Eclipse ingress and egress of the M33 HMXRB X-7 resolved by Chandra

ATel #633; M. Sasaki (CfA), W. Pietsch (MPE), F. Haberl (MPE), T. J. Gaetz (CfA), P. Ghavamian (JHU), K. S. Long (STScI), T. G. Pannuti (SSC), P. P. Plucinsky (CfA) for the CHASeM33 team
on 17 Oct 2005; 21:23 UT
Credential Certification: Manami Sasaki (msasaki@cfa.harvard.edu)

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We report on the detection of the eclipse in- and egress of the high-mass X-ray binary M33 X-7 with 3.45 d orbital period (for the latest ephemeris see Pietsch et al. 2004, A&A 413, 879) within the first Chandra CHASeM33 very large program observations of M33. M33 X-7 was in the field of view of four Chandra observations obtained from 2005 September 21 to October 2. For the first time, we monitored the source during its gradual transition (15 ks duration, 0.05 in binary phase) into eclipse (eclipse start JD 2453635.39) during the first observation. The eclipse ingress was also covered during the last observation. During the third observation, we observed the transition (within 6 ks, 0.02 in phase) out of eclipse (eclipse end JD 2453642.84). The epoch of mid eclipse is determined as JD 2453642.57. We refine the eclipse duration to 0.16 in binary phase and, combining our mid eclipse epoch with that given by Dubus et al. 1999, MNRAS 302, 731, determine an improved binary period of 3.45372 d.