GRB Observed by IBIS/PICsIT in the MeV Energy Range

GRB Observed by IBIS/PICsIT in the MeV Energy Range

V. Bianchin bianchin@iasfbo.inaf L. Foschini G. Di Cocco F. Gianotti D. Götz P. Laurent G. Malaguti F. Schiavone INAF/IASF-Bologna, via Gobetti 101, 40129 Bologna, Italy CEA,IRFU, SAp, 91191 Gif sur Yvette, France APC, Bâtiment Condorcet, 75205 Paris Cedex 13, France
Abstract

We present the preliminary results of a systematic search for GRB and other transients in the publicly available data for the IBIS/PICsIT detector on board INTEGRAL. Lightcurves in energy bands with time resolution from to have been collected and an analysis of spectral and temporal characteristics has been performed. This is the nucleus of a forthcoming first catalog of GRB observed by PICsIT.

keywords:
Gamma rays: bursts – Gamma rays: observations

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1 Introduction

PICsIT is the high-energy layer of the IBIS (Ubertini et al., 2003) instrument on-board the INTEGRAL satellite, operating in the energy range between  keV and  MeV (Di Cocco et al., 2003). Scientific pointings are acquired in Standard Mode, which is the combination of the two sub-modes: Spectral Imaging and Spectral Timing (ST), optimized for spatial and temporal resolution, respectively. For both modes, events are integrated over time and energy intervals according to the on-board settings; energy bins are derived from the original channels in conformity with the channel-to-energy relationship, defined in the on-board look-up tables (LUT) and derived from ground and in-flight calibration. ST mode allocates up to energy bins and time resolution down to  ms. However the limited telemetry budget requires data to be integrated over the whole detector surface, so that the event spatial information is lost. The on-board configuration history for the ST mode can be found at the INAF/IASF-Bologna web page111http://www.iasfbo.inaf.it/extras/Research/INTEGRAL/Documentation/Hardware.html.

2 Method

Spectral Timing data are processed by the specific software OSA 7.0 (Goldwurm et al., 2003). Each lightcurve in output is then scanned to search for excesses in each energy band. Excesses are selected where the background subtracted count rate is above a significance threshold defined as , where above  keV and it is set to below. The energy dependent threshold is applied to reduce spurious cosmic rays induced events which can mainly affect lightcurves below  keV. The background is computed, for each energy band and binning time, as the average rate over the pointing, neglecting saturation periods, telemetry gaps or oscillations at the Earth radiation belts passage. An excess is marked as a GRB candidate if the background corrected count rate remains above over a time interval of  s around the peak. It is worth noting that with the present criteria only long GRB are selected and a different approach is to be implemented for the search of short events, which is hampered by the strong impact of cosmic rays background (Segreto et al., 2003) and the lack of information on event position. Following the INTEGRAL Burst Alert System (IBAS) approach (Mereghetti et al., 2003), the procedure applies to the original binning lightcurve and to rebinned lightcurves with time intervals of , and  s. The effect of increasing binning time is to dilute cosmic rays induced spikes which have a typical time scale of tens ms (Segreto et al., 2003). Since the procedure is still in a test phase in several cases the lightcurve is visually inspected.

We cross-check PICsIT triggers with events of particle and solar origin, reported by the INTEGRAL Radiation Environment Monitor (IREM - Hajdas et al. (2003)) and by the Geostationary Operational Environmental Satellites (GOES)222http://www.swpc.noaa.gov. The most stringent criterion to validate an excess as a GRB is based on the directional information of the event. Although PICsIT Spectral Imaging (SI) mode is optimized for spatial resolution, events of typical GRB time scale are not detected in SI data since these are on-board integrated over the pointing duration ( s). In the present excess list only the very long ( s) GRB  was detected in both Spectral Imaging and Spectral Timing modes. Since the event position is not supplied by PICsIT Spectral Timing Mode data, to be confirmed as GRB, excesses are compared with GCN and other catalogs (KONUS-Wind, HETE, IBAS, IBIS/ISGRI).

Finally we point out that the sky coverage of PICsIT increases with energy, since the passive collimator between the coded mask and IBIS shields the detector against the cosmic background up to  keV and the active VETO surrounding the detector rejects spurious high-energy events up to  MeV (Quadrini et al., 2003).

3 Catalog status

GRB Time T GCN Acquisition Settings Energy Range
UTC (s) T bin (ms) - En Bin Num (keV)
030306 03:38:22 20 1930 62.5 - 2 208 - 676
030307 14:32:00 5 1937 62.5 - 2 208 - 676
030320 10:11:55 60 1941 62.5 - 2 208 - 676
030405 02:17:28 10 2126 62.5 - 2 208 - 676
030406 22:42:07 15 2127 62.5 - 2 208 - 676
030422 07:51:15 15 2162 62.5 - 2 208 - 676
041219 01:42:12 160 2866 4 - 4 156 - 676
050525A 00:02:53 8 3466 4 - 4 260 - 676
060901 18:43:51 10 5491 4 - 4 260 - 572
061122 07:56:49 8 5834 16 - 8 208 - 572
061222A 03:28:52 15 5954 16 - 8 208 - 2600
Table 1: GRB preliminary list. The table gives for each GRB: the start time of the event, the duration (T) in the PICsIT lightcurves, GCN number, the acquisition settings (time resolution and the number of energy bands), and the energy range (in keV) of the PICsIT detection.
Figure 1: GRB A lightcurve in the and  keV energy bands (right panel) and spectrum in the energy range  keV- MeV (left panel). This burst was not detected by IBIS/ISGRI.
Figure 2: The figure shows the double peaked GRB  together with a remarkable excess occurred 8 minutes later, in the two energy bands  keV and  keV. The nature of this GRB-like excess is still not clear.

At present our sample covers complete revolutions (one INTEGRAL orbit corresponds to  days), from revolution to , rev.  and from rev. to . Moreover the sample includes single pointings corresponding to IBIS/ISGRI GRB detections. IBIS/PICsIT observed 11 GRB documented by GCN (Tab. 1) among which GRB are observed in energy bands up to  keV, are observed in energy bands up to  keV and one GRB (061222A) seems to extend up to the highest energy band ( MeV). In particular GRB A satisfies the selection criteria in the energy range  keV, however several counts, probably related to the event, are present in all energy bins.

The updated list of events with lightcurves and spectra (if possible) is available at the web page333http://www.iasfbo.inaf.it/extras/Research/INTEGRAL/Catalogue/picsit_soucat.html. Spectral points are derived as the background subtracted count rate in each energy bin.

Among the confirmed events, we mention the very long GRB  (McBreen et al., 2006) that was also detected in Spectral Imaging Mode and GRB A (Butler et al., 2007) that appears up to  MeV. In Fig. 1 the lightcurve in two energy bands ( and  keV) and the spectrum of GRB A are shown. Despite the lightcurve suggests an energy dependent double peaked structure, the spectrum is integrated over the whole burst.

Besides documented GRB we found several excesses from unknown sources, either Galactic Transients, or GRB-like peaks. The most intriguing case is the excess found on March at  (UTC), shown in Fig.2 together with the documented GRB  (von Kienlin et al., 2003b), occurring  s before, in the same pointing. The same excess is also present in the lightcurve444http://www.mpe.mpg.de/gamma/science/grb/1ACSburst/2003_03_main.html of the Anti-Coincidence Shield SPI-ACS (von Kienlin et al., 2003a) however the imaging analysis with IBIS/ISGRI gives no detection. The satellite weekly report describes a nominal operational period, but no details about the IREM particle monitor are available. The solar activity was moderate-to-high over the whole week, however flares and particle events do not match with the excess. The event is not reported in any catalog and its nature is still under study.

4 Future work

  • The present sample of revolution will be extended to all public pointings.

  • The event catalog will include temporal and spectral information for each excess.

  • A procedure for the search of short and faint events will be implemented.

References

  • Butler et al. (2007) Butler N. R., Kocevski D., Bloom J. S., & Curtis J. L., A complete catalog of Swift gamma-ray burst spectra and durations: demise of a physical origin for pre-Swift high-energy correlation ApJ, 671, 656-677, 2007.
  • Di Cocco et al.  (2003) Di Cocco G., Caroli E., Celesti E., et al., IBIS/PICsIT in flight performances, A&A, 411, L189-L195, 2003.
  • Goldwurm et al.  (2003) Goldwurm A., David P., Foschini L., et al., The INTEGRAL/IBIS scientific analysis, A&A, 411, L223-L229, 2003.
  • Hajdas et al.  (2003) Hajdas W., Bühler P, eggel C., et al., Radiation environment along with the INTEGRAL orbit measured with the IREM monitor, A&A, 411, L43-L47, 2003.
  • McBreen et al.  (2006) McBreen S., Hanlon L., McGlynn S., et al., Observations of the intense and ultra-long GRB 041219a with the Germanium spectrometer on INTEGRAL, A&A, 455, 433-440, 2006.
  • Mereghetti et al.  (2003) Mereghetti S., Götz D., Borjowski J., et al., The INTEGRAL Burst Alert System, A&A, 411, L291-L297, 2003.
  • Quadrini et al.  (2003) Quadrini E. M., Bazzano A., Bird A. J., et al., IBIS Veto System Background rejection, instrument dead time and zoning performance A&A, 411, L153-L157, 2003.
  • Segreto et al.  (2003) Segreto A., Labanti C., Bazzano A., et al., Cosmic rays tracks on the PICsIT detector, A&A, 411, L215-L222, 2003.
  • Ubertini et al.  (2003) Ubertini P., Lebrun F., Di Cocco G., et al., IBIS: the Imager on-board INTEGRAL, A&A, 411, L131-L139, 2003.
  • von Kienlin et al.  (2003a) von Kienlin A., Beckmann V., Rau A., et al., INTEGRAL Spectrometer SPI’s GRB detection capabilities, A&A, 411, L299-L305, 2003a.
  • von Kienlin et al.  (2003b) von Kienlin A., Beckmann V., Corvino S., et al., INTEGRAL results on GRB 030320: A very long gamma-ray burst detected at the edge of the field of view, A&A, 411, L321-L325, 2003b.
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