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

ABSTRACT We present VLT/FORS2 time-series spectroscopy of the Wolf-Rayet star #41 in the Sculptorgroup galaxy NGC300. We confirm a physical association with N GC300 X–1, since radialvelocity variations of the He II λ4686 line indicate an orbital period of 32.3 ± 0.2 hr whichagrees at the 2σ level with the X-ray period from Carpano et al. We measure a radial velocitysemi-amplitudeof 267±8kms −1 , fromwhicha mass functionof 2.6±0.3 M ⊙ is obtained.Arevised spectroscopic mass for the WN-type companionof 26 +7−5 M ⊙ yields a black hole massof 20 ±4 M ⊙ for a preferred inclination of 60−75 ◦ . If the WR star provides half of the mea-sured 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/NGC300 X–1 represents only the second extragalactic Wolf-Rayet plus black-hole binary system, after IC10 X–1. In addition, the compact object respon-sible for NGC300 X–1 is the second highest stellar-mass black hole known to date, exceededonly by IC10 X–1.Key words: (galaxies:)individual:NGC 300– Stars: Wolf-Rayet – X-rays: binaries – X-rays:individual: NGC300 X–1