Swift and Supergiant Fast X-ray Transients: a novel monitoring approach

Abstract

We describe our monitoring strategy which best exploits the sensitivity and flexibility of Swift to study the long-term behaviour of Supergiant Fast X-ray Transients (SFXTs). We present observations of the recent outbursts from two objects of this class. IGR J16479-4514, underwent an outburst on 2008 March 19, reaching a peak luminosity of about 6E37 erg/s (0.5-100keV; at a distance of 4.9 kpc). We obtained a simultaneous broad-band spectrum (0.3-100 keV), the first for the SFXT class, which is fit with a heavily absorbed (column density 5E22 cm^-2) hard power-law with a high energy cut-off at about 7keV. This spectrum shows properties similar to the ones of accreting pulsars, although no X-ray pulsations were found. IGR J11215-5952, one of the only two periodic SFXT known to date, was observed with Swift several times, first with an intense 23-day long monitoring campaign around the 2007 February 9 outburst; then with a 26-day long monitoring around the unexpected July 24 outburst; finally with a deep exposure during the 2008 June 16 outburst. We present the whole dataset, which also includes observations which allowed us to firmly establish the outburst period at P~165 days. Thanks to our combined observations common characteristics to this class of objects are emerging, i.e., outburst lengths well in excess of hours, often with a multiple peaked structure, dynamic range ~3 orders of magnitude, and periodicities are starting to be found.