1 BeppoSAX and GRBs

One year and a half ago BeppoSAX discovered the X-ray afterglow source associated to GRB 980228 (Costa et al. 1997). Following this first discovery an impressive collection of new data and a real outburst of interpretation work has rejuvenated all this topic of Astrophysics. These mysterious objects are nowadays less elusive but the amount of problems open probably is not decreased. The expectation is now funded that the Astrophysics Gamma-Ray Burst is taking the place of the "Mystery of Gamma Ray Bursts". GRBs have become the crossroad of a network of knowledge including association with other phenomena, study of parent objects and probe of ancient universe at a scale of distance deeper than that of Supernovae. This discovery was possible with a special use of all the capabilities of BeppoSAX satellite. In the initial configuration that made the first result possible the sequence for the detection of GRB afterglows was:

• a) An onboard trigger from Gamma Ray Burst Monitor;
• b) A detection with Quick Look Analysis of a burst in the X-ray rate meters coincident with a);
• c) Imaging of the burst by processing data of the WFC;
• d) Pointing of the GRB direction with Narrow Field Instruments.

Now the situation has evolved. BeppoSAX is still the main producer of data on GRBs and their afterglow, but not only as an independent instrument. It is now part of a network that is significantly increasing the total amount of data and their significance. The major improvements (and the associated results) with respect to the original procedure are:

• a) Other triggers are used to search for the bursts in the WFC. In particular 2 GRBs have been detected with a trigger from BATSE experiment aboard CGRO;
• b) GRB positions are determined and distributed with increasing precision and decreasing delay, by an independent distribution system and by GCN. One of the transient sources (GRB 980326) has been detected in the optical band without any pointing with NFIs;
• c) Positions originated from other experiments (ASM-XTE, PCA, IPN) have been included in the alert loop and one X-ray transient has been found with BeppoSAX NFIs in a burst direction determined by ASM (GRB 980703);
• d) GRB 980613 has been detected without any on-board gamma-ray trigger but only from the Quick Look inspection of X/Gamma-rays light curves.

  

Table 1: GRBs detected and localized with BeppoSAX GRBM and WFCs. The peak fluxes are in units of 10^-7 erg cm^-2 s^-1. The Afterglow Sources are given in units of 10^-13 erg cm^-2 s^-1 around 8 hours after the GRB