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

S. Yamada, S. Torii, S. Mineshige, Y. Ueda, Aya Kubota, P. Gandhi, C. Done, H. Noda, A. Yoshikawa, K. Makishima

Research output: Contribution to journalArticle

6 Citations (Scopus)

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.

Original languageEnglish
Article numberL35
JournalAstrophysical Journal Letters
Volume767
Issue number2
DOIs
Publication statusPublished - 2013 Apr 20

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accretion
conjunction
stellar winds
clumps
transit
line of sight
calorimeters
dip
ionization
x rays
orbitals
monitoring
profiles

Keywords

  • stars: individual (Cyg X-1)
  • X-rays: binaries

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Highly ionized Fe-K absorption line from Cygnus X-1 in the high/soft state observed with Suzaku. / Yamada, S.; Torii, S.; Mineshige, S.; Ueda, Y.; Kubota, Aya; Gandhi, P.; Done, C.; Noda, H.; Yoshikawa, A.; Makishima, K.

In: Astrophysical Journal Letters, Vol. 767, No. 2, L35, 20.04.2013.

Research output: Contribution to journalArticle

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', Astrophysical Journal Letters, vol. 767, no. 2, L35. https://doi.org/10.1088/2041-8205/767/2/L35
Yamada, S. ; Torii, S. ; Mineshige, S. ; Ueda, Y. ; Kubota, Aya ; Gandhi, P. ; Done, C. ; Noda, H. ; Yoshikawa, A. ; Makishima, K. / Highly ionized Fe-K absorption line from Cygnus X-1 in the high/soft state observed with Suzaku. In: Astrophysical Journal Letters. 2013 ; Vol. 767, No. 2.
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N2 - 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.

AB - 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.

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