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

Abstract

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 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 × 10 10 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 × 10 13 G if spinning close to P eq and the disk is truncated close to the co-rotation surface.