Investigating supergiant fast X-ray transients with LOFT

Abstract

Supergiant Fast X-ray Transients (SFXT) are a class of High-Mass X-ray Binaries whose optical counterparts are O or B supergiant stars, and whose X-ray outbursts are about 4 orders of magnitude brighter than the quiescent state. LOFT, the Large Observatory For X-ray Timing, with its coded mask Wide Field Monitor (WFM) and its 10 m2 class collimated X-ray Large Area Detector (LAD), will be able to dramatically deepen the knowledge of this class of sources. It will provide simultaneous high S/N broad-band and time-resolved spectroscopy in several intensity states, and long term monitoring that will yield new determinations of orbital periods, as well as spin periods. We show the results of an extensive set of simulations performed using previous observational results on these sources obtained with Swift and XMM-Newton. The WFM will detect all SFXT flares within its field of view down to a 15–20 mCrab in 5 ks. Our simulations describe the outbursts at several intensities (F(2–10keV) = 5.9×10−9 to 5.5 × 10−10 erg cm−2 s−1), the intermediate and most common state (10−11 erg cm−2 s−1), and the low state (1.2 × 10−12 to 5 × 10−13 erg cm−2 s−1). We also considered large variations of NH and the presence of emission lines, as observed by Swift and XMM–Newton.