# CG X-1: an eclipsing Wolf-Rayet ULX in the Circinus galaxy

## TL;DR

This paper studies the eclipsing Wolf-Rayet ultraluminous X-ray source CG X-1 in the Circinus galaxy, revealing its orbital period, spectral properties, and suggesting it as a key object for understanding super-critical accretion and binary evolution.

## Contribution

It provides the first detailed analysis of CG X-1's timing, spectral variability, and wind interactions, establishing it as a unique Wolf-Rayet ULX candidate.

## Key findings

- Orbital period of approximately 7.2 hours with a measurable period derivative.
- Spectral analysis indicating complex absorption by neutral, ionized, and Compton-thick material.
- Luminosity varies between 4×10^{39} and 3×10^{40} erg/s, consistent with super-Eddington accretion.

## 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 lightcurves observed over 20 years, and obtained a best-fitting period $P = (25,970.0 \pm 0.1)$ s $\approx$7.2 hr, and a period derivative $\dot{P}/P = ( 10.2\pm4.6) \times 10^{-7}$ yr$^{-1}$. The X-ray lightcurve 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, absorbed by neutral and ionized Compton-thin material, and by a Compton-thick, partial-covering medium, 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 $\approx$4 $\times 10^{39}$ erg s$^{-1}$ and $\approx$3 $\times 10^{40}$ 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 super-critical accretion and close binary evolution; it is also a likely progenitor of gravitational wave events.