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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 our best current disc model, BHSPEC, assuming a 10 M ⊙  black hole with best‐fitting spin ∼0.4, declining from  L / L  Edd = 0.75 to 0.25. However, the data are better described by a two‐component model, where the disc emission is significantly affected by advection, and with an additional low‐temperature Comptonization component at high energies which becomes more important at high luminosities. This could simply indicate the limitations of our current disc models, though changes in the energy‐dependent variability also weakly supports a two‐component interpretation of the data.  Irrespective of the detailed interpretation of the spectral properties, these data support the presence of accretion on to a stellar mass black hole in a LMXB accreting in the Eddington regime. This allows an unambiguous connection of this object, and, by extension, similar low‐luminosity ULXs, to ‘standard’ X‐ray binaries.

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