About once per day, somewhere on the sky a Gamma-Ray Burst (GRB) flares up for typically a few seconds, often shining for a brief period of time brighter than all other gamma-ray sources together. While it has been clear for many years now that GRBs are observed from all across the universe, their origin and mechanisms are still subject for debate, with collapse of a massive star to a black hole and merging neutron stars being the most popular explanations for 'typical' and the subset of 'short' GRBs, respectively.
While INTEGRAL was not specifically built to observe GRBs, the wide field of view (FOV) of its main instruments leads to a detection of a GRB within their FOV every 1-2 months. In addition, the Anticoincidence Shield of the SPI instrument (SPI-ACS) also serves as an efficient GRB-detector, detecting roughly a GRB every second day, but lacking spatial and spectral information. For a recent overview of the global characteristics of GRBs observed with INTEGRAL see Foley et al. (2008).
IBAS - The INTEGRAL Burst Alert System
GRBs within the FOV of IBIS are largely detected in real-time by the INTEGRAL Burst Alert System (IBAS), running at the ISDC. For strong detections the burst positions are distributed automatically via a subscription mechanism to various recipients, e.g., to the GCN. This rapid dissemination allows direct follow-up observations at other wavelengths. In addition, weaker events are analyzed by a dedicated team who then distribute the information if a GRB is confirmed. All alerts and confirmed GRBs are publicly available from the IBAS pages at IASF Milano.
A full list of ACS triggers with the detected GRBs amidst various other rapidly varying events is maintained at the ISDC. While there is usually no position information and no spectral data, the information is used, e.g., in the 3rd Interplanetary Network of Gamma-ray burst detectors (IPN3) to find positions combining data across satellites. Sometimes the ACS data is also used directly in combination with data from other satellites (especially Swift) like for GRB 08062. The first the INTEGRAL SPI-ACS gamma-ray burst catalogue was published by Rau et al. (2005).
GRB 031203 - an unusually dim burst
GRB 031203 was detected by the IBAS system and the position distributed within 18 s of the trigger. This allowed follow-up observations by various other observatories, including XMM-Newton, Chandra and the VLA. These observations allowed the identification of the host galaxy and the determination of the redshift of only z=0.106, the closest GRB so far. While otherwise appearing like a typical long X-ray burst, the relative small distance makes GRB 031203 about three orders of magnitude less luminous than the usual GRB, similar to the unusual burst GRB 980425. The INTEGRAL results are reported in Sazonov et al. 2004.
GRBs outside the Field of View
Using the Compton scattering between the two layers of the IBIS instrument – ISGRI and PICsIT – Marcinkowski et al. (2006) obtained the location and spectrum of GRB 030406, despite the fact that the burst took place 36.9° off-axis. The analysis method is being used on further bursts and refined.