The viscosity parameter alpha and the properties of accretion disc outbursts in close binaries

Abstract

The physical mechanisms driving angular momentum transport in accretion discs are still unknown. Although it is generally accepted that, in hot discs, the turbulence triggered by the magneto-rotational instability is at the origin of the accretion process in Keplerian discs, it has been found that the values of the stress-to-pressure ratio (the alpha "viscosity" parameter) deduced from observations of outbursting discs are an order of magnitude higher than those obtained in numerical simulations. We test the conclusion about the observation-deduced value of alpha using a new set of data and comparing the results with model outbursts. We analyse a set of observations of dwarf-nova and AM CVn star outbursts and from the measured decay times determine the hot-disc viscosity parameter alpha_h. We determine if and how this method is model dependent. From the dwarf-nova disc instability model we determine an amplitude vs recurrence-time relation and compare it to the empirical Kukarkin-Parenago relation between the same, but observed, quantities. We found that all methods we tried, including the one based on the amplitude vs recurrence-time relation, imply alpha_h ~ 0.1 - 0.2 and exclude values an order of magnitude lower. The serious discrepancy between the observed and the MRI-calculated values of the accretion disc viscosity parameter alpha is therefore real since there can be no doubt about the validity of the values deduced from observations of disc outbursts.