Monitoring supergiant fast X-ray transients with swift. III. Outbursts of the prototypical supergiant fast X-ray transients IGR J17544-2619 and XTE J1739-302

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. These sources were observed during bright outbursts on 2008 March 31 and 2008 April 8, respectively, thanks to an ongoing monitoring campaign we are performing with Swift, started in 2007 October. Simultaneous observations with X-ray Telescope and Burst Alert Telescope allowed us to perform for the first time a broadband spectroscopy of their outbursts. The X-ray emission is well reproduced with absorbed cutoff power laws, 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 10 ³⁶ erg s^-1 is reached) than during the long-term low-level flaring activity (10³³-10³⁴ erg s^-1), 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, IGR J11215-5952 and IGR J16479-4514, reinforcing the connection among the members of this class of X-ray sources.