Clumped stellar winds in supergiant high-mass X-ray binaries: X-ray variability and photoionization
L.M. Oskinova1, A. Feldmeier1, P. Kretschmar2
1 Institute of Physics and Astronomy, University of Potsdam, Germany
2 European Space Astronomy Centre (ESA/ESAC), Villanueva de la Canada, Madrid, Spain
The clumping of massive star winds is an established paradigm, which is
confirmed by multiple lines of evidence and is supported by stellar
wind theory. The purpose of this paper is to bridge the gap between
detailed models of inhomogeneous stellar winds in single stars and the
phenomenological description of donor winds in supergiant high-mass
X-ray binaries (HMXBs). We use the results from time-dependent
hydrodynamical models of the instability in the line-driven wind of a
massive supergiant star to derive the time-dependent accretion rate on
to a compact object in the Bondi-Hoyle-Lyttleton approximation. The
strong density and velocity fluctuations in the wind result in strong
variability of the synthetic X-ray light curves. The model predicts a
large-scale X-ray variability, up to eight orders of magnitude, on
relatively short time-scales. The apparent lack of evidence for such
strong variability in the observed HMXBs indicates that the details of
the accretion process act to reduce the variability resulting from the
stellar wind velocity and density jumps. We study the absorption
of X-rays in the clumped stellar wind by means of a two-dimensional
stochastic wind model. The monochromatic absorption in the cool stellar
wind, depending on the orbital phase, is computed for realistic stellar
wind opacity. We find that the absorption of X-rays changes strongly at
different orbital phases. The degree of the variability resulting from
the absorption in the wind depends on the shape of the wind clumps,
and this is stronger for oblate clumps. We address the photoionization
in the clumped wind, and we show that the degree of ionization is
affected by the wind clumping. We derive a correction factor for the
photoionization parameter, and we show that the photoionization
parameter is reduced by a factor Χ compared to the smooth wind models
with the same mass-loss rate, where Χ is the wind inhomogeneity
parameter. We conclude that wind clumping must also be taken into
account when comparing the observed and model spectra of the
photoionized stellar wind.
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