Fast X-ray Transients and Their Connection to Gamma-Ray Bursts

Fast X-ray transients (FXTs) with timescales from seconds to hours have been seen by numerous space instruments. We have assembled archival data from Ariel-5, HEAO-1 (A-1 and A-2), WATCH, ROSAT, and Einstein to produce a global fluence-frequency relationship for these events. Fitting the log N-log S distribution over several orders of magnitude to simple power law we find a slope of -1.0. The sources of FXTs are undoubtedly heterogeneous, the -1 power law is an approximate result of the summation of these multiple sources. Two major contributions come from gamma-ray bursts and stellar flares. Extrapolating from the BATSE catalog of GRBs, we find that the fraction of X-ray flashes that can be the X-ray counterparts of gamma-ray bursts is a function of fluence. Certainly most FXTs are not counterparts of standard gamma-ray bursts. The fraction of FXTs from non-GRB sources, such as magnetic stars, is greatest for the faintest FXTs. Our understanding of the FXT phenomenon remains limited and would greatly benefit from a large, homogeneous data set, which requires a wide-field, sensitive instrument.