NGC 300 X-1 is a Wolf-Rayet/black hole binary

Abstract

We present Very Large Telescope/FORS2 time-series spectroscopy of the Wolf-Rayet (WR) 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 He II λ4686 line indicate an orbital period of 32.3 ± 0.2 h which agrees at the 2σ level with the X-ray period from Carpano et al. We measure a radial velocity semi-amplitude of 267 ± 8 kms-1, from which a mass function of 2.6 ± 0.3 M⊙ is obtained. A revised spectroscopic mass for the WN-type companion of 26+7-5 M⊙ 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+4-2.5 M⊙ (14.5+3-2.5 M.) would be inferred. As such, #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. © 2010 The Authors. Journal compilation © 2010 RAS.