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IGR J17544-2619 and XTE J1739-302 are considered the prototypical sources ofthe 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 31and 2008 April 8, respectively, thanks to an on-going monitoring campaign weare performing with Swift, started in October 2007. Simultaneous observationswith XRT and BAT allowed us to perform for the first time a broad bandspectroscopy of their outbursts. The X-ray emission is well reproduced withabsorbed cutoff powerlaws, similar to the typical spectral shape from accretingpulsars. IGR J17544-2619 shows a significantly harder spectrum during thebright flare (where a luminosity in excess of 1E36 erg/s is reached) thanduring the long-term low level flaring activity (1E33-1E34 erg/s), while XTEJ1739-302 displayed the same spectral shape, within the uncertainties, and ahigher column density during the flare than in the low level activity. Thelight curves of these two SFXTs during the bright flare look similar to thoseobserved during recent flares from other two SFXTs, IGRJ11215-5952 andIGRJ16479-4514, reinforcing the connection among the members of this class ofX-ray sources.

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