Unveiling supergiant fast X-ray transient sources with INTEGRAL

Supergiant high-mass X-ray binaries (SGXBs) are believed to be rare objects, as stars in the supergiant phase have a very short lifetime and to date only about a dozen of them have been discovered. They are known to be persistent and bright X-ray sources. INTEGRAL is changing this classical picture, as its observations are revealing the presence of a new subclass of SGXBs that have been labeled supergiant fast X-ray transients (SFXTs), since they are strongly characterized by fast X-ray outbursts lasting less than a day, typically a few hours. We report on IBIS detections of newly discovered fast X-ray outbursts from 10 sources, four of which have been recently optically identified as supergiant high-mass X-ray binaries. For one of them in particular, IGR J11215-5952, we observe fast X-ray transient behavior for the first time. The remaining six sources (IGR J16479-4514, IGR J16418-4532, IGR J16195-4945=AX J161929-4945, XTE J1743-363, AX J1749.1-2733, and IGR J17407-2808) are still unclassified; however, they can be considered candidate SFXTs because of their similarity to the known SFXTs.