Ultraviolet observations of the X-ray photoionized wind of Cygnus X-1 during X-ray soft/high state

TL;DR

This study uses ultraviolet observations to analyze the X-ray photoionized wind of Cygnus X-1 during its soft/high state, revealing wind properties and their relation to X-ray illumination and accretion processes.

Contribution

It provides the first detailed modeling of the stellar wind and X-ray photoionization effects in Cygnus X-1 using HST/STIS data, estimating wind parameters and accretion-related luminosity.

Findings

Wind velocity increases with radius, with v_infinity ≈ 1420 km/s.

X-ray luminosity to wind mass-loss rate ratio is approximately 0.33.

Predicted X-ray luminosity matches RXTE observations but is lower than spherical wind accretion estimates.

Abstract

(Shortened) Ultraviolet observations of the black hole X-ray binary Cygnus X-1 were obtained using the STIS on HSTubble. We detect P Cygni line features show strong, broad absorption components when the X-ray source is behind the companion star and noticeably weaker absorption when the X-ray source is between us and the companion star. We fit the P Cygni profiles using the SEI method applied to a spherically symmetric stellar wind subject to X-ray photoionization from the black hole. The Si IV doublet provides the most reliable estimates of the parameters of the wind and X-ray illumination. The velocity $v$ increases with radius $r$ according to $v=v_\infty(1-r_\star/r)^\beta$, with$\beta\approx0.75$ and $v_\infty\approx1420$ km s$^{-1}$.The microturbulent velocity was $\approx160$ km s$^{-1}$. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate of L$_{X,38}/\dot M_{-6} \approx 0.33$, measured at $\dot M_{-6}$ = 4.8. Our models determine parameters that may be used to estimate the accretion rate onto the black hole and independently predict the X-ray luminosity. Our predicted L$_x$ matches that determined by contemporaneous RXTE ASM remarkably well, but is a factor of 3 lower than the rate according to Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the energy of accretion may go into powering a jet.