M33 X-7: ChASeM33 Reveals the First Eclipsing Black Hole X-Ray Binary

• Published in: The Astrophysical journal Vol. 646; no. 1; pp. 420 - 428
• Main Authors: Pietsch, W, Haberl, F, Sasaki, M, Gaetz, T. J, Plucinsky, P. P, Ghavamian, P, Long, K. S, Pannuti, T. G
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 20.07.2006; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Eclipsing binary stars; galaxies: individual (M33); Galaxies; X-rays: binaries; X-ray binary stars; Black holes; black hole high-mass X-ray binary. binaries: eclipsing; Binary X ray source; Cosmic x-ray sources; Cosmology; X-rays: individual (M33 X-7)
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/504704

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 c 0.005) c N(3.453014 c 0.000020) and constrain the eclipse half-angle to 26.5 c 1.1. 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 super(-4) to 0.15 Hz. HST WFPC2 images resolve the optical counterpart, which can be identified as an 06 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 sub(z). 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.