Beppo SAX-WFC catalog of fast X-ray transients

We performed a search for fast X-ray transients (FXTs), defined for the purposes of this work as transients with durations of longer than one second and shorter than one day. We used data from the Wide Field Camera (WFC) instrument on board the Beppo SAX X-ray observatory collected between June 1996 and April 2002. The WFC sensitivity ranged from 10 −9 erg s −1 cm −2 (2-10 keV), for a timescale of 10 s, to a few times 10 −11 erg s −1 cm −2 , for a timescale of 10 5 s. The WFC location accuracy was 0.7-4.9′ at a 68% confidence. We focused our search on gamma-ray bursts (GRBs), X-ray flashes (XRFs), and X-ray flares from high-mass X-ray binaries and stellar flares. Overall, 149 FXTs were detected. Of these, 63 are new to the literature. 38 flares were identified with 22 nearby stars. Three stars in the sample have never been seen flaring before in X-rays or optical (NLTT 51688, GR Dra and UCAC4 255-003783). We find that the MeV transient GRO J1753+57 is most likely the same object as GR Dra, rather than an active galactic nucleus (AGN) as previously thought. Eleven flares were detected from known high-mass X-ray binaries with irregular wind accretion. 100 GRBs were identified. Of these, 24 had not been published before. We classified 38% of the X-ray detected GRBs as XRFs with a relatively high X-ray-to-gamma-ray flux ratio. The gamma rays were measured with the Beppo SAX Gamma Ray Burst Monitor. The duration and spectral hardness distribution of all FXTs are bimodal, separating the group roughly in transients that are either shorter or longer than 1 ksec and with relatively hard and soft spectra, respectively. We identified the “short” FXTs as GRBs and XRFs and the “long” FXTs as flares from nearby late-type stars and X-ray binaries. The Beppo SAX-WFC FXT sample is found to be consistent with the one observed by Einstein Probe when the sensitivity of the two instruments is taken into account. These results suggest that the bulk of the population of faints events disclosed by Einstein Probe could represent the extension of a similar population to lower luminosities.