The missing link: a low-mass X-ray binary in M31 seen as an ultraluminous X-ray source

A new, transient ultraluminous X-ray source (ULX) was recently discovered by Chandra in M31 with a luminosity at ∼5 × 10 39 erg s −1 . Here we analyse a series of five subsequent XMM–Newton observations. These show a steady decline in X-ray luminosity over 1.5 months, from 1.8 × 10 39 to 0.6 × 10 39 erg s −1 , giving an observed e-fold time-scale of ∼40 d. This is similar to the decay time-scales seen in multiple soft X-ray transients in our own Galaxy, supporting the interpretation of this ULX as a stellar mass black hole in a low-mass X-ray binary (LMXB), accreting at super-Eddington rates. This is further supported by the lack of detection of an O/B star in quiescence and the spectral behaviour of the XMM–Newton data being dominated by a disc-like component rather than the power law expected from a sub-Eddington intermediate-mass black hole. These data give the best sequence of high Eddington fraction spectra ever assembled due to the combination of low absorption column to M31 and well-calibrated bandpass down to 0.3 keV of XMM–Newton in full frame mode. The spectra can be roughly described by