The Einstein view of the Wolf-Rayet stars

A uniform analysis of all 48 Wolf-Rayet stars observed with the IPC of the Einstein Observatory shows that their X-ray luminosities cover a range of more than two orders of magnitude. Most of the brightest stars are either also sources of nonthermal radio radiation, in which case the measurements interpreted as Compton scattering of photospheric radiation by relativistic electrons imply surface magnetic fields of up to a few hundred Gauss, or massive binary systems where the X-rays could come from colliding winds. The single stars and many of the stars that have been proposed as low-mass binaries are geneally among the faintest objects and probably emit a lower proportion of their bolometric luminosities at X-ray frequencies than OB stars of similar absolute magnitude.