Swift observations of two supergiant fast X-ray transient prototypes in outburst

We report on the results from observations of the most recent outbursts of XTE J1739 302 and IGR J17544 2619, which are considered to be the prototypes of the supergiant fast X-ray transient (SFXT) class. They triggered the Swift/BAT on 2011 February 22 and March 24, respectively, and each time a prompt Swift slew allowed us to obtain the rich broad-band data we present. The XRT light curves show the descending portion of very bright flares that reached luminosities of � 2 × 10 36 and � 5 × 10 36 erg s 1 , respectively. The broad-band spectra, when fit with the usual phenomenological models ado pted for accreting neutron stars, yield values of both high energy cut-off and e-folding energy consistent with those obtained from previously reported outbursts from these sources. In the context of more physical models, the spectra of both sources can be well fitted either with a two -blackbody model, or with a single unsaturated Comptonization model. In the latter case, the model can be either a classical static Comptonization model, such as COMPTT, or the recently developed COMPMAG model, which includes thermal and bulk Comptonization for cylindrical accretion onto a magnetized neutron star. We discuss the possible accretion scenarios derived by the different models, and we also emphasize the fact that the electron density derived from the Comptonization models, in the regions where the X-ray spectrum presumably forms, is lower than that estimated using the continuity equation at the magnetospheric radius and the source X-ray luminosity, and we give some possible explanations.