Discovery of a 3 s Spinning Neutron Star in a 4.15 hr Orbit in the Brightest Hard X-Ray Source in M31

We report the discovery with XMM-Newton of 3 s X-ray pulsations from 3XMM J004232.1+411314, a dipping source that dominates the hard X-ray emission of M31. This finding unambiguously assesses the neutron star (NS) nature of the compact object. We also measured an orbital period of 4.15 hr and a projected semi-axis at a_X sin i = 0.6 lt-s, which implies a low-mass companion of about 0.2-0.3 M _⊙ assuming an NS of 1.5 M _⊙ and an orbital inclination i = 60°-80°. The barycentric orbit-corrected pulse period decreased by ∼28 ms in about 16 year, corresponding to an average spin-up rate of \dot{P}∼ -6× {10}^-11 s s^-1 pulse period variations, probably caused by X-ray luminosity changes, were observed on shorter timescales. We identify two possible extreme scenarios for the source: a mildly magnetic NS with B _p ≃ few × 10¹⁰ G if the pulsar is far from its equilibrium period P _eq and the disk is truncated at the value of the Alfvén radius corresponding to the observed luminosity, and a relatively young, highly magnetic NS with B _eq ≃ 2 × 10¹³ G if spinning close to P _eq and the disk is truncated close to the co-rotation surface.