004a) Ghirlanda G., Ghisellini G., Lazzati D., Firmani C., ApJ, 613, 13 (2004b) Ghirlanda G., Ghisellini G., Firmani C., NJPh, 8, 123 (2006) Ghirlanda G., Bosnjak Z., Ghisellini G. et al., MNRAS, 379, 73 (2007) Ghirlanda G., AIP conf. proc. "Probing stellar populations out to the distant universe, Cefalu' 2008" Vol. 1111, pp. 579-586 (2009) Ghirlanda G., Nava L., Ghisellini G., A&A, 511, 43 (2010) Ghisellini G., Ghirlanda G., Firmani C., Lazzati D., Avila-Reese V., NCimC,28, 639 (2005) Güver T., Özel F., Cabrera-Lavers A., Wroblewski P., ApJ, 712, 964 (2010) Guidorzi C., Frontera F., Montanari E., A&A, 401, 491 (2003)


performances


Pag.42/43

42

Heise J., in't Zand J.J.M., Kippen R.M., Woods P.M., in: “Gamma-Ray Bursts in the Afterglow Era”, ESO Astrophysics Symposia, p. 16, (2001) Hjorth J., Sollerman J., Møller P. et al., Nature, 423, 847 (2003) Hurley K., in: GAMMA-RAY BURSTS: 5th Huntsville Symposium. AIP Conference Proceedings, Volume 526, pp. 763-770 (2000) Ibrahim A.I., Safi-Harb S., Swank J.H., ApJ, 574, L51 (2002) Jager R., Mels W. A., Brinkman A. C. et al., A&AS, 125, 557 (1997) Kaspi V.M., Gavriil F.P., Woods P. M., ApJ, 588, L93 (2003) Kippen R.M., Woods P. M., Heise J., in: “Gamma-Ray Bursts in the Afterglow Era”, ESO Astrophysics Symposia, p. 22, (2001) Lazzati D., Ghisellini G., Amati L. et al., ApJ, 556, 471 (2001) Lazzati D., Perna R., MNRAS, 340, 694 (2003) Le Floc'h E., Duc P.-A., Mirabel I. F., ApJ, 581, L81 (2002) Liang E.W., Dai Z.G., Wu F., ApJ, 606, L29 (2004) Mereghetti S., proceedings of "44th Rencontres de Moriond on Very High Energy Phenomena in the Universe” (arXiv:09044880) (2009) Mereghetti S., nvited review presented at the NATO ASI Workshop ''The Neutron Star - Black Hole Connection'' Elounda, Crete - 7-18 June 1999 (arXiv: astro-ph/9911252) (1999) Mészaros P., Rep.Prog.Phys, 8, 2259 (2006) Mészaros P., Rees M.J., ApJ, 715, 967 (2010) Paczynski B., ApJ, 494, L45 (1998) Mochkovitch R. et al., in “in: “Gamma-Ray Bursts in the Afterglow Era”, ASP Conf. Series, Vol. 312, p.381 (2004) Pélangeon A., Atteia J.-L., Nakagawa Y. E., A&A, 491, 157 (2008) Preece R. D., Briggs M. S., Mallozzi R. S. et al., ApJS, 126, 19 (2000) Sakamoto T., Lamb D. Q., Kawai N., ApJ, 629, 311 (2005) Salvaterra R., Campana S., Chincarini G. et al., MNRAS, 385, 189 (2008) Salvaterra R., Della Valle M., Campana S. et al., Nature, 461, 1258 (2009) Soderberg A., Grindlay J.E., Bloom J.S. et al., Astro2010: The Astronomy and Astrophysics Decadal Survey, Science White Papers, no. 278 (arXiv: 0902.3674) (2010) Stanek K. Z., Matheson T., Garnavich P., ApJ, 591, L17 (2003) Strohmayer T.E., Ibrahim A.I., ApJ, 537, L111 (2000) Thompson C., Duncan R.C., MNRAS, 255, 275 (1995) Thompson C., Lyutikov M., Kulkarni S.R., ApJ, 574, 332 (2002) Vietri M., Stella L., ApJ, 547, L45 (1998)592, 457 (2003) Wang L., Baade D., Höflich P., Wheeler J. C., ApJ, Weth C., Mészáros P., Kallman T., Rees M. J, ApJ, 534, 581 (2000) Woods P.M., in “From X-ray Binaries to Gamma-Ray Bursts: Jan van Paradijs Memorial Symposium”, ASP Conference Proceedings, Vol. 308 (2003) Woods P.M., Kouveliotou C., Göğüş E., ApJ, 596, 464 (2003) Woosley S.E., Heger A., ApJ, 637, 914 (2006) in't Zand J. J. M. Kuiper L., Amati L. et al., ApJ, 559, 710 (2001) Zhang B., Mèszàros P., ApJ, 581, 1236 (2002)

an italian proposal for mirax science case, configuration ... · inaf istituto di astrofisica...

Documents