The 100-month Swift catalogue of supergiant fast X-ray transients I. BAT on-board and transient monitor flares

摘要

We investigate the characteristics of bright flares for a sample ofsupergiant fast X-ray transients and their relation to the orbital phase. Wehave retrieved all Swift/BAT Transient Monitor light curves, and collected alldetections in excess of $5\sigma$ from both daily- and orbital-averaged lightcurves in the time range of 2005-Feb-12 to 2013-May-31. We also considered allon-board detections as recorded in the same time span and selected those within4 arcmin of each source in our sample and in excess of $5\sigma$. We present acatalogue of over a thousand BAT flares from 11 SFXTs, down to 15-150keV fluxesof $\sim6\times10^{-10}$ erg cm$^{-2}$ s$^{-1}$ (daily timescale) and$\sim1.5\times10^{-9}$ erg cm$^{-2}$ s$^{-1}$ (orbital timescale, averaging$\sim800$s) and spanning 100 months. The great majority of these flares areunpublished. This population is characterized by short (a few hundred seconds)and relatively bright (in excess of 100mCrab, 15-50keV) events. In the hardX-ray, these flares last in general much less than a day. Clustering of hardX-ray flares can be used to indirectly measure the length of an outburst, evenwhen the low-level emission is not detected. We construct the distributions offlares, of their significance (in terms of sigma) and their flux as a functionof orbital phase, to infer the properties of these binary systems. Inparticular, we observe a trend of clustering of flares at some phases as$P_{\rm orb}$ increases, as consistent with a progression from tight, circularor mildly eccentric orbits at short periods, to wider and more eccentric orbitsat longer orbital periods. Finally, we estimate the expected number of flaresfor a given source for our limiting flux and provide the recipe for calculatingthem for the limiting flux of future hard X-ray observatories. (Abridged).