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Supergiant Fast X-ray Transients (SFXTs) are a sub-class of High Mass X-rayBinaries (HMXBs) associated with OB supergiant companions and displayingtransient X-ray activity. This behaviour is quite surprising since HMXBshosting supergiants were known to be persistent sources, until the INTEGRALdiscoveries obtained by means of the monitoring of the Galactic plane. We havebeen performing a monitoring campaign with Swift of four SFXTs with the mainaim of characterizing both the long-term behaviour of these transients and theproperties during bright outbursts. Here we discuss the properties of the X-rayemission observed outside the outbursts as well as the flares observed from twoSFXTs: IGRJ17544-2916 and XTEJ1739-302. Contrarily to what previously thought,Swift allowed us to discover that SFXTs spend most of the time in accretion ata low level, even outside the bright outbursts, with an accretion luminosity of1E33-1E34 erg/s, and that the quiescent level at ~1E32 erg/s is a much rarerstate.

Swift monitoring of Supergiant Fast X-ray Transients: the out-of-outburst behaviour and the flares from IGRJ17544-2916 and XTEJ1739-302