XMM-Newton Discovery of 217 s Pulsations in the Brightest Persistent Supersoft X-Ray Source in M31

We report on the discovery of a periodic modulation in the bright supersoft X-ray source XMMU J004252.5+411540 detected in the 2000-2004 XMM-Newton observations of M31. The source exhibits X-ray pulsations with a period P ~ 217.7 s and a quasi-sinusoidal pulse shape and pulsed fraction ~7%-11%. We did not detect statistically significant changes in the pulsation period on the timescale of 4 yr. The X-ray spectra of XMMU J004252.5+411540 are extremely soft and can be approximated with an absorbed blackbody of temperature 62-77 eV and a weak power-law tail of photon index Γ ~ 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 counterpart brighter than 21 mag suggest that it belongs to M31. The estimated bolometric luminosity of the source varies between ~2 × 10³⁸ and ~8 × 10³⁸ ergs s^-1 at 760 kpc, depending on the choice of spectral model. The X-ray pulsations and supersoft spectrum of XMMU J004252.5+411540 imply that it is almost certainly an accreting white dwarf, steadily burning hydrogen-rich material on its surface. We interpret X-ray pulsations as a signature of the strong magnetic field of the rotating white dwarf. Assuming that the X-ray source is powered by disk accretion, we estimate its surface field strength to be in the range 4 × 10⁵ G < B₀ < 8 × 10⁶ G. XMMU J004252.5+411540 is the second supersoft X-ray source in M31 showing coherent pulsations, after the transient supersoft source XMMU J004319.4+411758 with an 865.5 s pulsation period.