Abstract
We investigate the characteristics of bright flares for a sample of supergiant fast X-ray transients and their relation to the orbital phase. We have retrieved all Swift/BAT Transient Monitor light curves, and collected all detections in excess of $5\sigma$ from both daily- and orbital-averaged light curves in the time range of 2005-Feb-12 to 2013-May-31. We also considered all on-board detections as recorded in the same time span and selected those within 4 arcmin of each source in our sample and in excess of $5\sigma$. We present a catalogue of over a thousand BAT flares from 11 SFXTs, down to 15-150keV fluxes of $\sim6\times10^{-10}$ 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 The great majority of these flares are unpublished. This population is characterized by short (a few hundred seconds) and relatively bright (in excess of 100mCrab, 15-50keV) events. In the hard X-ray, these flares last in general much less than a day. Clustering of hard X-ray flares can be used to indirectly measure the length of an outburst, even when the low-level emission is not detected. We construct the distributions of flares, of their significance (in terms of sigma) and their flux as a function of orbital phase, to infer the properties of these binary systems. In particular, we observe a trend of clustering of flares at some phases as $P_{\rm orb}$ increases, as consistent with a progression from tight, circular or mildly eccentric orbits at short periods, to wider and more eccentric orbits at longer orbital periods. Finally, we estimate the expected number of flares for a given source for our limiting flux and provide the recipe for calculating them for the limiting flux of future hard X-ray observatories. (Abridged).
Highlights
Supergiant fast X-ray transients (SFXTs) are a class of high mass X-ray binaries (HMXBs) associated with OB supergiant stars brought to the foreground by INTEGRAL (Smith et al 2004; Sguera et al 2005; Negueruela et al 2006b)
We observe a trend of clustering of flares at some phases as Porb increases, which is consistent with a progression from tight circular or mildly eccentric orbits at short periods to wider and more eccentric orbits at longer orbital periods
We have presented a catalogue of over a thousand BAT flares on 11 SFXTs, down to 15–150 keV fluxes of ∼6×10−10 erg cm−2 s−1 and ∼1.5×10−9 erg cm−2 s−1 spanning the initial 100 months of the Swift mission
Summary
Supergiant fast X-ray transients (SFXTs) are a class of high mass X-ray binaries (HMXBs) associated with OB supergiant stars brought to the foreground by INTEGRAL (Smith et al 2004; Sguera et al 2005; Negueruela et al 2006b). In quiescence (e.g. in’t Zand 2005; Bozzo et al 2010), they have a luminosity of ∼1032 erg s−1; one defining characteristic of SFXTs is the dynamic range of 3–5 orders of magnitude. During outburst their hard X-ray spectra resemble those of HMXBs that host accreting neutron stars (NS), with hard power laws below 10 keV, which are combined with high energy cut-offs at ∼15–30 keV and sometimes strongly absorbed at soft energies (Walter et al 2006). The mechanism producing the outbursts is still subject to debate but it is probably related to either the properties of the wind from the supergiant companion (in’t Zand 2005; Walter & Zurita Heras 2007; Negueruela et al 2008; Sidoli et al 2007)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
