1 research outputs found
Two years of monitoring supergiant fast X-ray transients with Swift
We present results based on 2 yr of intense Swift monitoring of three supergiant fast X-ray transients (SFXTs), IGR J16479โ4514, XTE J1739โ302 and IGR J17544โ2619, which we started in 2007 October. Our out-of-outburst intensity-based X-ray (0.3โ10 keV) spectroscopy yields absorbed power laws characterized by hard photon indices (ฮโผ 1 โ2). The broad-band (0.3โ150 keV) spectra of these sources, obtained while they were undergoing new outbursts observed during the second year of monitoring, can be fitted well with models typically used to describe the X-ray emission from accreting neutron stars in high-mass X-ray binaries. We obtain an assessment of how long each source spends in each state using a systematic monitoring with a sensitive instrument. By considering our monitoring as a casual sampling of the X-ray light curves, we can infer that the time these sources spend in bright outbursts is between 3 and 5 per cent of the total. The most probable X-ray flux for these sources is โผ(1 โ2) ร 10โ11 erg cmโ2 sโ1 (2โ10 keV, unabsorbed), corresponding to luminosities of the order of a few 1033 to a few 1034 erg sโ1 (two orders of magnitude lower than the bright outbursts). In particular, the duty-cycle of inactivity is โผ19, 39 and 55 per cent (โผ5 per cent uncertainty) for IGR J16479โ4514, XTE J1739โ302 and IGR J17544โ2619, respectively. We present a complete list of BAT onboard detections, which further confirm the continued activity of these sources. This demonstrates that true quiescence is a rare state and that these transients accrete matter throughout their life at different rates. Variability in the X-ray flux is observed at all time-scales and intensity ranges we can probe. Superimposed on the day-to-day variability is intraday flaring, which involves flux variations up to one order of magnitude that can occur down to time-scales as short as โผ1 ks, and which can be naturally explained by the accretion of single clumps composing the donor wind with masses Mclโผ (0.3 โ2) ร 1019 g. Thanks to the Swift observations, the general picture we obtain is that, despite individual differences, common X-ray characteristics of this class are now well defined, such as outburst lengths well in excess of hours, with a multiple peaked structure, and a high dynamic range (including bright outbursts), up to approximately four orders of magnitude