Discovery of 3 s pulsations from 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 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 $\times 10^{10}$ 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 $P_{\mathrm{eq}}$.

5 pages, 3 figures, 2 tables, Submitted to the Astrophysical Journal Letters