Monitoring Supergiant Fast X-ray Transients with Swift. III. Outbursts of the prototypical SFXTs IGR J17544-2619 and XTE J1739-302

TL;DR

This study presents broad band X-ray spectroscopy of outbursts from two prototypical Supergiant Fast X-ray Transients, revealing spectral similarities with accreting pulsars and insights into their flare behaviors.

Contribution

First simultaneous broad band spectroscopy of SFXT outbursts, demonstrating spectral properties and flare characteristics that link these sources within the class.

Findings

Spectra are well fitted by absorbed cutoff powerlaws, typical of accreting pulsars.

IGR J17544-2619 shows a harder spectrum during bright flares.

XTE J1739-302 exhibits higher column density during flares.

Abstract

IGR J17544-2619 and XTE J1739-302 are considered the prototypical sources of the new class of High Mass X-ray Binaries, the Supergiant Fast X-ray Transients (SFXTs).These sources were observed during bright outbursts on 2008 March 31 and 2008 April 8, respectively, thanks to an on-going monitoring campaign we are performing with Swift, started in October 2007. Simultaneous observations with XRT and BAT allowed us to perform for the first time a broad band spectroscopy of their outbursts. The X-ray emission is well reproduced with absorbed cutoff powerlaws, similar to the typical spectral shape from accreting pulsars. IGR J17544-2619 shows a significantly harder spectrum during the bright flare (where a luminosity in excess of 1E36 erg/s is reached) than during the long-term low level flaring activity (1E33-1E34 erg/s), while XTE J1739-302 displayed the same spectral shape, within the uncertainties, and a higher column density during the flare than in the low level activity. The light curves of these two SFXTs during the bright flare look similar to those observed during recent flares from other two SFXTs, IGRJ11215-5952 and IGRJ16479-4514, reinforcing the connection among the members of this class of X-ray sources.