Two years of monitoring supergiant fast X-ray transients with Swift

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

doi:10.1111/j.1365-2966.2010.17564.x

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)

Highlights

Supergiant fast X–ray transients (SFXTs) constitute a new class of High Mass X–ray Binaries (HMXBs)
In our previous papers of this series, we described the longterm X–ray emission outside the bright outbursts based on the first 4 months of data (Sidoli et al 2008b, Paper I); the outbursts of IGR J16479−4514 (Romano et al 2008d, Paper II; Romano et al 2009b, Paper V); and the prototypical IGR J17544−2619 and XTE J17391−302 (Sidoli et al 2009c, Paper III; Sidoli et al 2009a, Paper IV)
XTE J1739−302 triggered the BAT on 2009 March 10 at 18:39:55 UT

Summary

XRT light curves and inactivity duty cycle

We considered the following three states, i) BATdetected outburst, ii) intermediate state (all observations yielding a firm detection excluding outburst ones), iii) ‘non detections’ (detections with a significance below 3σ) From the latter state we excluded all observations that had a net exposure below 900 s [corresponding to [2,3,4,5,6,7,8,9,10] keV flux limits that vary between 1 and 3×10−12 erg cm−2 s−1 (3σ), depending on the source, see Romano et al (2009b)]. ∆TΣ is sum of the exposures accumulated in all observations, each in excess of 900 s, where only a 3σ upper limit was achieved; Pshort is the percentage of time lost to short observations; IDC is the duty cycle of inactivity, i.e., the time each source spends undetected down to a flux limit of 1–3×10−12 erg cm−2 s−1; Rate∆TΣ is detailed in the text As we can consider our monitoring as a casual sampling of the light curve, we can infer that the time these sources spend in bright outbursts is between 3 and 5 % of the total (estimated error of ∼ 5 %)

BAT transient monitor data and on-board detections

UVOT light curves

Findings

DISCUSSION