Monitoring Supergiant Fast X-Ray Transients with Swift . II. Rise to the Outburst in IGR J16479-4514

Abstract

IGR J16479-4514 is a supergiant fast X-ray transient (SFXT), a new class of high-mass X-ray binaries, whose number is rapidly growing thanks to the INTEGRAL observations of the Galactic plane. It has been regularly monitored with Swift XRT since 2007 November to study the quiescent emission, the outburst properties, and their recurrence. A new bright outburst, reaching fluxes above 10-9 erg cm-2 s-1, was caught by the Swift BAT. Swift immediately repointed at the target with the narrow-field instruments so that, for the first time, an outburst from an SFXT where a periodicity in the outburst recurrence is unknown could be observed simultaneously in the 0.2-150 keV energy band. The X-ray emission is highly variable and spans almost 4 orders of magnitude in count rate during the Swift XRT observations covering a few days before and after the bright peak. The X-ray spectrum in outburst is hard and highly absorbed. The power-law fit resulted in a photon index of 0.98 +/- 0.07, and in an absorbing column density of ~5 × 1022 cm-2. These observations demonstrate that in this source (similarly to what was observed during the 2007 outburst from the periodic SFXT IGR J11215-5952), the accretion phase lasts much longer than a few hours.