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 modulation of 4.15 h and a projected semi-axis at $a_{ \mathrm{ X}} \sin i= 0.6$ lt-s, which implies a low-mass companion of about 0.2$-$0.3$M_{ \odot}$ assuming a NS of 1.5 $M_{ \odot}$ and an orbital inclination $i=70^{ \circ}-80^{ \circ}$. The barycentric orbit-corrected pulse period decreased by $\sim$28 ms in about 16 yr, corresponding to an average spin-up rate of $\dot{ P} \sim -6 \times 10^{ -11}$ s s$^{ -1}$ ; pulse period variations, probably caused to by X-ray luminosity changes, were observed on shorter time scales. We identify two possible scenarios for the source: a mildly magnetic NS with $B_{ \mathrm{ p}}\simeq$ few $\times10^{ 10}$ G if the pulsar is far from its equilibrium period $P_{ \mathrm{ eq}}$, and a relatively young highly magnetic NS with $B_{ \mathrm{ eq}}\simeq 10^{ 13}$ G if spinning close to $P_{ \mathrm{ eq}}$.
Categories: astro-ph.HE