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

Free access article

Issue		A&A Volume 461, Number 1, January I 2007


Page(s)		L9 - L12
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:20066527

A&A 461, L9-L12 (2007)
DOI: 10.1051/0004-6361:20066527

Letter

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

S. Carpano¹, A. M. T. Pollock¹, J. Wilms², M. Ehle¹, and M. Schirmer³

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 $0\farcs11$ $\pm$ $0\farcs45$ . 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 $\Gamma\sim2.45$ 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 $\times$ $10^{38}~{\rm erg~s}^{-1}$ .
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 $L_{0.2-10~{\rm keV}}\sim1$ $\times$ $10^{39}~{\rm erg~s}^{-1}$ , suggesting the presence of a black hole.

Key words: galaxies: individual: NGC 300 -- X-rays: binaries -- stars: Wolf-Rayet

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