CG X-1: An Eclipsing Wolf-Rayet ULX in the Circinus Galaxy

• Published in: The Astrophysical journal 2019-05, Vol.877 (1), p.57
• Main Authors: Qiu, Yanli, Soria, Roberto, Wang, Song, Wiktorowicz, Grzegorz, Liu, Jifeng, Bai, Yu, Bogomazov, Alexey, Stefano, Rosanne Di, Walton, Dominic J., Xu, Xiaojie
• Format: Article
• Language: English
• Subjects: accretion, accretion disks; Astrophysics; Cataclysmic variables; Deposition; Eclipses; Galaxies; Gravitational waves; Helium; Light curve; Luminosity; Power law; Stars & galaxies; stars: Wolf-Rayet; Stellar winds; Wind; X ray binaries; X ray sources; X ray spectra; X ray stars; X-rays: binaries; X-rays: individual (CGX-1); XMM (spacecraft)
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ab16e7

We investigated the time-variability and spectral properties of the eclipsing X-ray source Circinus Galaxy X-1 (GG X-1), using Chandra, XMM-Newton and ROSAT. We phase-connected the light curves observed over 20 yr, and we obtained a best-fitting period P = (25,970.0 0.1) s 7.2 hr, and a period derivative P ˙ P = ( 10.2 4.6 ) × 10 − 7 yr−1. The X-ray light curve shows asymmetric eclipses, with sharp ingresses and slow, irregular egresses. The eclipse profile and duration vary substantially from cycle to cycle. We show that the X-ray spectra are consistent with a power-law-like component, which is absorbed by neutral and ionized Compton-thin material, and by a Compton-thick, partial-covering medium, which is responsible for the irregular dips. The high X-ray/optical flux ratio rules out the possibility that CG X-1 is a foreground Cataclysmic Variable. In agreement with previous studies, we conclude that it is the first example of a compact ultraluminous X-ray source fed by a Wolf-Rayet star or stripped Helium star. Its unocculted luminosity varies between 4 × 1039 erg s−1 and 3 × 1040 erg s−1. Both the donor star and the super-Eddington compact object drive powerful outflows. We suggest that the occulting clouds are produced in the wind-wind collision region and in the bow shock in front of the compact object. Among the rare sample of Wolf-Rayet X-ray binaries, CG X-1 is an exceptional target for studies of supercritical accretion and close binary evolution; it is also a likely progenitor of gravitational wave events.