The M31 X-ray source RX J0042.3+4115 was originally identified as a black hole 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 , an HST ACS/WFC observation , and a 60 ks XMM-Newton observation has supplied new evidence that RX J0042.3+4115 is indeed a black hole binary .
The brightest optical star within 3 \sigma of the position of RXJ0042.3+4115 had a F435W ( \sim B ) magnitude of 25.4 \pm 0.2 ; M _ { B } > - 0.4 , hence we find a low mass donor likely .
RX J0042.3+4115 was persistently bright over \sim 12 years .
Spectral fits revealed characteristic black hole binary states : a low/hard state at 2.08 \pm 0.08 \times 10 ^ { 38 } erg s ^ { -1 } , and a steep power law state at 2.41 \pm 0.05 \times 10 ^ { 38 } erg s ^ { -1 } ( 0.3–10 keV ) .
The high luminosity low state suggests a \sim 20 M _ { \odot } primary ; this is high , but within the range of known stellar black hole masses .
The inner disk temperature during the steep power law state is 2.24 \pm 0.15 keV , high but strikingly similar to that of GRS 1915+105 , the only known Galactic black hole 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 disc temperature are rejected .