Introduction

Since their discovery (late 60s), the Gamma-Ray Bursts (GRBs) still remain mysterious, although in the last decade some fundamental steps have been done in the comprehension of the involved physical mechanisms, and some hints on their progenitors' nature, at least for some of them, have been gained.

These unpredictable, intense and relatively short pulses of elecromagnetic radiation, typically lasting from $\sim$ few ms to $\sim$ few $10^3$ s, carry their energy in the hard X- and $\gamma$-ray range (mostly above 100 keV). It often happens that, in the time they burst, they overcome the overall sky brightness in this energy band.

The complexity, the high time variability and the great variety of shapes of the GRB time profiles make it difficult a classification into different classes.

The primary subjects of this thesis are the search, the identification and the analysis of the Gamma-Ray Bursts (GRBs) from the overall data archive of the Gamma-Ray Burst Monitor (GRBM) onboard the Italian-Dutch BeppoSAX satellite, during five years of mission, from its beginning (April 1996) to date (August 2001).

This work has been carried out with the high-energy astrophysics group of the Physics Department of the University of Ferrara (Italy), in collaboration with the BeppoSAX/GRBM team of the Istituto di Tecnologie e Studio di Radiazioni Extraterrestri (ITeSRE, C.N.R., Bologna) and of the Istituto di Astrofisica Spaziale (IAS, C.N.R., Rome).

In the following, I outline the importance of the GRB topic among the most debated and still open subjects of modern high-energy astrophysics, the key roles of successful missions like BATSE/CGRO and BeppoSAX. Then I describe the motivation of this work, its applications and results.

Besides GRBs, the search for different X- and $\gamma$-ray transient events has produced other minor catalogs corresponding to their different origins: mainly, the non-GRBs responsible for an increase in the GRBM units' count rates have been classified into separate classes, the most important of which are the following: Soft-Gamma Repeaters (SGRs), solar X-ray flares, $\gamma$-ray flashes caused by the Earth atmosphere, high-energy charged particle-induced flashes, occultation steps of well known bright celestial sources (mostly, Crab and CygX-1) and other whose origin is still uncertain.