2012A&A...545A.115K

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 α ``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 α 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 α_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 α_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 α is therefore real since there can be no doubt about the validity of the values deduced from observations of disc outbursts.