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We present a multi-wavelenght study of the recently discovered UltraluminousX-ray transient XMMUJ004243.6+412519 (ULX2 hereafter) in M31, based on Swiftdata and the 1.8-m Copernico Telescope in Asiago (Italy). Undetected untilJanuary 2012, the source suddenly showed a powerful X-ray emission with aluminosity of 1e38 erg/s (assuming a distance of 780 kpc). In the followingweeks, its luminosity overcame 1e39 erg/s, remaining fairly constant for atleast 40 days and fading below 1e38 erg/s in the next 200 days. The spectrumcan be well described by a single multi-color disk blackbody model whichprogressively softened during the decay (from kT=0.9 keV to 0.4 keV). Noemission from ULX2 was detected down to 22 mag in the optical band and to 23-24mag in the near ultraviolet. We compare its properties with those of otherknown ULXs and Galactic black hole transients, finding more similarities withthe latter.

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