Energy-Dependent Orbital Phases in NGC 300 X-1

TL;DR

This study investigates the X-ray variability and spectral properties of NGC 300 X-1, a Wolf-Rayet and black hole binary, revealing insights into accretion processes, wind interactions, and challenges in mass estimation.

Contribution

The paper provides new Chandra and HST observations, analyzes spectral variability, and discusses the impact of stellar winds on X-ray emission and black hole mass measurements.

Findings

X-ray variability inconsistent with star occultation, linked to accretion disk or wind structure.

Spectral modeling shows changes in partial covering fraction and absorption during eclipse phases.

Reanalysis suggests wind ionization affects radial velocity measurements, questioning black hole mass estimates.

Abstract

NGC~300 X-1 is a Wolf Rayet + 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 (totaling $\sim$130 ks) along with ACS imaging data from the Hubble Space Telescope. We observe significant short-term variability in the X-ray emission that is inconsistent with an occultation by the donor star, but is consistent with structure in the outer accretion disk or the wind of the donor star. We simultaneously fit a partially-covered disk blackbody and Comptonized corona model to the eclipse egress and non-eclipsing portions of the X-ray spectrum. We find that the only model parameters that varied between the eclipse egress and non-eclipsing portions of the spectra were the partial covering fraction ($\sim$86% during eclipse egress and $\sim$44% during non-eclipse) and absorbing column ($\sim$12.3$\times10^{22}$ cm$^{-2}$ during eclipse egress, compared to $\sim$1.4$\times10^{22}$ 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 AGB 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.