Highly Ionized Fe-K Absorption Line from Cygnus X-1 in the High/Soft State Observed with Suzaku. (arXiv:1304.1970v1 [astro-ph.HE])

Highly Ionized Fe-K Absorption Line from Cygnus X-1 in the High/Soft State Observed with Suzaku. (arXiv:1304.1970v1 [astro-ph.HE]):
We present observations of a transient He-like Fe K alpha 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 and 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 ~10^(10-12) 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 the 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.