Discovery of 3 s pulsations from 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 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 \(P_{\mathrm{eq}}\).