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 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.
