Supergiant Fast X-ray Transients: a New Class of High Mass X-ray Binaries Unveiled by Integral

INTEGRAL monitoring of the Galactic Plane is revealing a growing number of recurrent X-ray transients, characterised by short outbursts with very fast rise times (∼ tens of minutes) and typical durations of a few hours. Here we show that several of these transients are associated with OB supergiants and hence define a new class of massive X-ray binaries which we call Supergiant Fast X-ray Transients. Many other transient X-ray sources display similar X-ray characteristics, suggesting that they belong to the same class. Since they are difficult to detect and their number is growing fast and steadily, they could represent a major class of X-ray binaries.