RODRIGUEZ CASTILLO, Guillermo AndresGuillermo AndresRODRIGUEZ CASTILLOISRAEL, Gian LucaGian LucaISRAELESPOSITO, PAOLOPAOLOESPOSITOPAPITTO, ALESSANDROALESSANDROPAPITTOSTELLA, LuigiLuigiSTELLATIENGO, ANDREAANDREATIENGODE LUCA, AndreaAndreaDE LUCAMARELLI, MARTINOMARTINOMARELLI2021-02-222021-02-2220182041-8205http://hdl.handle.net/20.500.12386/30507We 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 × 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.STAMPAenArticle10.3847/2041-8213/aacf402-s2.0-85049992871000438612300006https://iopscience.iop.org/article/10.3847/2041-8213/aacf40https://arxiv.org/abs/1804.108582018ApJ...861L..26RFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_10 High energy and particle astronomy – X-rays, cosmic rays, gamma rays, neutrinos