Transient outburst mechanisms

The recent discovery of a new class of recurrent and fast X–ray transient sources, the Supergiant Fast X–ray Transients, poses interesting questions on the possible mechanisms responsible for their transient X–ray emission. The association with blue supergiants, the spectral properties similar to those of accreting pulsars and the detection, in a few cases, of X-ray pulsations, confirm that these transients are High Mass X-ray Binaries. Their transient behaviour is quite surprising since neutron stars accreting from the winds of supergiant companions were seen as persistent sources. I review the different mechanisms proposed to explain their transient outbursts and the link to the persistent sources. The different explanations for these sources can be divided into two classes: the first one deals with the structure of the supergiant wind, while other hypotheses are related with the properties of the accreting compact object. I discuss the different models proposed, in light of the new observational results coming from an on-going monitoring campaign of four