What is a GRB ?

Burst Optical Observer and Transient Exploring System    

( A world wide Network of Robotic Telescopes )


In 1967-73, the four VELA spacecraft (named after the spanish verb {velar}, to keep watch), that where originally designed for verifying whether the former Soviet Union abided by the Limited Nuclear Test Ban Treaty of 1963, observed 16 peculiarly strong events.

On the basis of arrival time differences, it was determined that they were related neither to the Earth nor to the Sun, but they were of cosmic origin. Therefore they were named cosmic Gamma-Ray Bursts (GRBs hereafter).

GRBs appear as brief flashes of cosmic high energy photons, emitting the bulk of their energy above 0.1 MeV. They are detected by instruments somewhat similar to those used by the particle physicists at their laboratories. The difference is that GRB detectors have to be placed onboard balloons, rockets or satellites.

Gamma-ray bursts and Optical Transients

It is well known that an important clue for resolving the gamma-ray burst (GRB) puzzle is the detection of transient optical emission associated with the bursts. This might be favoured by an optimum system capable of performing rapid movements towards the position of the event. After years of search, the first fading optical counterparts were found in 1997 beggining 3-20 hr after the onset of the high energy events thanks to the accurate positions provided by the satellites BeppoSAX and RossiXTE. This has led to prove their extragalactic origin.

An additional mistery surrounds the reliability of Optical Transients (OTs), as some of them could be related to GRBs. Under the assumption of the bursters being a repeating phenomena, archival plates have been used in order to look for optical transient emission in the smallest GRB error boxes, and about 50 candidates have been identified so far, but most of them were rejected as they turned out to be plate defects. Nevertheless, there are still few events (OTs) that might be associated with the GRB sources, but with the exception of the V = 9 mag optical transient discovered by Rotse simultaneously to GRB 990123, none of them can be definitively proven as optical emission related to the GRB. Three could be related to underlaying faint Active Galactic Nuclei (AGNs). In fact, a test was performed on the variable source PKS 0420-014, and was detected several times as short optical flashes lasting less than one day. Most (82%) blazars show variability on time scales of days or less; however the available observations are not dense enough to resolve the faster variations.

If the OT connection to QSO/AGNs proves solid, it will substantially extend the known amplitude of AGN variability. There are hints that rapid and sudden flares may occur, though of much smaller amplitude.

The Explosive Transient Camera (ETC), a system with 16 wide-field cameras, each with a field of view of 20 x 15 deg² recorded about 100000 flashes brighter than 11 mag. in 2.5 yr of operation in Kitt Peak. Most of the events were due to satellite glints, but about 0.5% remain unidentified This is the fraction that might be recorded with the Optical Monitoring Camera (OMC) on the European Space Agency's INTEGRAL satellite, when launched in September 2002.