Supergiant Fast X-ray Transients in outburst: new Swift observations of XTE J1739−302, IGR J17544−2619 and IGR J08408−4503

We report on new X‐ray outbursts observed with Swift from three Supergiant Fast X‐ ray Transients (SFXTs): XTE J1739 302, IGR J17544 2619, and IGR J08408 4503. XTE J1739 302 underwent a new outburst on 2008, August 13, IGR J17544 2619 on 2008, September 4, while IGR J08408 4503 on 2008, September 21. While the XTE J1739 302 and IGR J08408 4503 bright emission triggered the Swift/Burst Alert Telescope, IGR J17544 2619 did not, thus we could perform a spectral investigation only of the spectrum below 10 keV. The broad band spectra from XTE J1739 302 and IGR J08408 4503 were compatible with the X‐ray spectral shape displayed during the previous flares. A variable absorbing column density during the flare was observed in XTE J1739 302 for the first time. The broad-band spectrum of IGR J08408 4503 requires the presence of two distinct photon populations, a cold one (�0.3 keV) most likely from a thermal halo around the neutron star and a hotter one (1.4‐1.8 keV) from the accreting column. The outburst from XTE J1739 302 could be monitored with a very good sampling, thus revealing a shape which can be explained with a second wind component in this SFXT, in analogy to what we have suggested in the periodic SFXT IGR J11215‐5952. The outburst recurrence timescale in IGR J17544 2619 during our monitoring campaign with Swift suggests a long orbital period of �150 days (in an highly eccentric orbit), compatible with what previously observed with INTEGRAL.