2024ApJ...961..174S

θ Mus is a remarkable spectroscopic binary (SB) consisting of a carbon-type Wolf-Rayet star and OV companion (WC6+O6-7V) in a ≈19 days orbit. In addition an O-supergiant is visually detected at a small offset of 46 mas and if gravitationally bound to the SB system would have an orbital period of many decades. θ Mus is X-ray bright and a nonthermal radio source as commonly observed in massive colliding wind (CW) binaries. We present new Chandra X-ray observations of θ Mus, which complement previous XMM-Newton observations. The X-ray emission consists of a cool, nearly steady, weakly absorbed plasma component with broad redshifted emission lines located in an extended region far from the SB system. Hotter plasma is also present traced by Fe XXV emission. The observed flux in the ≈2-5 keV range dropped significantly on a timescale of ≤5 yr. The flux decrease can be attributed to an increase in absorption toward the hotter plasma which is likely located in the confined wind interaction region of the short-period SB system. The X-ray emission of θ Mus is remarkably similar to the WC+O binary γ² Vel including carbon recombination spectral lines but both systems show unusual line centroid properties that challenge CW models.