Galactic and extragalactic fast X-ray transient sources

Fast X-ray Transients (FXTs) have long been an enigma in x-ray astronomy. These non-recurrent and bright x-ray sources, lasting up to a day and without quiescent x-ray emission, have been studied with the Wide Field Cameras (WFCs) during the 6 years of operational life of BeppoSAX. They were among the prime motivations for building the instrument. The WFC observations allow for the determination of duration, lightcurves and spectra, while the positional information is sufficiently good to search for counterparts. The results show the heterogeneous nature of FXTs. If we exclude the x-ray counterparts of Gamma Ray Bursts, at least two types can be distinguished, both seen at high galactic latitude with a spatial distribution consistent with isotropy. A galactic population of FXTs has a typical duration of an hour and is associated with stellar sources exhibiting coronal activity. An extragalactic population of FXTs, called X-ray Flashes, has a duration of order minutes. They are explosive events whose origin has most likely a connection with the origin of Gamma Ray Bursts. Recently the first spectroscopic distance determination from the optical afterglow of an X-ray Flash has been found at z=0.25. These results, together with recent developments in GRBs, indicate a two component cosmic explosion in which the prompt ultra-relativistic gamma-ray energy can be minor, whereas the total energy release of the explosion is similar in XRFs and GRBs.