Title: NGC 300 X-1 is a Wolf-Rayet/Black-Hole binary

P. A. Crowther (1), R. Barnard (2), S. Carpano (3), J. S. Clark (2), V. S. Dhillon (1), A. M. T. Pollock (3)

1) Sheffield UK; (2) Open Univ UK; (3) ESAC, Spain)

We present VLT/FORS2 time-series spectroscopy of the Wolf-Rayet star #41 in the Sculptor group galaxy NGC 300. We confirm a physical association with NGC 300 X-1, since radial velocity variations of the HeII 4686 line indicate an orbital period of 32.3 +/- 0.2 hr which agrees at the 2 sigma level with the X-ray period from Carpano et al. We measure a radial velocity semi-amplitude of 267 +/- 8 km/s, from which a mass function of 2.6 +/- 0.3 Msun is obtained. A revised spectroscopic mass for the WN-type companion of 26+7-5 Msun yields a black hole mass of 20 +/- 4 Msun for a preferred inclination of 60-75 deg. If the WR star provides half of the measured visual continuum flux, a reduced WR (black hole) mass of 15 +4 -2.5 Msun (14.5 +3 -2.5 Msun) would be inferred. As such, #41/NGC 300 X-1 represents only the second extragalactic Wolf-Rayet plus black-hole binary system, after IC 10 X-1. In addition, the compact object responsible for NGC 300 X-1 is the second highest stellar-mass black hole known to date, exceeded only by IC 10 X-1.

Reference: MNRAS Letters in press
Status: Manuscript has been accepted

Weblink: http://lanl.arxiv.org/abs/1001.4616

Comments: Images and Simulations available from ESO: http://www.eso.org/public/news/eso1004/

Email: Paul.Crowther@sheffield.ac.uk