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Abstract
I review observational constraints on accretion disc viscosity inferred from changes of disc structure with time and from disc flickering distributions. The radial run of the disc viscosity parameter in four cases are presented and discussed.
Highlights
Accretion discs are cosmic devices where angular momentum and gravitational energy are extracted from matter by an anomalous, still unknown viscosity mechanism, allowing it to be accreted onto a central star (Frank, King & Raine 2002)
Baptista et al (2011) analyzed an extensive data set of optical light curves of HT Cas
Their observations frame a 2 d transition from a low state back to quiescence, allowing the application of both time-lapse and eclipse mapping techniques to estimate αss and to compare the independently derived results. They find that, in the low state, the gas stream hits the disc at the circularization radius rcirc, and the accretion disc has its smallest possible size
Summary
Accretion discs are cosmic devices where angular momentum and gravitational energy are extracted from matter by an anomalous, still unknown viscosity mechanism, allowing it to be accreted onto a central star (Frank, King & Raine 2002). In U Gem, flickering is stronger at orbital hump maximum and disappears during the eclipse of the bright spot (BS) – where the mass transfer stream hits the outer edge of its accretion disc. The flickering in HT Cas disappears during eclipse of the central source and recovers well before the egress of its BS (Patterson 1981), indicating that it originates in the inner disc regions or at the WD-disc boundary layer (BL) Possible explanations for this disc+BL flickering include MHD turbulence + convection (Geertsema & Achterberg 1992), unsteady WD accretion (Bruch 1992) or events of magnetic reconnection at the disc atmosphere (Kawagushi et al 2000). The values of n and fc vary from one object to the other and for the same object at different brightness levels (Fig. 1)
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