Stellar Wind Variations During the X-ray High and Low States of Cygnus X-1

TL;DR

This study uses Hubble UV spectroscopy to analyze how X-ray states in Cyg X-1 affect stellar wind ionization and mass transfer, revealing that X-ray photoionization significantly alters wind structure and accretion processes.

Contribution

It provides the first detailed model of wind ionization effects during different X-ray states, highlighting the role of shadowed wind regions in accretion variability.

Findings

Wind ionization extends near the X-ray hemisphere of the supergiant.

Mass transfer is dominated by a focused wind flow along the star-black hole axis.

X-ray photoionization suppresses the stellar wind contribution, influencing accretion rates.

Abstract

We present results from Hubble Space Telescope UV spectroscopy of the massive X-ray binary system, HD226868 = Cyg X-1. The spectra were obtained at both orbital conjunction phases in two separate runs in 2002 and 2003 when the system was in the X-ray high/soft state. The stellar wind lines suffer large reductions in strength when the black hole is in the foreground due to the X-ray ionization of the wind ions. A comparison of HST and archival IUE spectra shows that similar photoionization effects occur in both the X-ray states. We constructed model UV wind line profiles assuming that X-ray ionization occurs everywhere in the wind except the zone where the supergiant blocks the X-ray flux. The good match between the observed and model profiles indicates that the wind ionization extends to near to the hemisphere of the supergiant facing the X-ray source. The H-alpha emission strength is generally lower in the high/soft state compared to the low/hard state, but the He II 4686 emission is relatively constant between states. The results suggest that mass transfer in Cyg X-1 is dominated by a focused wind flow that peaks along the axis joining the stars and that the stellar wind contribution is shut down by X-ray photoionization effects. The strong stellar wind from the shadowed side of the supergiant will stall when Coriolis deflection brings the gas into the region of X-ray illumination. This stalled gas component may be overtaken by the orbital motion of the black hole and act to inhibit accretion from the focused wind. The variations in the strength of the shadow wind component may then lead to accretion rate changes that ultimately determine the X-ray state.