Volume 413, Number 3, January III 2004
|Page(s)||879 - 887|
|Published online||07 January 2004|
Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Hubble Fellow
4 XMM-Newton Science Operations Centre, ESA, Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
5 Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
Corresponding author: W. Pietsch, firstname.lastname@example.org
Accepted: 29 September 2003
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 1037 (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 . 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.
Key words: galaxies: individual: M 33 / X-rays: individuals: M 33 X-7 / X-rays: binaries / binaries: eclipsing
© ESO, 2004
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