A New Explanation for the Supergiant Fast X-ray Transients' Outbursts

The physical mechanism responsible for the short outbursts in a recently recognized class of High Mass X-ray Binaries, the Supergiant Fast X-ray Transients (SFXTs), is still unknown. Recent observations performed with Swift/XRT, XMM-Newton and INTEGRAL of the 2007 outburst from IGR J11215-5952, the only SFXT known to exhibit periodic outbursts, suggest a new explanation for the outburst mechanism in this class of transients; the outbursts could be linked to the possible presence of a second wind component in the supergiant companion, in the form of an equatorial wind. The applicability of the model to the short outburst durations of all other Supergiant Fast X-ray Transients, where a clear periodicity in the outbursts has not been found yet, is discussed. The scenario we are proposing also includes the persistently accreting supergiant High Mass X-ray Binaries.