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

¹ Astronomical Observatory, Jagellonian University, ul. Orla 171, 30-244 Kraków, Poland
e-mail: Iwona.Kotko@uj.edu.pl
² Institut d’Astrophysique de Paris, UMR 7095 CNRS, UPMC Université Paris 6, 98bis Bd Arago, 75014 Paris, France

Received: 16 May 2012
Accepted: 6 August 2012

Abstract

Context. 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.

Aims. We test the conclusion about the observation-deduced value of α using a new set of data and comparing the results with model outbursts.

Methods. 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.

Results. 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.

Conclusions. 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.