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We report on a multi-wavelength study of the recently discovered X-raytransient XMMU J004243.6+412519 in M31, based on data collected with Swift andthe 1.8-m Copernico Telescope at Cima Ekar in Asiago (Italy) between 2012February and August. Undetected in all previous observations, in 2012 JanuaryXMMU J004243.6+412519 suddenly turned on, showing powerful X-ray emission witha luminosity of 1E+38 erg/s (assuming a distance of 780 kpc). In the followingweeks, it reached a luminosity higher than 1E+39 erg/s, in the typical range ofultraluminous X-ray sources (ULXs). For at least 40 days the source luminosityremained fairly constant, then it faded below 1E+38 erg/s in the following 200days. The source spectrum, which can be well described by multi-color diskblackbody model, progressively softened during the decay (the temperaturechanged from kT = 0.9 keV to 0.4 keV). No emission from XMMU J004243.6+412519was detected down to 22 mag in the optical band and of 23-24 mag in the nearultraviolet. We compare the properties of XMMU J004243.6+412519 with those ofother known ULXs and Galactic black hole transients, finding more similaritieswith the latter.

Swift observations of the ultraluminous X-ray source XMMU J004243.6+412519 in M31