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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.