A Survey of X-ray Emission from Wolf-Rayet Stars

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It is well-known that massive WR + OB binaries are bright X-ray sources with typical luminosities log Lx 33 ergs/s. Some of their high-temperature X-ray emission likely originates in colliding wind shocks between the binary components, but intrinsic emission from the stars themselves may also contribute. However, much less is known about the X-ray emission of single WR stars. We present results of a pilot X-ray survey of single WR stars with Chandra and XMM-Newton aimed at defining their X-ray properties. None of our targets is so far known to be a binary system. We have detected X-rays from nitrogen-rich WN stars spanning all subtypes (WN2 - WN9). Typical luminosities are log Lx = 32.5 (+-1.0) ergs/s and their spectra invariably show high-temperature plasma (kT > 2 keV). Such hot plasma is not predicted by radiative wind shock models, implying that other as yet unidentified X-ray production mechanisms are at work. We also report a recent Chandra detection of the rare oxygen-type WO star WR 142 in the young stellar cluster Berkeley 87. Its ACIS spectrum is faint and very hard and can be fitted by either a nonthermal (power-law) model or an ultra-hot thermal plasma. Carbon-rich WC stars have so far not been detected in X-rays, but the observed sample is small. Their dense metal-enriched winds likely act as strong absorbers of any X-ray emission produced near the star.

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