# The First Candidate Colliding-Wind Binary in M33

## TL;DR

This paper reports the discovery and analysis of the first candidate colliding-wind binary in M33, characterized by X-ray and optical observations, revealing a bright, thermal plasma source with a Wolf-Rayet star and an O star companion.

## Contribution

It is the first identification and detailed multi-wavelength characterization of a colliding-wind binary in M33, including spectral modeling and long-term light curve analysis.

## Key findings

- The candidate CWB is among the brightest observed, with an X-ray luminosity of ~3 x 10^{35} erg/s.
- Spectroscopy confirms the primary as a carbon-rich Wolf-Rayet star with an O star companion.
- X-ray spectrum fits a ~1 keV thermal plasma model.

## Abstract

We present the detection of the first candidate colliding-wind binary (CWB) in M33, located in the giant H II region NGC 604. The source was first identified in archival {\it Chandra} imaging as a relatively soft X-ray point source, with the likely primary star determined from precise astrometric alignment between archival {\it Hubble Space Telescope} and {\it Chandra} imaging. The candidate primary star in the CWB is classified for the first time in this work as a carbon-rich Wolf-Rayet star with a likely O star companion based on spectroscopy obtained from Gemini-North. We model the X-ray spectrum using {\it Chandra} and {\it XMM-Newton} observations, and find the CWB is well-fit as a $\sim$ 1 keV thermal plasma with a median unabsorbed luminosity in the 0.5--2.0 keV band of $L_{\rm X}$ $\sim$ 3 $\times$ 10$^{35}$ erg s$^{-1}$, making this source among the brightest of CWBs observed to date. We present a long term light curve for the candidate CWB from archival {\it Chandra} and {\it XMM-Newton} observations, and discuss the constraints placed on the binary by this light curve, as well as the X-ray luminosity at maximum. Finally, we compare this candidate CWB in M33 to other well-studied, bright CWBs in the Galaxy and Magellanic Clouds, such as $\eta$ Car.