The Wolf–Rayet + Black Hole Binary NGC 300 X-1: What is the Mass of the Black Hole?

• Published in: The Astrophysical journal Vol. 910; no. 1; p. 74
• Main Authors: Binder, Breanna A., Sy, Janelle M., Eracleous, Michael, Christodoulou, Dimitris M., Bhattacharya, Sayantan, Cappallo, Rigel, Laycock, Silas, Plucinsky, Paul P., Williams, Benjamin F.
• Format: Journal Article
• Language: English
• Published: Philadelphia IOP Publishing 01-03-2021
• Subjects: Accretion disks; Astrophysics; Binary stars; Black holes; Emission lines; Emissions; Hubble Space Telescope; Inclination angle; Light curve; Line spectra; Orbits; Overflow; Radial velocity; Space telescopes; Spectroscopy; Velocity curve; Wind; X ray telescopes
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/abe6a9

Abstract
We present new X-ray and UV observations of the Wolf–Rayet + black hole (BH) binary system NGC 300 X-1 with the Chandra X-ray Observatory and the Hubble Space Telescope Cosmic Origins Spectrograph. When combined with archival X-ray observations, our X-ray and UV observations sample the entire binary orbit, providing clues to the system geometry and interaction between the BH accretion disk and the donor star wind. We measure a binary orbital period of 32.7921 ± 0.0003 hr, in agreement with previous studies, and perform phase-resolved spectroscopy using the X-ray data. The X-ray light curve reveals a deep eclipse, consistent with inclination angles of
i
= 60°–75°, and a pre-eclipse excess consistent with an accretion stream impacting the disk edge. We further measure radial velocity variations for several prominent far-UV spectral lines, most notably H
ii
λ
1640 and C
iv
λ
1550. We find that the He
ii
emission lines systematically lag the expected Wolf–Rayet star orbital motion by a phase difference of Δ
ϕ
∼ 0.3, while C
iv
λ
1550 matches the phase of the anticipated radial velocity curve of the Wolf–Rayet donor. We assume the C
iv
λ
1550 emission line follows a sinusoidal radial velocity curve (semi-amplitude = 250 km s
−1
) and infer a BH mass of 17 ± 4
M
⊙
. Our observations are consistent with the presence of a wind-Roche lobe overflow accretion disk, where an accretion stream forms from gravitationally focused wind material and impacts the edge of the BH accretion disk.