Several persistent , low luminosity ( L _ { X } \sim 10 ^ { 34 } erg s ^ { -1 } ) , long spin period ( P > 100 s ) High Mass X-ray Binaries have been reported with blackbody components with temperatures > 1 keV .
These hot thermal excesses have correspondingly small emitting regions ( < 1 km ^ { 2 } ) and are attributed to the neutron star polar caps .
We present a recent XMM-Newton target of opportunity observation of the newest member of this class , Swift J045106.8-694803 .
The period was determined to be 168.5 \pm 0.2 s as of 17 July 2012 ( MJD = 56125.0 ) .
At L _ { X } \sim 10 ^ { 36 } erg s ^ { -1 } , Swift J045106.8-694803 is the brightest member of this new class , as well as the one with the shortest period .
The spectral analysis reveals for the first time the presence of a blackbody with temperature kT _ { BB } = 1.8 ^ { +0.2 } _ { -0.3 } keV and radius R _ { BB } = 0.5 \pm 0.2 km .
The pulsed fraction decreases with increasing energy and the ratio between the hard ( > 2 keV ) and soft ( < 2 keV ) light curves is anticorrelated with the pulse profile .
Simulations of the spectrum suggest that this is caused by the pulsations of the blackbody being \sim \pi out of phase with those of the power law component .
Using a simple model for emission from hot spots on the neutron star surface , we fit the pulse profile of the blackbody component to obtain an indication of the geometry of the system .