A Newly Identified Transient Ultraluminous X-ray Source in M31

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 $\sim 10^{39}\mathrm{erg}{\mathrm{s}}^{-1}$. 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 $\sim 41-61$ days. The inferred mass-accretion rate is super-Eddington for a $1.4M_{\odot }$ neutron star but approximately Eddington for a $10M_{\odot }$ stellar-mass black hole. Nearby optical sources from the Hubble Source Catalog suggest a possible giant donor with an age of ${\log }_{10}(age/yr)\sim 8.5$ and a mass of $\sim 3.2M_{\odot }$, when compared with stellar isochrones with an assumed $A_V=1.58$. We estimate a transient ULX occurrence rate in M31 of $0.18\pm 0.10\mathrm{y}{\mathrm{r}}^{-1}$. 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.

20 pages, 9 figures, 2 tables, submitted to ApJ