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The eclipsing massive X-ray binary M33 X-7: New X-ray observations and optical identification

W. Pietsch·B. J. Mochejska·Z. Misanovic·F. Haberl·M. Ehle·G. Trinchieri
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Abstract

The eclipsing X-ray binary M33 X-7 was in the field of view during several observations of our XMM-Newton M33 survey and in the archival Chanfdra observation 1730 which cover a large part of the 3.45 d orbital period. We detect emission of M33 X-7 during eclipse and a soft X-ray spectrum of the source out of eclipse that can best be described by bremsstrahlung or disk blackbody models. No significant regular pulsations of the source in the range 0.25-1000 s were found. The average source luminosity out of eclipse is 5E37 erg/s (0.5-4.5 keV). In a special analysis of DIRECT observations we identify as optical counterpart a B0I to O7I star of 18.89 mag in V which shows the ellipsoidal heating light curve of a high mass X-ray binary with the M33 X-7 binary period. The location of the X-ray eclipse and the optical minima allow us to determine an improved binary period and ephemeris of mid-eclipse as HJD (2451760.61+-0.09)+- N * (3.45376+-0.00021). The mass of the compact object derived from orbital parameters and the optical companion mass, the lack of pulsations, and the X-ray spectrum of M33 X-7 may indicate that the compact object in the system is a black hole. M33 X-7 would be the first detected eclipsing high mass black hole X-ray binary.

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W. Pietsch, B. J. Mochejska, Z. Misanovic, F. Haberl, M. Ehle, G. Trinchieri. 2003-10-04. The eclipsing massive X-ray binary M33 X-7: New X-ray observations and optical identification. https://doi.org/10.1051/0004-6361%3A20034081

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