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Durham University

Department of Physics

Staff profile

Publication details for Prof Chris Done

Yamada, S., Torii, S., Mineshige, S., Ueda, Y., Kubota, A., Gandhi, P., Done, C., Noda, H., Yoshikawa, A. & Makishima, K. (2013). Highly Ionized Fe-K Absorption Line from Cygnus X-1 in the High/Soft State Observed with Suzaku. The Astrophysical Journal Letters 767(2): L35.

Author(s) from Durham

Abstract

We present observations of a transient He-like Fe Kα 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 to 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 ∼1010–12 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 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.