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

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 0.11 arcsec ± 0.45 arcsec. 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 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 × 10³⁸~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 × 10³⁹~erg~s^-1, suggesting the presence of a black hole.