Monitoring of the eclipsing Wolf-Rayet ULX in the Circinus galaxy

Abstract

We studied the eclipsing ultraluminous X-ray source CG X-1 in the Circinus galaxy, re-examining two decades of {\it Chandra} and {\it XMM-Newton} observations. The short binary period (7.21 hr) and high luminosity ($L_{\mathrm{X}}\approx 10^{40}$ erg s${}^{-1}$) suggest a Wolf-Rayet donor, close to filling its Roche lobe; this is the most luminous Wolf-Rayet X-ray binary known to-date, and a potential progenitor of a gravitational-wave merger. We phase-connect all observations, and show an intriguing dipping pattern in the X-ray lightcurve, variable from orbit to orbit. We interpret the dips as partial occultation of the X-ray emitting region by fast-moving clumps of Compton-thick gas. We suggest that the occulting clouds are fragments of the dense shell swept-up by a bow shock ahead of the compact object, as it orbits in the wind of the more massive donor.

10 pages, 4 figures. This manuscript is a slightly expanded and updated version of our contribution to the Proceedings of IAU Symposium 346, "High Mass X-ray Binaries: illuminating the passage from massive binaries to merging compact objects" (Vienna, August 2018)

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