XMM‐NewtonDetection of Hard X‐Ray Emission in the Nitrogen‐Type Wolf‐Rayet Star WR 110

We have used the excellent sensitivity of XMM-Newton to obtain the firsthigh-quality X-ray spectrum of a Wolf-Rayet (WR) star which is not known to bea member of a binary system. Our target, the nitrogen-type star WR 110 (= HD165688) was also observed and detected with the VLA at four differentfrequencies. The radio data are in excellent agreement with that expected forfree-free wind emission. and the ionized mass-loss rate is derived. The X-rayemission measure distribution shows a dominant contribution from cool plasma atkT$_{cool}$ = 0.5 keV (6 MK) which is only weakly absorbed. We argue that thiscool emission originates at hundreds of radii if the wind is spherical andhomogeneous and derive shock velocities and the X-ray filling factor usingradiative shock models. A surprising result is the unambiguous detection of ahard X-ray component clearly seen in the hard-band images and the spectra. Thishard component accounts for about half of the observed flux and can beacceptably fitted by a hot optically thin thermal plasma or a power-law model.If the emission is thermal, then a temperature kT$_{hot}$ $\geq$ 3 keV isderived. Such high temperatures are not predicted by current instability-drivenwind shock models. We examine several alternatives and show that the hardemission could be accounted for by the WR wind shocking onto a close stellarcompanion which has so far escaped detection. However, until persuasiveevidence for binarity is found we are left with the intriguing possibility thatthe hard X-ray emission is produced entirely by the Wolf-Rayet star.Comment: 2 tables, 7 figure