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Highly ionized Fe-K absorption line from Cygnus X-1 in the high/soft state observed with Suzaku.

Yamada, S. and Torii, S. and Mineshige, S. and Ueda, Y. and Kubota, A. and Gandhi, P. and Done, C. and Noda, H. and Yoshikawa, A. and Makishima, K. (2013) 'Highly ionized Fe-K absorption line from Cygnus X-1 in the high/soft state observed with Suzaku.', Astrophysical journal letters., 767 (2). L35.

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.

Item Type:Article
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1088/2041-8205/767/2/L35
Publisher statement:© 2013. The American Astronomical Society. All rights reserved.
Date accepted:23 February 2013
Date deposited:20 March 2018
Date of first online publication:08 April 2013
Date first made open access:No date available

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