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Optical modulation of the new eclipsing XRB in M33

ATEL # 913; A. Shporer (Wise Obs.), J. Hartman (CfA), T. Mazeh (Wise Obs.), W. Pietsch (MPE)
on 11 Oct 2006; 12:13 UT
Password Certification: Avi Shporer (

Subjects: Optical, Binaries

The variable XMM-Newton source [PMH2004] 47 was recently identified by Pietsch et al. (ATel #905) as an eclipsing XRB with an orbital period of 1.73245 days. Based on its position and color, Pietsch et al. identified star #4571 of Massey et al. (2006, AJ 131, 2478) as the optical counterpart of the X-ray source.

This star was not identified as variable in the M33 CFHT variability survey (Hartman et al. 2006, MNRAS, 371, 1405). To search for any small optical modulation, we derived the light curves of this star in the Sloan g', r' & i' bands directly from the CFHT images. Folding the 31-33 measurements in each of these colors with the X-ray period reveals a clear periodicity. The folded light curve is of double sinusoidal shape, with one of the minima at the phase of mid X-ray eclipse, suggesting an ellipsoidal modulation of a high-mass optical companion.

The detection of an optical modulation at the X-ray period confirms the identification of the optical counterpart. This opens the way to estimate the two masses and determine whether the primary is a neutron star or a black hole.

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