Letter to the Editor

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

¹ XMM-Newton Science Operations Centre, ESAC, European Space Agency, Apartado 50727, 28080 Madrid, Spain e-mail: scarpano@sciops.esa.int
² Dr. Remeis Sternwarte, Astronomisches Institut der FAU Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
³ Isaac Newton Group of Telescopes, 38700 Santa Cruz de La Palma, Spain

Received: 9 October 2006
Accepted: 6 November 2006

Abstract

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  ± . The X-ray light curves show irregular variability. During one XMM-Newton observation, the flux increased by about a factor of ten in 10 h. The spectrum can be modelled by a power-law with 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  .

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   , suggesting the presence of a black hole.