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

Published online by Cambridge University Press:  30 December 2019

Yanli Qiu  and
Roberto Soria
Yanli Qiu
Affiliation:
National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Rd, Beijing 100101, China email: qiuyanli824@163.com
Roberto Soria
Affiliation:
College of Astronomy and Space Sciences, University of the Chinese Academy of Sciences, Beijing 100049, China email: rsoria@nao.cas.cn
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Abstract

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. We studied the eclipsing ultraluminous X-ray source CG X-1 in the Circinus galaxy, re-examining two decades of Chandra and XMM-Newton observations. The short binary period (7.21 hr) and high luminosity (LX ≈ 1040 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.

Keywords

X-rays: binariesX-rays: individual (Circinus Galaxy X-1)stars: Wolf-Rayetbinaries: eclipsing

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