1998A&A...329..906P

In this paper we present the observations performed by the BeppoSAX Gamma-Ray Burst Monitor (GRBM) and Wide Field Cameras (WFC) of GB960720. We derive a precise localization (3arcmin radius) and fast broad band (2-700keV) spectral evolution of the event. A search in the catalogues at all wavelengths in the error box yields a unique outstanding source: the bright radio quasar 4C 49.29. Although the probability of finding such a source by chance is very low (∼2x10^–4), the absence of similar counterparts in other small error boxes suggests a chance occurrence. We also find that the duration-energy relationship for bursts previously observed above 25keV (Fenimore et al., 1995ApJ...448L.101F) extends down to 1.5keV. This result suggests that the same radiation mechanism is operating from X-rays to gamma-rays and is in agreement with radiative cooling by synchotron emission. A fast evolution of the spectrum is found, in which the ratio of X- to gamma-ray intensities varies over three orders of magnitude. Furthermore, the spectrum in the initial phase of the event betrays the presence of an optically thick source rapidly evolving in a thin configuration. No other class of sources in the universe shows such a fast and extreme evolution. These results pose new and tighter constraints on theoretical models for gamma-ray bursts.