Crowther, P. A.; Barnard, R.; Carpano, S.; Clark, J. S.; Dhillon, V. S. and Pollock, A. M. T
NGC300 X-1 is a Wolf-Rayet/Black-Hole binary.
Monthly Notices of the Royal Astronomical Society, 403(1) L41-L45.
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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?4686 line indicate an orbital period of 32.3 ± 0.2?h which agrees at the 2? level with the X-ray period from Carpano et al. We measure a radial velocity semi-amplitude of 267 ± 8?km?s?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 ± 4?M? 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.
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