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The M31 X-ray source RX J0042.3+4115 was originally identified as a blackhole binary because it displayed characteristic low state variability atconspicuously high luminosities; unfortunately, this variability was laterfound to be artificial. However, analysis of 84 Chandra ACIS observations, anHST ACS/WFC observation, and a 60 ks XMM-Newton observation has supplied newevidence that RX J0042.3+4115 is indeed a black hole binary. The brightestoptical star within 3 sigma of the position of RX J0042.3+4115 had a F435W (~B)magnitude of 25.4+/-0.2; M_B > -0.4, hence we find a low mass donor likely. RXJ0042.3+4115 was persistently bright over ~12 years. Spectral fits revealedcharacteristic black hole binary states: a low/hard state at 2.08+/-0.08 E+38erg/s, and a steep power law state at 2.41+/-0.05 E+38 erg/s (0.3--10 keV). Thehigh luminosity low state suggests a ~20 M_Sun primary; this is high, butwithin the range of known stellar black hole masses. The inner disk temperatureduring the steep power law state is 2.24$\pm$0.15 keV, high but strikinglysimilar to that of GRS 1915+105, the only known Galactic black hole binary witha low mass donor t o be persistently bright. Therefore RX J0042.3+4115 may bean analog for GRS 1915+105; however, other mechanisms may account for itsbehavior. We find compelling evidence for an extended corona during the steeppower law state, because compact corona models where the seed photons forComptonization are tied to the inner disc temperature are rejected.

Reinstating the M31 X-ray system RX J0042.3+4115 as a black hole X-ray binary, and compelling evidence for an extended corona