M33 X-7: ChASeM33 Reveals the First Eclipsing Black Hole X-ray Binary
The first observations conducted as part of the Chandra ACIS survey of M33 (ChASeM33) sampled the eclipsing X-ray binary M33 X-7 over a large part of the 3.45 day 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 (2; 453; 639:119 0:005) N(3:453014 0:000020) and constrain the eclipse half-angle to 26N5 1N1. 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 104 to 0.15 Hz. HST WFPC2 images resolve the optical counterpart, which can be identified as an O6 III star with the help of extinction and color 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. This mass, the shape of the X-ray spectrum, and the short-term X-ray time variability identify M33 X-7 as the first eclipsing black hole high-mass X-ray binary.
The Astrophysical Journal, Vol. 646, 420-428, 2006 July 20.