The eclipsing massive X-ray binary M 33 X-7: New X-ray observations and optical identification

Abstract

The eclipsing X-ray binary M 33 X 7 was in the field of view during several observations of our XMM-Newton M 33 survey and in the archival Chandra observation 1730 which cover a large part of the 3.45 d orbital period. We detect emission of M 33 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 5 10 37 erg s 1 (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 M 33 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 (2 451 760:61 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 M 33 X 7 may indicate that the compact object in the system is a black hole. M 33 X 7 would be the first detected eclipsing high mass black hole X-ray binary.