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Several persistent, low luminosity (L_X 10^{34} erg s^{-1}), long spin period(P>100 s) High Mass X-ray Binaries have been reported with blackbody componentswith temperatures >1 keV. These hot thermal excesses have correspondingly smallemitting 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 newestmember of this class, Swift J045106.8-694803. The period was determined to be168.5+/-0.2 s as of 17 July 2012 (MJD = 56125.0). At $L_X 10^{36} erg s^{-1},Swift J045106.8-694803 is the brightest member of this new class, as well asthe one with the shortest period. The spectral analysis reveals for the firsttime the presence of a blackbody with temperature kT_{BB}=1.8^{+0.2}_{-0.3} keVand radius R_{BB}=0.5+/-0.2 km. The pulsed fraction decreases with increasingenergy and the ratio between the hard (>2 keV) and soft (<2 keV) light curvesis anticorrelated with the pulse profile. Simulations of the spectrum suggestthat this is caused by the pulsations of the blackbody being pi out of phasewith those of the power law component. Using a simple model for emission fromhot spots on the neutron star surface, we fit the pulse profile of theblackbody component to obtain an indication of the geometry of the system.

XMM-Newton observation of the highly magnetised accreting pulsar Swift J045106.8-694803: Evidence of a hot thermal excess