The XMM-Newton view of Supergiant Fast X-ray Transients: the case of
  IGRJ16418-4532
  We report on a 40 ks long, uninterrupted X-ray observation of the candidate
supergiant fast X-ray transient (SFXT) IGRJ16418-4532 performed with XMM-Newton
on February 23, 2011. This high mass X-ray binary lies in the direction of the
Norma arm, at an estimated distance of 13 kpc. During the observation, the
source showed strong variability exceeding two orders of magnitudes, never
observed before from this source. Its X-ray flux varied in the range from 0.1
counts/s to about 15 counts/s, with several bright flares of different
durations (from a few hundreds to a few thousands seconds) and sometimes with a
quasi-periodic behavior. This finding supports the previous suggestion that
IGRJ16418-4532 is a member of the SFXTs class. In our new observation we
measured a pulse period of 1212+/-6 s, thus confirming that this binary
contains a slowly rotating neutron star. During the periods of low luminosity
the source spectrum is softer and more absorbed than during the flares. A soft
excess is present below 2 keV in the cumulative flares spectrum, possibly due
to ionized wind material at a distance similar to the neutron star accretion
radius. The kind of X-ray variability displayed by IGRJ16418-4532, its dynamic
range and time scale,together with the sporadic presence of quasi-periodic
flaring, all are suggestive of a transitional accretion regime between pure
wind accretion and full Roche lobe overflow. We discuss here for the first time
this hypothesis to explain the behavior of IGRJ16418-4532 and, possibly, of
other SFXTs with short orbital periods.
