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

  • Crowther P
  • Barnard R
  • Carpano S
 et al. 
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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 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 +/- 8kms-1, from which a mass function of 2.6 +/- 0.3Msolar is obtained. A revised spectroscopic mass for the WN-type companion of 26+7-5Msolar yields a black hole mass of 20 +/- 4Msolar 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+4-2.5Msolar (14.5+3-2.5Msolar) 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. Based on observations made with European Southern Observatory (ESO) telescopes at the Paranal Observatory under programme ID 384.D-0093(A). E-mail:

Author-supplied keywords

  • Galaxies: individual: NGC 300
  • Stars: wolf-rayet
  • X-rays: binaries
  • X-rays: individual: NGC 300 X-1

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  • P. A. Crowther

  • R. Barnard

  • S. Carpano

  • J. S. Clark

  • V. S. Dhillon

  • A. M.T. Pollock

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