Highly Ionized Fe-K Absorption Line from Cygnus X-1 in the High/Soft State Observed with Suzaku

TL;DR

This study reports on the detection and analysis of a transient Fe K alpha absorption line in Cygnus X-1, revealing clumpy stellar wind accretion dynamics during the high/soft state with implications for understanding black hole accretion environments.

Contribution

First detailed spectral mapping of a transient Fe K alpha absorption event in Cygnus X-1, linking wind clumpiness to accretion variability.

Findings

Absorption line depth varies with orbital phase, indicating clumpy stellar wind.

Line consistent with slightly redshifted Fe XXV, suggesting complex ionization states.

Clump properties estimated at ~10^10-12 cm distance with specific density range.

Abstract

We present observations of a transient He-like Fe K alpha absorption line in Suzaku observations of the black hole binary Cygnus X-1 on 2011 October 5 near superior conjunction during the high/soft state, which enable us to map the full evolution from the start and the end of the episodic accretion phenomena or dips for the first time. We model the X-ray spectra during the event and trace their evolution. The absorption line is rather weak in the first half of the observation, but instantly deepens for ~10 ks, and weakens thereafter. The overall change in equivalent width is a factor of ~3, peaking at an orbital phase of ~0.08. This is evidence that the companion stellar wind feeding the black hole is clumpy. By analyzing the line with a Voigt profile, it is found to be consistent with a slightly redshifted Fe XXV transition, or possibly a mixture of several species less ionized than Fe XXV. The data may be explained by a clump located at a distance of ~10^(10-12) cm with a density of ~10^((-13)-(-11)) g cm^-3, which accretes onto and/or transits the line-of-sight to the black hole, causing an instant decrease in the observed degree of the ionization and/or an increase in density of the accreting matter. Continued monitoring for individual events with future X-ray calorimeter missions such as ASTRO-H and AXSIO will allow us to map out the accretion environment in detail and how it changes between the various accretion states.