The eclipsing massive X-ray binary M33 X−7: New X-ray observations and optical identificationThis work is based on observations obtained with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA)

[1]×101 erg cm−2 s−1 [1]10 erg cm−2 s−1 [1]×101 erg s−1 [1]10 erg s−1 [1]×101 cm−2 [1]10 cm−2 [1]×101 [1]10 kTbb kTbr NH Lx L x ct s−1 [1]×101 ct s−1 arcmin−2 r90 MLexi document The eclipsing massive X-ray binary M33 X−7: New X-ray observations and optical identificationThis work is based on observations obtained with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). W. Pietsch1 B. J. Mochejska 2,3 Z. Misanovic1 F. Haberl1 M. Ehle4 G. Trinchieri5 Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, US Hubble Fellow XMM-Newton Science Operations Centre, ESA, Villafranca del Castillo, P.O. Box 50727, 28080 Madrid, Spain Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy W. Pietsch wnp@mpe.mpg. de Received; accepted The eclipsing massive X-ray binary M33 X−7 The eclipsing X-ray binary M33 X−7 was in the field of view during several observations of our XMMNewton M33 survey and in the archival Chandra 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 537 (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 (245 1760.61±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. Galaxies: individual: M33 X-rays: individuals: M33 X−7 X-rays: binaries binaries: eclipsing Introduction M33 X−7 (hereafter X−7) was detected as a variable source with a luminosity brighter than 38 in Einstein observations 1981ApJ.246L. 61L,1983ApJ.275. 571M,1988ApJ.325. 531T,1988ApJ.329. 1037T. 1989ApJ.336. 140P suggested that the X−7 variability pattern can be explained by an eclipsing X-ray binary (XRB) with an orbital period of 1.7 d and an eclipse duration of ∼ 0.4 d. This finding was the first identification of a close accreting binary system with an X-ray source in an external galaxy other than the Magellanic Clouds. It was confirmed combining Einstein observatory and first ROSAT data 1993ApJ.418L. 67S,1994ApJ.426L. 55S. With the inclusion of more ROSAT and ASCA data 1999MNRAS.302. 731DD99 [][ hereafter 1999MNRAS.302. 731D]1997AJ.113. 618L,1999MNRAS.302. 731D the orbital period turned out to be twice as long. The shape of the eclipse could be described by a slow ingress (∆Φingress = 0.10±0.05), an eclipse duration of ∆Φeclipse = 0.20±0.03, and a fast eclipse egress (∆Φegress = 0.01 ± 0.01) with an ephemeris for the mid-eclipse time of HJD 244 8631.5±0.1 + N×(3.4535±0.0005). In addition, 1999MNRAS.302. 731D discovered evidence for a 0.31 s pulse period. The orbital period, pulse period and observed X-ray luminosity are remarkably similar to those of the Small Magellanic Cloud neutron star XRB SMC X−1 2000AAS.147. 25L. However, if the pulse period of X−7 can not be confirmed, the source could also resemble high mass black hole XRBs (BHXB) like LMC X−1 or LMC X−3. It would be the first eclipsing object within this rare class of XRBs. The position of X−7 correlates with the dense O–B association HS13 1980APJS.44. 319H and therefore no individual counterpart could be identified based on position only. However, its location in HS13 is consistent with the expectation of a massive companion. As 1999MNRAS.302. 731D point out, the optical counterpart is likely to show ellipsoidal and/or X-ray heating variations 1986AA.154. 77T which can be used for the optical identification. Variable optical sources within M33 were systematically searched for in the DIRECT project 2001AJ.122. 2477MM01b [see e.g.][ hereafter 2001AJ.122. 2477M]2001AJ.122. 2477M. Many eclipsing binaries, Cepheids, and other periodic, possibly long-period or nonperiodic variables were detected. X−7 is located in DIRECT field M33B. The variability of the optical counterpart was not detected in the previous analysis due to the limitations of the variable search strategy for such small amplitude variables in crowded regions.