The First Candidate Colliding-wind Binary in M33

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 Chandra imaging as a relatively soft X-ray point source, with the likely primary star determined from precise astrometric alignment between archival Hubble Space Telescope and 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 Chandra and XMM-Newton observations, and find the CWB to be well fit as a ∼1 keV thermal plasma with a median unabsorbed luminosity in the 0.5-2.0 keV band of L _X ∼ 3 × 10³⁵ 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 Chandra and 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 η Car.