Evidence for colliding winds in WR 25 from XMM-Newton observations of X-ray variability

The Wolf-Rayet star WR 25 in the Carina Nebula is a surprisingly bright X-ray source, and amongst the brightest WR stars in X-rays. It is a suspected binary star, though its binary nature has been a matter of controversy. We report here observations of WR 25 from the XMM-Newton archive which showed an increase in X-ray luminosity of more than a factor of two. The X-ray absorption also increased. Such a large change in X-ray output is unknown in single massive stars, though large changes in X-ray brightness are seen in binaries. The most likely explanation of the observed X-ray variability is as a result of colliding-wind emission in a moderately eccentric binary with a period of about 4 years. The next periastron passage is expected in 2007. The X-ray spectrum is similar to that of the archetype colliding-wind binary WR 140.