Two years of monitoring Supergiant Fast X-ray Transients with Swift

P. Romano; V. La Parola; S. Vercellone; G. Cusumano; L. Sidoli; H. A. Krimm; C. Pagani; P. Esposito; E. A. Hoversten; J. A. Kennea; K. L. Page; D. N. Burrows; N. Gehrels

doi:10.1111/j.1365-2966.2010.17564.x

arXiv · 1009.1146

Two years of monitoring Supergiant Fast X-ray Transients with Swift

P. Romano·V. La Parola·S. Vercellone·G. Cusumano·L. Sidoli·H. A. Krimm·C. Pagani·P. Esposito·E. A. Hoversten·J. A. Kennea·K. L. Page·D. N. Burrows·N. Gehrels

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Abstract

We present two years of intense Swift monitoring of three SFXTs, IGR J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007). Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed power laws with by hard photon indices (G~1-2). Their outburst broad-band (0.3-150 keV) spectra can be fit well with models typically used to describe the X-ray emission from accreting NSs in HMXBs. We assess how long each source spends in each state using a systematic monitoring with a sensitive instrument. These sources spend 3-5% of the total in bright outbursts. The most probable flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19, 39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively. We present a complete list of BAT on-board detections further confirming 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. X-ray variability is observed at all timescales and intensities we can probe. Superimposed on the day-to-day variability is intra-day flaring which involves variations up to one order of magnitude that can occur down to timescales as short as ~1ks, and whichcan be explained by the accretion of single clumps composing the donor wind with masses M_cl~0.3-2x10^{19} g. (Abridged)

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P. Romano, V. La Parola, S. Vercellone, G. Cusumano, L. Sidoli, H. A. Krimm, C. Pagani, P. Esposito, E. A. Hoversten, J. A. Kennea, K. L. Page, D. N. Burrows, N. Gehrels. 2010-09-06. Two years of monitoring Supergiant Fast X-ray Transients with Swift. https://doi.org/10.1111/j.1365-2966.2010.17564.x

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