2010MNRAS.403L..41C

We present Very Large Telescope/FORS2 time-series spectroscopy of the Wolf-Rayet (WR) star {num}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 λ4686 line indicate an orbital period of 32.3±0.2h which agrees at the 2σ level with the X-ray period from Carpano et al. We measure a radial velocity semi-amplitude of 267±8km/s, from which a mass function of 2.6±0.3M_☉ is obtained. A revised spectroscopic mass for the WN-type companion of 26⁺⁷_–5M_☉ yields a black hole mass of 20±4M_☉ for a preferred inclination of 60°-75°. If the WR star provides half of the measured visual continuum flux, a reduced WR (black hole) mass of 15⁺⁴_–2.5M_☉(14.5⁺³_–2.5M_☉) would be inferred. As such, {num}41/NGC 300 X-1 represents only the second extragalactic WR 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.