A Newly Identified Transient Ultraluminous X-ray Source in M31

Ferdinand; Murray Brightman; Jean-Marie Hameury; Elias Kammoun; Dominic J. Walton; Matthew J. Middleton; Hannah Earnshaw; Felix Fürst; Wynn V. Jacobson-Galán; Jacob E. Jencson; Margaret Lazzarini; Mark T. Reynolds

arXiv · 2610.08918

A Newly Identified Transient Ultraluminous X-ray Source in M31

Ferdinand·Murray Brightman·Jean-Marie Hameury·Elias Kammoun·Dominic J. Walton·Matthew J. Middleton·Hannah Earnshaw·Felix Fürst·Wynn V. Jacobson-Galán·Jacob E. Jencson·Margaret Lazzarini·Mark T. Reynolds

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Abstract

Ultraluminous X-ray sources (ULXs) are off-nuclear X-ray sources with luminosities exceeding the Eddington limit of typical stellar-mass compact objects. While most known ULXs are relatively persistent sources, transient ULXs have emerged as a distinct population that offer unique insight into extreme, super-Eddington accretion. We report the discovery and X-ray characterization of a newly identified transient ULX in M31. First detected serendipitously on 2021 June 9, the source reached a peak unabsorbed $0.3 - 10$ keV luminosity of $\sim10^{39}\ \rm erg\ s^{-1}$. Its spectral evolution suggests increasing advection at later epochs, opposite to the expected transition from a super-Eddington slim disk toward a standard thin disk during the decay of the outburst. The observed X-ray light curve can be reproduced using the disk-instability model of Hameury & Lasota, with characteristic timescales of $\sim41-61$ days. The inferred mass-accretion rate is super-Eddington for a $1.4\,M_{\odot}$ neutron star but approximately Eddington for a $10\,M_{\odot}$ stellar-mass black hole. Nearby optical sources from the Hubble Source Catalog suggest a possible giant donor with an age of $\log_{10}({\rm age/yr})\sim8.5$ and a mass of $\sim3.2\,M_{\odot}$, when compared with stellar isochrones with an assumed $A_V=1.58$. We estimate a transient ULX occurrence rate in M31 of $0.18\pm0.10\ {\rm yr^{-1}}$. This rate is lower than those reported for other galaxies in previous studies. M31 ULX-3 expands the small population of known transient ULXs in M31 and highlights the importance of continued, regular X-ray monitoring of nearby galaxies.

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Ferdinand, Murray Brightman, Jean-Marie Hameury, Elias Kammoun, Dominic J. Walton, Matthew J. Middleton, Hannah Earnshaw, Felix Fürst, Wynn V. Jacobson-Galán, Jacob E. Jencson, Margaret Lazzarini, Mark T. Reynolds. 2026-10-06. A Newly Identified Transient Ultraluminous X-ray Source in M31. https://arxiv.org/abs/2610.08918

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