A stellar-mass BH in a transient, low luminosity ULX in M31?

TL;DR

This study analyzes a transient ultraluminous X-ray source in M31, revealing a stellar-mass black hole candidate with properties similar to Galactic black hole transients, based on multi-wavelength observations and spectral analysis.

Contribution

It provides the first detailed multi-wavelength characterization of ULX2, suggesting it is a stellar-mass black hole with transient behavior similar to Galactic black hole systems.

Findings

Luminosity peaked at over 10^39 erg/s and remained stable for 40 days.

Spectrum softened from 0.9 keV to 0.4 keV during decay.

No optical or UV counterpart detected down to deep magnitude limits.

Abstract

We present a multi-wavelenght study of the recently discovered Ultraluminous X-ray transient XMMUJ004243.6+412519 (ULX2 hereafter) in M31, based on Swift data and the 1.8-m Copernico Telescope in Asiago (Italy). Undetected until January 2012, the source suddenly showed a powerful X-ray emission with a luminosity of 1e38 erg/s (assuming a distance of 780 kpc). In the following weeks, its luminosity overcame 1e39 erg/s, remaining fairly constant for at least 40 days and fading below 1e38 erg/s in the next 200 days. The spectrum can be well described by a single multi-color disk blackbody model which progressively softened during the decay (from kT=0.9 keV to 0.4 keV). No emission from ULX2 was detected down to 22 mag in the optical band and to 23-24 mag in the near ultraviolet. We compare its properties with those of other known ULXs and Galactic black hole transients, finding more similarities with the latter.