Probing the Stellar Wind of the Wolf-Rayet Star in IC 10 X-1

TL;DR

This study analyzes the stellar wind of the Wolf-Rayet star in IC 10 X-1 using spectral data, revealing wind structures influenced by the black hole's radiation and accretion flow, with implications for understanding gravitational wave progenitors.

Contribution

It provides new insights into the wind dynamics of the WR star in IC 10 X-1 and compares different models explaining spectral line variations in this binary system.

Findings

He II line profile correlates with X-ray hardness ratio.

Skewness of the line profile varies with orbital phase.

Spectral analysis supports wind interaction models.

Abstract

IC 10 X-1 is an eclipsing high mass X-ray binary (HMXB) containing a stellar-mass black hole (BH) and a Wolf-Rayet (WR) donor star with an orbital period of P = 34.9 hr. This binary belongs to a group of systems that can be the progenitors of gravitational wave sources, hence understanding the dynamics of systems such as IC 10 X-1 is of paramount importance. The prominent He II 4686 emission line (previously used in mass estimates of the BH) is out of phase with the X-ray eclipse, suggesting that this line originates somewhere in the ionized wind of the WR star or in the accretion disk. We obtained 52 spectra from the GEMINI/GMOS archive, observed between 2001 and 2019. We analyzed the spectra both individually, and after binning them by orbital phase to improve the signal-to-noise ratio. The RV curve from the stacked data is similar to historical results, indicating the overall parameters of the binary have remained constant. However, the He II line profile shows a correlation with the X-ray hardness-ratio values, also, we report a pronounced skewness of the line-profile, and the skewness varies with the orbital phase. These results support a paradigm wherein the He II line tracks structures in the stellar wind that are produced by interactions with the BH's ionizing radiation and the accretion flow. We compare the observable signatures of two alternative hypotheses proposed in the literature: wind irradiation plus shadowing, and accretion disk hotspot; and we explore how the line-profile variations fit into each of these models.