Introduction

Since their discovery in the late 60s, cosmic gamma-ray bursts (GRBs) represented a challenge for astrophysicists, who tried to give a full description of the sources and of the physical mechanisms responsible for the emission.

GRBs are unpredictable flashes of electromagnetic radiation in the X- and $\gamma$-ray energy domain (mostly around 100 keV), coming from any directions: typically, their time durations range from few ms up to $10^3$ s, and during the time they last, they outshine all other $\gamma$ celestial sources combined.

The mean rate of detected GRBs is around one per day; when the sky exposure time and the actual solid angle of the detectors are taken into account, this mean rate typically raises up to a few (2-3) per day.

The very first detections of GRBs were done by the US Vela satellites since 1967, although they were published only in 1973 ([Klebesadel et al., 1973]), once their cosmic origin was established definitely. Mainly because of their brightness, the first candidate sources were thought to be galactic neutron stars: the identification of some spectral features, like cyclotron lines, that were claimed to be observed by some early missions ([Mazets et al., 1981a,Murakami et al., 1988]), in addition to the relatively low luminosity requirements in the case of near sources, supported this assumption until the 90s, when the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma-Ray Observatory (CGRO) yielded the first revolutionary findings on the GRB properties.

BATSE, which detected 2704 bursts from April 1991 (launch) to June 2000 (reentry), gave the first evidence of an isotropic distribution (fig. ), although some earlier indications had been suggested by the results of the Konus instrument on Venera 11 and 12 ([Mazets et al., 1981a,Atteia, 1987]): therefore, the absence of a concentration along the galactic plane was in favour of isotropy and suggested a more distant class of sources. Also hypotetical populations of GRB sources distributed all over the galactic halo were considered, but again no dipole or quadrupole momenta in the direction distribution were found, as expected from this model, owing to the off-center position of the solar system. Another property shown by the BATSE GRB sample is that, if one supposes that GRBs are standard candles, the number of faint bursts, which in this picture correspond to the distant ones, are fewer than expected for a homogeneous distribution in an euclidean space.

If BATSE was the first experiment to strongly support a cosmological origin of GRBs, the real breakthrough in the GRB knowledge occurred in 1997 thanks to BeppoSAX. In fact, for the first time a GRB, occurred on February 28, was localized within a few hours, with an error box of a few arcmins; just eight hours after the burst it was possible to make BeppoSAX itself promptly slew and point its more sensitive Narrow Field Instruments (NFIs), that discovered a new fading X-ray source, immediately identified as the X-ray afterglow ([Costa et al., 1997]). Some hours later, an optical afterglow counterpart (V $\sim 21.3$) was discovered as well ([Groot et al., 1997], [van Paradijs et al., 1997]). The X-ray fluence of GRB970228 was $\sim$ 40% of the $\gamma$-ray fluence ([Costa et al., 1997]): this implies that the X-ray afterglow is not only the low-energy tail of the burst, but also another significant way for dissipating energy on a very different timescale. Furthermore, the non-thermal origin of the burst itself and of the X-ray afterglow ([Frontera et al. 1998]) was another important result with strong implications on the interpretation of the radiation mechanisms.

In several cases, the discovery of the optical afterglow allowed the identification of the host galaxy and, therefore, an estimate of the gamma-ray burster distance became possible by simply measuring the galaxy redshift.

Since then, $\sim$ 120 bursts have been localized in time for possible counterpart searches in many wavelengths: to date (September 2001), the two main experiments, that have been contributing to localize bursts, are the InterPlanetary Network (IPN, 60 GRBs) and BeppoSAX (47 GRBs), while the remaining bursts were localized by other experiments, like All Sky Monitor (ASM) on board the Rossi X-ray Transient Experiment (RXTE), the PCA on board RXTE, and since 2001 the HETE-II satellite, as well . As for the bursters' distances, one of the most outstanding results made possible by the BeppoSAX detections is that for all the 20 bursts for which an estimate of the distance has been obtained, the redshifts have resulted to be cosmological, but one, GRB980425, which is peculiar also for other reasons: it is the only burst which seems to be associated to SN1998bw, an unusual supernova of type Ic ([Galama et al., 1998]).

These fast and precise localizations made it possible to detect and study the afterglow counterparts of several bursts at many wavelengths, thus giving important clues on the nature and the mechanisms of what nowadays are generally thought as among the most powerful explosions taking place in the Universe after the Big Bang. Actually the situation of the afterglow search is quite complex; not all GRBs promptly localized show afterglow emission: out of the followed up GRBs, 80-90% show X-ray afterglow, $\sim$ 50% optical afterglow and only $\sim$ 4% a radio afterglow. About 50% of GRBs with X-ray afterglows have their optical counterpart sources (if any) below the detection limits: these bursts have been called ``dark bursts''.

For a detailed overview of the pre-1997 state of affairs, see the review by [Fishman & Meegan, 1995], while other recent reviews describe the current status of the GRB knowledge, especially concerning the afterglows: [Kulkarni et al., 2000b], [van Paradijs et al., 2000], [Klose, 2000], [Castro-Tirado, 2001], [Cheng and Lu, 2001], [Djorgovski et al., 2001], [Ghisellini, 2001], [Pian, 2001]. The following reviews concern the interpretation, according to the most accounted theoretical models: [Mészáros, 1999], [Mészáros, 2001], [Piran, 1999], [Mészáros, 2002].