Supergiant Fast X-ray Transients as transient sources in High Mass X-ray
  Binaries
  Supergiant Fast X-ray Transients (SFXTs) represent the most extreme case of
X-ray variability in High Mass X-ray Binaries hosting blue supergiant
companions. Mainly discovered thanks to the INTEGRAL monitoring of the Galactic
plane, these hard X-ray transients display dynamic ranges which can span five
orders of magnitude. This intensity variability is associated with accretion
onto Galactic compact objects (mostly neutron stars) through a physical
mechanism which is still poorly understood. A review of the current status of
our understanding on these sources is presented and discussed. Finally, I
present recent XMM-Newton results about the SFXT IGRJ16418-4532, proposing that
its variability behavior is due to a transitional accretion regime,
intermediate between pure wind accretion and Roche Lobe Overflow. This same
regime could explain the X-ray activity of other "intermediate" SFXTs with
narrow orbits.
