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-001v4
Issue
1
Date
8 October 2010
Revision
3
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TABLE OF CONTENTS
1.
1.1.
1.2.
2.
2.1.
2.2.
2.3.
INTRODUCTION ........................................................................................................................................... 4
CONTEXT .................................................................................................................................................... 4
SCOPE OF THIS DOCUMENT ............................................................................................................................ 6
SCIENCE CASE....................................................................................................................................... 7
SCIENCE OBJECTIVES SUMMARY ................................................................................................................... 7
GAMMA-RAY BURSTS ................................................................................................................................... 8
X-RAY ALL-SKY MONITORING ........................................................................................................................ 17
3.
SCIENCE REQUIREMENTS .............................................................................................................. 24
4.
MAIN FEATURES OF THE INSTRUMENTATION PROPOSED ...................................................................... 26
4.1.
4.2.
4.3.
5.
5.1.
5.2.
5.3.
6.
INSTRUMENT SUMMARY ............................................................................................................................... 26
THE X-RAY ALL-SKY MONITOR (XRM) ............................................................................................................ 27
THE SOFT GAMMA-RAY SPECTROMETER (SGS) ............................................................................................... 28
EXPECTED PERFORMANCE ...................................................................................................................... 31
GAMMA-RAY BURSTS ................................................................................................................................. 31
COMPARISON TO OTHER GRB EXPERIMENTS .................................................................................................. 33
ALL-SKY MONITORING ................................................................................................................................. 36
REFERENCES............................................................................................................................................ 39
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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 multipayload 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
Power
Orbit
Telemetry
~500 kg (PMM total), ~120 kg (MIRAX payload)
~240 W (total), ~100 W (MIRAX payload)
Near equatorial (15 o ) , circular, ~600 km, 4 years
Telemetry: X-band, ~2-3 Mbps TBD
Launch
not earlier than 2015; launcher to be selected
Instrument
parameters
Energy range
Angular Resolution
Detector type
Spectral resolution
Hard X-ray Imager
(HXI) - INPE
10-200 keV
< 1o TBD
2mm-thick CZT array
< 5 kev @ 60 keV (FWHM)
Localization
< ~ 10 arcmin (10 ) – TBD < ~ 1 arcmin (10 ) in 2-50 keV
Field-of-view
20o x 20o FWHM – TBD
3.5 sr
Sensitivity
~ 10 mCrab (1 day, 5 )
< 5 mCrab (1 day, 5 ) in 2-50 keV
GRB / X-ray All-Sky Monitor
(INAF/INFN/Univ.)
1 keV – > 10 MeV
4 arcmin in 1-50 keV
Silicon drift chamber/NaI+CsI
~1 keV @ 60 keV, 200 eV @ 6 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 PIship of the BeppoSAX PDS and GRBM instruments; c) the experience acquired from the PI-ship of the SuperAGILE experiment; d) the excellent competence acquired in the data analysis and interpretation of the GRB
and celestial source observations.
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Figure 1.1: A schematic view of the Lattes satellite and key orientations
Table 2: The Italian MIRAX collaboration as of September 2010
INAF
IASF Roma Marco Feroci**, Andrea Argan, Riccardo Campana**, Imma Donnarumma, Yuri
Evangelista**, 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
INFN
IASF
Bologna
OAB,
Merate
Sez. Trieste
Sez.
Bologna
LNGS
University
Ferrara
Como
L’Aquila
Pavia
Pisa
Lorenzo Amati, Claudio Labanti, Martino Marisaldi, Fabio Fuschino, Mauro
Orlandini, Alessandro Traci
Gabriele Ghisellini, Giancarlo Ghirlanda
Andrea Vacchi, Gianluigi Zampa, Nicola Zampa, Alexander Rashevski, Valter
Bonvicini
Giuseppe Baldazzi
Francesco Vissani, Giulia Pagliaro
Filippo Frontera*, **, Alessandro Drago**, Cristiano Guidorzi*, Ruben Farinelli*,
Lev Titarchuk
Ruben Salvaterra*
Francesco Villante**
Piero Malcovati, Luca Picolli, Marco Grassi
Ignazio Bombaci**
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ICRANET
SNS, Pisa
Pescara
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Mario Vietri*
Remo Ruffini and his team
* = INAF associate, ** = INFN associate)
.
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 ASIINAF for the support of the high energy astrophysics studies [DA 04] and partially by other ASI contracts for
the support of R&D.
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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
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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
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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.
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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 BAT 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. Collapse
of massive fast rotating stars (hypernova model, e.g., Paczynski 1998) or delayed collapse of a rotationally
stabilized neutron star (supranova model, Vietri & Stella 1998) are among the favoured scenarios for the origin
of these events. It has also been proposed (Berezhiani et al. 2003; Drago et al. 2008) that the GRB may be a
consequence of the transition of a pure hadronic (neutron) star, product of a core collapse supernova, to a
quark star or to a hybrid hadron-quark star. 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). Actually the discovery of GRB030329, found to be connected
with the energetic supernova SN2003dh (e.g., Hijorth et al. 2003, Stanek 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 Xrays. 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 lowenergy cut-offs are expected to be transient.
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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, 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).
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).
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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
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”). 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
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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).
short
sub-en.
XRFs
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.
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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 Xray (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
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).
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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).
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 – E (orange shaded region)
correlations . Right: confidence contours in the M – plane as determined with the Ep,i – E correlation compared
with those obtained with SN Ia (see text for more details). Figures from Ghirlanda et al. (2006).
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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 25 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
(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
provide constraints on cosmological parameters with an accuracy comparable, or even better, than the Ep-Eγ
correlation. 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 Δz/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
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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.
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.
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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
“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.
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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 quasiperiodic 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
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state transitions of the BHC GX 339-4, as observed through the wide field experiments RXTE/ASM (soft Xrays) 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 Xray 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.
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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
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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 ioncyclotron 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
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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).
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3. SCIENCE REQUIREMENTS
In order to achieve the scientific goals discussed and summarized in the previous section,
instrumentation required should show the following features.
the
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 (5, 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;
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•
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 s 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
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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.
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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.
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 arcminlocalization 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.
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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
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
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)
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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
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
2-50 keV
From 250 to 500 eV FWHM
10 µs
Energy Range
Energy Resolution
Time Resolution
Effective Area
Angular Resolution
Point
Source
Accuracy
Field of View
550 cm2 in FCFoV (spectroscopy)
5 arcminutes
Location <1 arcminute
Sensitivity
(5-, 1 detector, imaging,
for spectroscopy 1.4 better)
Source Transit Time in FoV
Telemetry (orbit average)
2.5 steradians FCFoV
3.5 steradians PCFoV
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
700 s /orbit
3800 kbits/s
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Table 5.1: Main characteristics of the MIRAX/SGS
Energy Range
Energy Resolution
Time Resolution
Effective Area (on axis)
Field of View
Sensitivity (5-)
Telemetry (orbit average)
Expected Value
15 - 10000 keV
15% @ 60 keV , 8% #662 keV
~ 1 µs
1000 cm2 up
~ 700 cm2 up
2.5 steradians
~ 1 Crab in 1s
40 kbits/s
to 300 keV,
to 1000 keV
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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.
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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 . 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 .
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.
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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 s, will allow the proposed MIRAX instrumentation
to achieve the unique and fundamental scientific goals described above.
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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 cm 2).
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.
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Figure 5.5 Probability of detecting a GRB/year at z > 10 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 Field
of GRB
(keV)
view (sr) Location
accuracy
CGRO
BATSE
25 - 2000
9
> 3°
BeppoSAX WFC
2 – 28
0.26
3’– 5’
GRBM
40 - 700
6
> 10°
HETE-2
WXM
FREGATE
Swift
BAT
WIND
Konus
Fermi
GBM
INTEGRAL ISGRI
MAXI (1)
GSC
SSC
MIRAX
XRM
SGS
2 - 25
7 - 400
15 - 150
15 - 10000
8 - 30000
20 - 200
2 - 30
0.5 - 12
1 - 50
20 - 5000
0.8
1.74
1.4
4

0.1
1.5ox160o
1.5ox90o
3.5
3.5
3’
> 3°
1.5
0.1o
0.1o
1’
-
Max. eff. Energy
#GRB / Nominal
area
resolution
year operation
/
2
(cm )
(keV)
launch
2000 15@100 keV 300 1991 - 2000
180
1.5@6 keV
15
1996 - 2002
700
20@100 keV 200
70
40
2600
250
300
1300
5350
200
550
1100
1.8@6 keV
15@100 keV
3@60 keV
15@100 keV
15@100 keV
3@60keV
1@5.9 keV
0.15@5.9 keV
0.2@3 keV
10@100 keV
15
70
120
250
250
10
5
2
200
200
2000 - 2006
2004 2008 2003 20092015 -
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).
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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
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of the Neutron Star (by measuring the “touchdown flux” and the BB normalization). Left: the case of 4U 160852 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.
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