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

## TL;DR

This study analyzes two decades of X-ray observations of the eclipsing Wolf-Rayet ultraluminous X-ray source CG X-1 in the Circinus galaxy, revealing its binary properties, dipping behavior, and possible gas cloud occultations.

## Contribution

It provides the first detailed phase-connected analysis of CG X-1, identifying the nature of dips and proposing a model involving bow shock fragments as occulting clouds.

## Key findings

- CG X-1 has a 7.21 hr binary period and luminosity around 10^40 erg/s.
- Dipping patterns are variable and interpreted as occultations by gas clumps.
- The system likely involves a Wolf-Rayet donor close to Roche lobe filling.

## 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 ($L_{\rm X} \approx 10^{40}$ 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.