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We present new X-ray and UV observations of the Wolf-Rayet + black holebinary system NGC 300 X-1 with the Chandra X-ray Observatory and the HubbleSpace Telescope Cosmic Origins Spectrograph. When combined with archival X-rayobservations, our X-ray and UV observations sample the entire binary orbit,providing clues to the system geometry and interaction between the black holeaccretion disk and the donor star wind. We measure a binary orbital period of32.7921$\pm$0.0003 hr, in agreement with previous studies, and performphase-resolved spectroscopy using the X-ray data. The X-ray light curve revealsa deep eclipse, consistent with inclination angles of $i=60-75^{\circ}$, and apre-eclipse excess consistent with an accretion stream impacting the disk edge.We further measure radial velocity variations for several prominent FUVspectral lines, most notably He II $\lambda$1640 and C IV $\lambda$1550. Wefind that the He II emission lines systematically lag the expected Wolf-Rayetstar orbital motion by a phase difference $\Delta \phi\sim0.3$, while C IV$\lambda$1550 matches the phase of the anticipated radial velocity curve of theWolf-Rayet donor. We assume the C IV $\lambda$1550 emission line follows asinusoidal radial velocity curve (semi-amplitude = 250 km s$^{-1}$) and infer aBH mass of 17$\pm$4 M$_{\odot}$. Our observations are consistent with thepresence of a wind-Roche lobe overflow accretion disk, where an accretionstream forms from gravitationally focused wind material and impacts the edge ofthe black hole accretion disk.

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