NGC 300 X-1 is a Wolf-Rayet/Black-Hole binary

TL;DR

This paper confirms NGC 300 X-1 as a Wolf-Rayet/black-hole binary system in NGC 300, providing detailed measurements of the orbital parameters and black hole mass, making it only the second such extragalactic system known.

Contribution

First detailed spectroscopic analysis confirming the Wolf-Rayet/black-hole binary nature of NGC 300 X-1 and measuring its black hole mass.

Findings

Orbital period of 32.3 hours confirmed.

Black hole mass estimated at 20 +/- 4 solar masses.

Second highest stellar-mass black hole known.

Abstract

We present VLT/FORS2 time-series spectroscopy of the Wolf-Rayet 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 4686 line indicate an orbital period of 32.3 +/- 0.2 hr which agrees at the 2 sigma level with the X-ray period from Carpano et al. We measure a radial velocity semi-amplitude of 267 +/- 8 km/s, from which a mass function of 2.6 +/- 0.3 Msun is obtained. A revised spectroscopic mass for the WN-type companion of 26+7-5 Msun yields a black hole mass of 20 +/- 4 Msun for a preferred inclination of 60-75 deg. If the WR star provides half of the measured visual continuum flux, a reduced WR (black hole) mass of 15 +4 -2.5 Msun (14.5 +3 -2.5 Msun) would be inferred. As such, #41/NGC 300 X-1 represents only the second extragalactic Wolf-Rayet 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.