BeppoSAX-WFC catalog of fast X-ray transients
J. J. M. in 't Zand, C. Guidorzi, J. Heise, L. Amati, E. Kuulkers, F. Frontera, G. Gianfagna, L. Piro
TL;DR
The paper delivers a systematic BeppoSAX-WFC FXT catalog, identifying 149 fast X-ray transients with a substantial fraction new to the literature. By combining WFC data with simultaneous GRBM and BATSE observations, the study assigns source types across GRBs, XRFs, stellar and HMXB flares, and unambiguous stellar identifications, revealing a bimodal distribution in duration and spectral hardness that separates short FXTs (GRBs/XRFs) from long FXTs (stellar and binary star flares). Spectral analyses favor power-law fits, with Fe-K lines detected in several stellar flares and distances enabling energy estimates that span $10^{34}$–$10^{51}$ erg across classes. The results position BeppoSAX-WFC as a cornerstone for wide-field FXT studies, providing a flux-limited baseline that informs and harmonizes comparisons with current and future all-sky monitors like EP-WXT and MAXI, and clarifying the FXT landscape including the XRF–GRB connection and the population of SFXTs and stellar flares.
Abstract
We performed a search for fast X-ray transients (FXTs), with durations longer than one second and less than one day, through data of the Wide Field Camera (WFC) instrument onboard the BeppoSAX X-ray observatory collected between June 1996 and April 2002. (..) We focused our search on gamma-ray bursts (GRBs), X-ray flashes (XRFs), X-ray flares from high-mass X-ray binaries and stellar flares, while Type-I and II X-ray bursts from Galactic neutron stars were excluded. 149 such fast transient events were detected. 63 of these are new to the literature. 38 flares are identified with 22 nearby stars. Three stars 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 AGN as previously thought. Eleven flares were detected from known high-mass X-ray binaries with irregular wind accretion (four of which are of the subclass of supergiant fast X-ray transients). 100 GRBs were identified of which 24 have not been published before. We classify 37% of the X-ray detected GRBs as XRFs with relatively large X-ray to gamma-ray flux ratio, gamma-rays being measured with the BeppoSAX Gamma Ray Burst Monitor. The duration/spectral hardness distribution of all FXTs is bimodal, separating the group roughly in transients shorter and longer than 1 ksec and with relatively hard and soft spectra, respectively. We identify the 'short' FXTs as GRBs and XRFs and the `long' FXTs as flares from nearby late-type stars and X-ray binaries. The BeppoSAX-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.