2011ApJ...743..185B

The M31 X-ray source RX J0042.3+4115 was originally identified as a black hole (BH) binary because it displayed characteristic low-state variability at conspicuously high luminosities; unfortunately, this variability was later found to be artificial. However, analysis of 84 Chandra ACIS observations, a Hubble Space Telescope Advanced Camera for Surveys (ACS)/WFC observation, and a 60 ks XMM-Newton observation has supplied new evidence that RX J0042.3+4115 is indeed a BH binary. The brightest optical star within 3σ of the position of RXJ0042.3+4115 had a F435W (∼B) magnitude of 25.4±0.2; M_B > -0.4, hence we find a low-mass donor likely. RX J0042.3+4115 was persistently bright over ∼12 years. Spectral fits revealed characteristic BH binary states: a low/hard state at 2.08±0.08x10³⁸ erg/s and a steep power-law state at 2.41±0.05x10³⁸ erg/s (0.3-10 keV). The high-luminosity low state suggests a ∼20 M_☉primary; this is high, but within the range of known stellar BH masses. The inner disk temperature during the steep power-law state is 2.24±0.15 keV, high but strikingly similar to that of GRS 1915+105, the only known Galactic BH binary with a low-mass donor to be persistently bright. Therefore, RX J0042.3+4115 may be an analog for GRS 1915+105; however, other mechanisms may account for its behavior. We find compelling evidence for an extended corona during the steep power-law state, because compact corona models where the seed photons for Comptonization are tied to the inner disk temperature are rejected.