The First x-ray localization of a gamma-ray burst by BeppoSAX and its fast spectral evolution

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(3arcminradius)andfastbroadband(2 700keV) spectral evolution of the event. A search in the catalogues at all wavelengthsintheerrorboxyieldsauniqueoutstandingsource: the bright radio quasar 4C 49.29. Although the probability of nding such a source by chance is very low ( 2 10 4 ), the absence of similar counterparts in other small error boxes suggests a chance occurrence. We also nd that the duration- energyrelationshipforburstspreviouslyobservedabove25keV (Fenimore et al. 1995) extends down to 1.5 keV. This result suggests that the same radiation mechanism is operating from X-raystogamma-raysandisinagreementwithradiativecooling 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 conguration. 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.