Monitoring supergiant fast X-ray transients with Swift: results from the first year

The advent of Swift has allowed, for the first time, the possibility to give supergiant fast X-ray transients (SFXTs), the new class of high-mass X-ray binaries discovered by the International Gamma-Ray Astrophysics Laboratory, non-serendipitous attention throughout most phases of their life. In this paper, we present our results based on the first year of intense Swift monitoring of four SFXTs, IGR J16479-4514, XTE J1739-302, IGR J17544-2619 and AX J1841.0-0536. We obtain the first assessment of how long each source spends in each state using a systematic monitoring with a sensitive instrument. The duty-cycle of inactivity is similar to 17, 28, 39 and 55 per cent (similar to 5 per cent uncertainty), for IGR J16479-4514, AX J1841.0-0536, XTE J1739-302 and IGR J17544-2619, respectively, so that true quiescence, which is below our detection ability even with the exposures we collected in 1 yr, is a rare state, when compared with estimates from less sensitive instruments. This demonstrates that these transients accrete matter throughout their lifetime at different rates. AX J1841.0-0536 is the only source which has not undergone a bright outburst during our monitoring campaign. Although individual sources behave somewhat differently, common X-ray characteristics of this class are emerging, such as outburst lengths well in excess of hours, with a multiple peaked structure. A high dynamic range (including bright outbursts) of similar to 4 orders of magnitude has been observed in IGR J17544-2619 and XTE J1739-302, of similar to 3 in IGR J16479-4514 and of about 2 in AX J1841.0-0536 (this lowest range is due to the lack of bright flares). We also present a complete list of Burst Alert Telescope (BAT) on-board detections, which complements our previous work, and further confirms the continuous activity of these sources. We performed out-of-outburst intensity-based spectroscopy. In particular, spectral fits with an absorbed blackbody always result in blackbody radii of a few hundred metres, consistent with being emitted from a small portion of the neutron star surface, very likely the neutron star polar caps. We used the whole BAT data set, since the beginning of the mission, to search for periodicities due to orbital motion and found P(orb) = 3.32 d for IGR J16479-4514, confirming previous findings. We also present the Ultraviolet/Optical Telescope (UVOT) data of these sources; we show the UVOT light curves of AX J1841.0-0536 and the ones of XTE J1739-302 before, during and after the outbursts.