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

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 1038 erg s-1 (assuming a distance of 780 kpc). In the following weeks, its luminosity overcame 1039 erg s-1, remaining fairly constant for at least 40 days and fading below 1038 erg s-1 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 \textit{mag} in the optical band and to 23-24 \textit{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 latte