What is a GRB ?

Burst Optical Observer and Transient Exploring System    

( A world wide Network of Robotic Telescopes )

General properties

Temporal Properties

The time profiles of the bursts are very different, with some GRBs lasting a few ms and others lasting for several minutes.  In general, there was no evidence of periodicity in the time histories of GRBs.  However there was indication of a bimodal distribution of burst durations, with 25 % of bursts having durations around 0.2 s and 75 % with durations around 30 s.

 

Distribución bimodal de los GRBs presentes en el catálogo 4B de BATSE

GRB Location

The KONUS instrument on Veneras 11 and 12 gave the first indication that GRB sources were isotropically distributed in the sky.  Based on a much larger sample, this result was nicely confirmed by BATSE on board the CGRO satellite, launched in the spring of 1991, an instrument that has revolutioned the GRB field. About 800 GRBs are detected on a yearly basis, but only few of them are localized accurately. The apparent isotropy of the bursts in the sky ruled out the models dealing with neutron stars in the Galactic Plane, and it was rather interpreted in terms of GRBs arising at cosmological distances, although the possibility of a small fraction of the sources lying nearby, within a galactic disc scale of few hundred pc, or in the halo of the Galaxy, could not be discarded by that time.

Another result was that the time profiles of the bursts are very different, with some GRBs lasting a few ms and others lasting for several minutes.  In general, there was no evidence of periodicity in the time histories of GRBs.  However there was indication of a bimodal distribution of burst durations, with about 25 % of bursts having durations around 0.2 s and about 75 % with durations around 30 s.

 

Isotropic distribution of GRBs in the sky (galactic coordinates)

 

A deficiency of weak events was noticed in the log N-log S diagram, as the GRB distribution deviates from the -3/2 slope of the straight line expected for an homogeneous distribution of sources assuming an Euclidean geometry.

All these observational data led many researchers to believe that GRBs are indeed at cosmological distances. However, the origin of GRB remains unknown for more than 30 years.

 

Diagrama log N-log S

 

It was well known that an important clue for solving the GRB puzzle was going to be the detection of transient emission -at longer wavelengths- associated with the bursts.