A Wolf - Rayet / black - hole X - ray binary candidate in NGC 300

Context. Wolf Rayet/black hole binaries are believed to exist as a later evolutionary product of high-mass X-ray binaries. Hundreds of such binaries may exist in galaxies, but only a few of them are close enough to be observed as X-ray binaries. Only a couple of candidates have been reported so far. Aims. Based on XMM-Newton observations, we report the positional coincidence of the brightest X-ray source in NGC 300 (NGC 300 X-1) with a Wolf-Rayet candidate. Temporal and spectral analysis of the X-ray source is performed. Methods. We determine an accurate X-ray position of the object, and derive light curves, spectra and flux in four XMM-Newton observations. Results. The positions of the X-ray source and the helium star candidate coincide within 0. ′′ 11 ± 0. ′′ 45. The X-ray light curves show irregular variability. During one XMM-Newton observation, the flux increased by about a factor of ten in 10 hours. The spectrum can be modelled by a power-law with Γ ∼ 2.45 with additional relatively weak line emission, notably around 0.95 keV. The mean observed (absorbed) luminosity in the 0.2–10 keV band is ∼ 2 × 10 38 erg s −1. Conclusions. NGC 300 X-1 is a good candidate for a Wolf-Rayet/black-hole X-ray binary: its position coincides with a Wolf-Rayet candidate and the unabsorbed X-ray luminosity reached L 0.2−10 keV ∼ 1 × 10 39 erg s −1 , suggesting the presence of a black hole.