Highly Ionized Fe-K Absorption Line from Cygnus X-1 in the High/Soft State Observed with Suzaku
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 2013
- DOI:
- 10.1088/2041-8205/767/2/L35
- arXiv:
- arXiv:1304.1970
- Bibcode:
- 2013ApJ...767L..35Y
- Keywords:
-
- stars: individual: Cyg X-1;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 6 pages, 4 figures, ApJL accepted on February 28, 2013