Energy-dependent orbital phases in NGC 300 X-1

NGC 300 X-1 is a Wolf-Rayet (WR)+black hole binary that exhibits periodic decreases in X-ray flux. We present two new observations of NGC 300 X-1 from the Chandra X-ray Observatory (totalling ∼130 ks) along with Advanced Camera for Surveys imaging data from the Hubble Space Telescope (HST). We observe significant short-term variability in the X-ray emission that is inconsistent with an occultation by the donor star, but is consistent with structure in the outer accretion disc or the wind of the donor star. We simultaneously fit a partially covered disc blackbody and Comptonized corona model to the eclipse egress and non-eclipsing portions of the X-ray spectrum. We find that the only model parameters that varied between the eclipse egress and non-eclipsing portions of the spectra were the partial covering fraction (∼86 per cent during eclipse egress and ∼44 per cent during non-eclipse) and absorbing column (∼12.3 × 10²² cm^-2 during eclipse egress, compared to ∼1.4 × 10²² cm^-2 during non-eclipse). The X-ray spectra are consistent with the movement of the X-ray source through the dense stellar winds of the companion star. From our new HST imaging, we find the WR star within the X-ray error circle, along with additional optical sources including an asymptotic giant branch star and an early-type main-sequence star. Finally, we use our egress measurement to rephase previous radial velocity measurements reported in the literature, and find evidence that the velocities are strongly affected by the ionization of the wind by the compact object. Thus, we argue the inferred mass of the black hole may not be reliable.