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Black holes are named 'black' because the light cannot overcome their
gravitational pull. Yet some of them are among the brightest X-ray
sources in the sky. What in reality shines is the flow of matter to
the black hole. In the inner region the flow becomes very hot, giving
rise to X-ray emission. Observations suggest that in fact two kinds of
flows may simultaneously be present around the black hole. One flow is
relatively cool and resembles a very thin disk, while the other is
much hotter and is almost quasi spherical. These two flows are
responsible for the two drastically different components in the
spectra of black hole candidates (Fig.1). The variability properties
of the two components are also different - the cooler component is
very stable, while the hotter component varies strongly over a broad
range of time scales (Fig.2). The nature of this variability remains
a puzzle.
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Researchers at the Max-Planck-Institute for Astrophysics suggested
that variability properties of these spectral components are caused by
the difference in the internal dynamics of the flows. Both flows are
believed to be turbulent with a similar characteristic turbulence time
scales -- comparable to the orbital period at a given radius. The
difference lays in the dynamics of the radial motion. The radial flow
of matter in the cool flow (thin disk) is extremely slow and it
effectively smears out and erases any perturbations introduced to it,
while the radial velocity in the hotter flow is orders of magnitude
faster. Such ``faster'' flow can advect perturbation, added to the
flow at large distance from the black hole, down to the innermost
region where most X-rays are emitted. This assumption then allows one
to probe the geometry of the flow at a range of distances around the black
hole, even in the regions where the flow is too cool to produce
observable emission in X-rays. This model naturally explains how the
characteristic amplitude of variations depends on the time scale of
interest and how changes in the geometry of the accretion flow affect
the observed variability (Fig.3 and Fig.4). Furthermore it predicts
that shorter perturbations, which are introduced to the flow at small
radii should come on top of longer perturbations produced at much
larger radii. This way the amplitude of shortest variations "knows"
about slower changes of the X-ray flux -- in excellent agreement with
observations.
E. Churazov, M. Gilfanov, M. Revnivtsev
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R E S E A R C H H I G H L I G H T S
