HIGHLY IONIZED Fe-K ABSORPTION LINE FROM CYGNUS X-1 IN THE HIGH/SOFT STATE OBSERVED WITH <i>SUZAKU</i>

Abstract

We present observations of a transient He-like Fe Kα absorption line in Suzaku observations of the black hole&#13;\nbinary Cygnus X-1 on 2011 October 5 near superior conjunction during the high/soft state, which enable us to&#13;\nmap the full evolution from the start to the end of the episodic accretion phenomena or dips for the first time. We&#13;\nmodel the X-ray spectra during the event and trace their evolution. The absorption line is rather weak in the first&#13;\nhalf of the observation, but instantly deepens for ∼10 ks, and weakens thereafter. The overall change in equivalent&#13;\nwidth is a factor of ∼3, peaking at an orbital phase of ∼0.08. This is evidence that the companion stellar wind&#13;\nfeeding the black hole is clumpy. By analyzing the line with a Voigt profile, it is found to be consistent with a&#13;\nslightly redshifted Fe xxv transition, or possibly a mixture of several species less ionized than Fe xxv. The data&#13;\nmay be explained by a clump located at a distance of ∼1010–12 cm with a density of ∼10(−13)–(−11) g cm−3, which&#13;\naccretes onto and/or transits the line of sight to the black hole, causing an instant decrease in the observed degree&#13;\nof ionization and/or an increase in density of the accreting matter. Continued monitoring for individual events with&#13;\nfuture X-ray calorimeter missions such as ASTRO-H and AXSIO will allow us to map out the accretion environment&#13;\nin detail and how it changes between the various accretion states.