GRBM Science Case for Lobster-ISS

Transcription

1 GRBM Science Case for Lobster-ISS Prepared by F. Frontera, L. Amati, E. Caroli, and D. Lazzati Issue 3.0 October 2003

2 1. Executive Summary. We describe the scientific capabilities of the Gamma Ray Burst Monitor (GRBM) for Lobster-ISS. Lobster-ISS is designed to be flown as an attached payload on the European Columbus module of the International Space Station. This version of the science case is that submitted as part of the PI data deliverables for the Lobster Phase-A final review being undertaken at the request of ESA, with the addition of the section 2 which summarizes the GRBM main spec.s.

3 2. The scientific capabilities of the GRBM for Lobster-ISS 2.1 Introduction Lobster-ISS is proposed as an attached payload on the zenith platform exposed payload facility of the ESA Columbus module of the International Space Station (ISS). The GRBM for Lobster-ISS consists of four X-ray detection units. Each unit is made of an array of cooled (250 K) CdZnTe, surmounted by a passive collimator which defines the Field of View (FOV) of each unit. In the plane perpendicular to the ISS direction of motion, the detector axes are misaligned with each other in order to monitor the entire X-ray sky once per 90 minute ISS orbit. The GRBM allows the identification of all types of cosmic fast transients, like Gamma Ray Bursts (GRBs), X-ray Flashes (XRFs) and Soft Gamma Ray Repeaters. Thanks to the GRBM, Lobster-ISS also provides a powerful alert capability to be used to generate target-of-opportunity (TOO) alerts, which can trigger pointed observations from contemporary ground and space-based facilities. The scientifically-relevant specifications of the GRBM are listed in Table 2.1. Table 2.1. Scientifically-relevant specifications of the GRBM for Lobster-ISS. No. of units 4 Detection area/unit (cm 2 ) 184 Instantaneous field of view 35 x 55 (fwhm) per module, 35 x 240 for 4 modules Bandpass 3 to 300 kev Exposed area to a given point 130 to 240 (cm 2 ) Detector Cooled ( 250 K) CZT Energy resolution 6 kev Minimum (non-zero) exposure ~1150 s time for a given point, per orbit Diffuse X-ray background rate See Table 2.2 Internal background See Table 2.2 Flux sensitivity See Fig. 2.1 and Table 2.3 Positioning capability See Figs. 2.2 and Background level per detection unit The mean background level which is expected at the ISS orbit is reported in Table 2.2. A FOV of 55 x 35 (FWHM) is assumed for each detection unit. The intrinsic background spectrum is assumed to have a power law shape with count index of -1.4 and normalization derived from the assumption that the count intensity in the kev range is 3x10-3 cts/(cm 2 s kev).

4 Table 2.2 Expected mean background level/detection unit Energy band (kev) Diffuse cosmic BKG (cts/s) Intrinsic BKG (cts/s) Given the wide field of view of the GRBM, the contribution of bright galactic surces to the total background level is not negligible. In particular, when the Galactic Bulge is inside the FOV of a detection unit, the background level is expected to increase of about 30%. This has been taken into account in the estimates and simulations reported in the next sections. 2.3 GRBM Flux Sensitivity The 5 σ sensitivity to GRB events for both on-line and off-line data analysis is reported in Table 2.3 for the same configurations assumed in Table 2.1. Given that a GRB in the FOV of the Lobster telescope is viewed by at least 2 GRBM detection units, we assumed the background level corresponding to an area of 2 x 184 = 368 cm 2 as input for the on-board source localization and detection algorithms (on-line analysis). For the off-line analysis we expect to be able to identify the pixels (or groups of pixels) illuminated by the source. In this case the detection area contributing to the background level will be reduced to that exposed to the source; here we assumed a minimum exposed area/unit of 129 cm 2 corresponding to 70% of the area of each detection unit. As source spectrum we assumed a typical Band function with α = -1, β = -2 and E 0 = 200 kev. Table 2.3 Flux sensitivity in 1 s integration time (5 σ) Energy band (kev) Total background level (cts/cm 2 /s) Flux sensitivity (1 s) (ph/cm 2 s) Flux sensitivity (1 s) (10-8 erg/ cm 2 s) On-line Off-line On-line Off-line This sensitivity matches the kev Lobster sensitivity for classical GRBs. The 5σ sensitivity of the GRBM to a Crab-like source as a function of exposure, with on-line data analysis and always assuming a total illuminated area of 129 cm 2, is shown in Figure 2.1 for the cases of a source located in the galactic center region and of a source with high galactic latitude.

5 Figure 2.1 The 5σ sensitivity of the GRBM to a Crab-like source as a function of exposure time for the on-line data analysis and assuming a total illuminated area of 129 cm 2 (worst case). Two cases have been considered: a source located in the galactic center region and source at high galactic latitude. 2.4 Source location accuracy The simulated source location accuracy (90% confidence level, c.l.) as a function of the burst fluence in the kev energy band is shown in Figs. 2.2 and 2.3 for the cases of a short (1 s duration) and a long (20 s duration) GRB, respectively. In each figure, the upper line corresponds to the on-line analysis, whereas the lower line corresponds to the off-line analysis technique. The dashed vertical line indicates the fluence corresponding to the sensitivity limit of the Lobster telescope. In these simulations we assumed the values of the total detector area illuminated by the source, background level and source spectrum assumed for the flux sensitivity evaluation. We used as source location reconstruction algorithm that expected to be used during the flight.

6 Figure 2.2 Localization accuracy as a function of fluence for a short (20s) burst. Figure 2.3 Localization accuracy as a function of fluence for a long (20s) burst.

7 3. Study of the prompt emission from Gamma Ray Bursts and X-Ray Flashes 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. Both 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). Recently the discovery of GRB030329, found to be connected with the energetic supernova SN2003dh (e.g., Hijorth et al. 2003, Stanek et al. 2003) points toward the 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 X--ray burst spectrum, which 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. In fact, with the Wide Field Cameras (WFCs, 2-28 kev, Jager et al. 1997) and the GRBM ( kev, Frontera et al. 1997) aboard the BeppoSAX satellite, variable absorption has already been detected in the X--ray spectrum of the prompt emission of a few GRBs (GRB980329, Frontera et al. 2000; GRB010222, in t Zand et al. 2001; GRB010214, Guidorzi et al. 2003), and 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 has been reported. These features are present in the first part (GRB rise) of the burst and fade 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 GRB could be derived (0.86±0.17) and later confirmed from the optical redshift (z opt = 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/Fe sun = 75±19, which is typical of a supernova environment. An alternative explanation of the transient absorption feature from GRB 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, E rest = 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). With the combination of the Lobster telescope with energy passband from 0.1 to 3.5 kev and the proposed GRBM with passband from 3 to 300 kev and better energy resolution than the BeppoSAX WFCs, a more sensitive study of absorption cut-offs and absorption features in the X-ray ( kev) prompt emission spectrum of GRBs can be performed (e.g., Fig. 3.1). The

8 study of variable absorption can 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 (e.g., Gehrels 2001), in spite of the enormous effort to promptly follow-on (slew time 1 min) in the kev band the GRBs detected with the GRB localization telescope BAT ( kev) aboard, is unable to study the early phases of the 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. Thus these features are only accessible to Lobster-ISS. The early study of the spectral evolution of the burst/afterglow 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, when the photon flux is the largest (e.g. Lazzati & Perna 2003). Figure 3.1 The spectrum of GRB in the earliest 13 s (Amati et al. 2000) as expected could be measured with the proposed GRBM for Lobster-ISS. The fit with a simple powerlaw, shown in the figure, is clearly unacceptable ( χ 2 /dof = 1066/654). The absorption edge at ~3.8 kev is apparent at a >12σ significance. With the WFCs plus GRBM aboard BeppoSAX, also a transient emission feature in the prompt emission of GRB has been detected (Frontera et al. 2001). The feature was interpreted as

9 evidence of blackbody emission from the fireball photosphere. Given its broad energy passband ( kev) and high sensitivity, the combination of the Lobster Figure 3.2 The spectrum of GRB990712, as expected to be measured with the proposed GRBM for Lobster-ISS, in the time interval in which a blackbody component with kt ~1.3 kev was measured with BeppoSAX WFC plus the GRBM aboard the BeppoSAX satellite (Frontera et al. 2001b). The fit with a simple power-law, shown in the figure, is clearly unsatisfactory and the blackbody component is detected at 8σ significance level. telescope and the GRBM instrument becomes an ideal mission for the study of these thermal components (see, e.g., Fig. 3.2). The polarization measurement of the prompt gamma-ray emission from GRBs is recognized to be one of the major objectives of the GRB astronomy, of key importance to establish the emission mechanism of the radiation. The recent discovery of a very high polarization level (80±20%) from GRB (Coburn & Boggs 2003) strongly points to the synchrotron radiation as emission mechanism of the radiation, but it should be of key importance to confirm this result with other detections. Besides the confirmation of the Coburn & Boggs (2003) result, it is important to measure: i) the spectral dependence of the linear polarized fraction and ii) the temporal dependence of the position angle. These quantities would allow to clearly identify the emission mechanism, the geometry of the advected magnetic field and the origin of the lightcurve variability. In addition, it is clearly of primary importance to measure the level of polarization in the GRBs for which the observed spectrum is supposed to be in violation of the synchrotron properties. The proposed GRBM, given its multi-pixel

10 configuration, can also be exploited to measure the polarization of the GRB prompt emission by optimizing the CZT thickness for this goal. With the present configuration, the expected minimum modulation factor due to linear polarization which can be detected, for a GRB with kev fluence similar to that of GRB (2.9 x10-5 erg/cm 2 ), is 60% in the kev energy range and 30% in the kev band. In the last few years, mainly with BeppoSAX, a new class of fast transients (X-Ray Flashes, XRFs) has been discovered (e.g., Heise et al. 2001). Likely this class is strictly related to that of GRBs extending it. 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. 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, 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 firaball ). The SWIFT mission, given the hard energy band of the GRB localizator, is not expected to give a relevant contribution to the study of the XRFs (expected localization rate of about 1 event/year ). Lobster-ISS, thanks to the broad energy band of operation ( kev) is the best mission to study XRFs and their sites. In addition to these primary goals, the following goals on GRB prompt emission can be achieved: To correlate the emission/absorption features with the absorption column behaviour. The correlation study between the time behaviour of the emission/absorption features and the GRB spectral evolution. Evolution of the GRB spectrum in the kev energy band. This study, never peroformed before in such a broad band, is of key importance for deriving the GRB continuum components. In addition to synchrotron, Inverse Compton and blackbody components are expected, but other mechanisms cannot be excluded. Study of GRB energetics with energy: high energies vs. soft energies (<3.5 kev), never exploited. Study of the GRB time profiles and their dependence on photon energy. Study of the GRB erratic time variability and its dependence on energy. Study of short GRBs (<2 s), whose origin is still unknown. 4 Soft Gamma Ray Repeaters Soft-Gamma Repeaters (SGRs, see Hurley 2000 and Woods 2003 for reviews) are X/gammaray 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 four sources (SGR , SGR , SGR and SGR ) plus one candidate, SGR (only two bursts detected) with position not well known (area of 80 arcmin 2 ) 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 10 4 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 SGR which is in the Large Magellanic Cloud.

11 Typically, bursts from SGRs have short durations(~0.1 s), recurrence times of seconds to years, energies of ~10 41 D 2 10 ergs, assuming a distance D = 10 D 10 kpc. Their hard X--ray spectra (>25 kev) are consistent with an Optically Thin Thermal Bremsstrahlung (OTTB) with temperatures of kev. During quiescence, persistent X-ray emission (< 10 kev) has been observed from three of them (SGR , SGR , SGR ) with luminosities of D 2 10 erg/s and power-law spectral shapes. In the case of SGR , an additional blackbody component (kt bb ~ 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 s/s have been detected. In the case of SGR and SGR , evidence of X--ray lines has been reported: an emission line at ~6.5 kev from the former source (Strohmayer & Ibrahim 2000), while anabsorption--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 D 2 10 erg/s. Giant flares have been observed from SGR and SGR On the basis of their locations and their spectral and temporal properties, in the absence of companion stars, SGRs are thought to be young (< 10 4 yrs) isolated neutron stars (NS) with ultrastrong magnetic fields (B dipole >10 14 gauss), or ``magnetars''. The magnetar model (Thompson & Duncan 1995) considers a young neutron star with a very strong magnetic field ( 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 SGR 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~ (e.g., Kaspi et al. 2003) and 1E (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). Lobster-ISS, thanks to its borad energy band ( kev) and its better spectroscopic performance, can perform un unprecedented study the X-ray spectra and time variability of the transient X/gammaray emission from these sources, with the determination of the broad band continuum spectrum possible discovery of 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 The spectral information, extended down to 0.1 kev, will shed new light in the emission mechanisms from these still enigmatic sources. References Amati, L., Frontera, F., Vietri, M., et al., 2000, Science, 290, 398 Coburn, W. & Boggs, S. E. 2003, Nature, 423, 415 Frontera, F. et al. 1997, A&AS, 122, 357

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