M 33 X-7: ChASeM33 reveals the first eclipsing black hole X-ray binary

W. Pietsch; F. Haberl; M. Sasaki; T. J. Gaetz; P. P. Plucinsky; P. Ghavamian; K. S. Long; T. G. Pannuti

doi:10.1086/504704

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M 33 X-7: ChASeM33 reveals the first eclipsing black hole X-ray binary

W. Pietsch·F. Haberl·M. Sasaki·T. J. Gaetz·P. P. Plucinsky·P. Ghavamian·K. S. Long·T. G. Pannuti

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Abstract

The first observations conducted as part of the Chandra ACIS survey of M 33 (ChASeM33) sampled the eclipsing X-ray binary M 33 X-7 over a large part of the 3.45 d orbital period and have resolved eclipse ingress and egress for the first time. The occurrence of the X-ray eclipse allows us to determine an improved ephemeris of mid-eclipse and binary period as HJD (2453639.119+-0.005) +- N x (3.453014+-0.000020) and constrain the eclipse half angle to (26.5+-1.1) degree. There are indications for a shortening of the orbital period. The X-ray spectrum is best described by a disk blackbody spectrum typical for black hole X-ray binaries in the Galaxy. We find a flat power density spectrum and no significant regular pulsations were found in the frequency range of 10^{-4} to 0.15 Hz. HST WFPC2 images resolve the optical counterpart, which can be identified as an O6III star with the help of extinction and colour corrections derived from the X-ray absorption. Based on the optical light curve, the mass of the compact object in the system most likely exceeds 9 M_sun. This mass, the shape of the X-ray spectrum and the short term X-ray time variability identify M 33 X-7 as the first eclipsing black hole high mass X-ray binary.

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W. Pietsch, F. Haberl, M. Sasaki, T. J. Gaetz, P. P. Plucinsky, P. Ghavamian, K. S. Long, T. G. Pannuti. 2006-03-25. M 33 X-7: ChASeM33 reveals the first eclipsing black hole X-ray binary. https://doi.org/10.1086/504704

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