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We report on the discovery of a periodic modulation in the bright supersoftX-ray source XMMU J004252.5+411540 detected in the 2000-2004 XMM-Newtonobservations of M31. The source exhibits X-ray pulsations with a period P~217.7s and a quasi-sinusoidal pulse shape and pulsed fraction ~7-11%. We did notdetect statistically significant changes in the pulsation period on the timescale of 4 years. The X-ray spectra of XMMU J004252.5+411540 are extremely softand can be approximated with an absorbed blackbody of temperature 62-77 eV anda weak power law tail of photon index ~1.7-3.1 in the 0.2-3.0 keV energy band.The X-ray properties of the source and the absence of an optical/UV counterpartbrighter than 19 mag suggest that it belongs to M31. The estimated bolometricluminosity of the source varies between ~2e38 and ~8e38 ergs/s at 760 kpc,depending on the choice of spectral model. The X-ray pulsations and supersoftspectrum of XMMU J004252.5+411540 imply that it is almost certainly anaccreting white dwarf, steadily burning hydrogen-rich material on its surface.We interpret X-ray pulsations as a signature of the strong magnetic field ofthe rotating white dwarf. Assuming that the X-ray source is powered by diskaccretion, we estimate its surface field strength to be in the range 4e5 G<B_{0}<8e6 G. XMMU J004252.5+411540 is the second supersoft X-ray source in M31showing coherent pulsations, after the transient supersoft source XMMUJ004319.4+411758 with 865.5 s pulsation period.

XMM-Newton discovery of 217 s pulsations in the brightest persistent supersoft X-ray source in M31