On the nature of Supergiant Fast X-ray Transients

More than a decade after fast x-ray transients with an OB supergiant counterpart were identified as a distinct class of wind-accreting sources, we still have not reached a consensus on the physical origin of their similarities and differences with persistent sources. Both kinds seem to extend over the same range of every relevant parameter. Here I argue that, despite this overall overlap, persistent sources have - on average - later-type, more evolved counterparts, and discuss the hypothesis that SFXTs are - on average - a younger population, as well as some of its possible implications.