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

Abstract

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 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. RX J0042.3+4115 was persistently bright over ~12 years. Spectral fits revealed characteristic black hole binary states: a low/hard state at 2.08+/-0.08 E+38 erg/s, and a steep power law state at 2.41+/-0.05 E+38 erg/s (0.3--10 keV). The high luminosity low state suggests a ~20 M_Sun 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 t o 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.

Accepted for publication in ApJ, 8 pages, 5 figures

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Cited by in corpus (7)

• X-ray source variability study of the M 31 central field using Chandra HRC-I
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• Chandra identification of 26 new black hole candidates in the central region of M31
• 50 M31 black hole candidates identified by Chandra and XMM-Newton
• The second ULX transient in M31: Chandra, HST and XMM observations, and evidence for an extended corona
• Energy-dependent evolution in IC10 X-1: hard evidence for an extended corona, and implications
• The first extragalactic ultra-compact X-ray binary : a candidate black hole-white dwarf system