Discovery of 3 s pulsations from the Brightest Hard X-ray Source in M31

TL;DR

This paper reports the discovery of 3-second X-ray pulsations from a bright source in M31, confirming it hosts a neutron star, and analyzes its orbital and spin properties to infer magnetic field strength and evolutionary state.

Contribution

First detection of 3-s X-ray pulsations from M31's brightest hard X-ray source, establishing its neutron star nature and characterizing its orbital and spin evolution.

Findings

Confirmed neutron star nature through pulsation detection.

Measured orbital period and semi-major axis, indicating a low-mass companion.

Observed long-term spin-up rate and short-term period variations.

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 $\times10^{10}$ 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 $P_{\mathrm{eq}}$.