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

Abstract

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 × 10 39 erg s −1 and ≈3 × 10 40 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.