We report on a 40 ks long , uninterrupted X–ray observation of the candidate supergiant fast X–ray transient ( SFXTs ) IGR J16418–4532 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 \sim 0.1 counts s ^ { -1 } to \sim 15 counts s ^ { -1 } , with several bright flares of different durations ( from a few hundreds to a few thousands seconds ) and sometimes with a quasi-periodic behaviour .
This finding supports the previous suggestion that IGR J16418–4532 is a member of the SFXTs class .
In our new observation we measured a pulse period of 1212 \pm 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 IGR J16418–4532 , 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 behaviour of IGR J16418–4532 and , possibly , of other SFXTs with short orbital periods .