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Strong X-ray variability of black holes - a puzzle or natural phenomenon?

Researchers at the Max-Planck-Institute for Astrophysics propose that a strong X-ray variability of black hole candidates is related to the rapid accretion of matter in a hot quasi-spherical flow. This model naturally explains the link between the variability and spectral properties of these objects.


Figure 1: Two components in the X-ray energy spectrum of a black hole candidate Cygnus X-1.


Figure 2: X-ray light curve of Cygnus X-1, demonstrating strong variability of the hard X-ray flux.

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.


Figure 3:Expected power density spectra of X-ray flux variability for different states of the source. This plot shows how the amplitude of flux variations depends on the considered time scale T. It is conventionally plotted as function of frequency, which is 1/T.


Figure 4:Observed power density spectra in the different spectral states of Cygnus X-1

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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