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

Abstract

The advent of Swift has allowed, for the first time, the possibility to give Super giant Fast X‐ray Transients (SFXTs), the new class of High Mass X‐ray Binaries discovered by INTEGRAL, 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 i n each state using a systematic monitoring with a sensitive instrument. The duty-cycle of inactivity is � 17, 28, 39, 55 % (�5 % 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 one year, 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 � 4 orders of magnitude have been observed in IGR J17544 2619 and XTE J1739 302, of �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 B AT on-board detections, which complements our previous work, and further confirms the cont inuous 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 meters, 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 dataset, since the beginning of the mission, to search for periodicities due to orbital motion and found Porb = 3.32 d for IGR J16479 4514, confirming previous findings. We also present the UVOT data of these sour ces; we show the UVOT light curves of AX J1841.0 0536 and the ones of XTE J1739 302 before, during, and after the outbursts.