On the physical nature of accretion disc viscosity

Understand

We use well-established observational evidence to draw conclusions about the fundamental nature of the viscosity in accretion discs.

• To do this, we first summarise the observational evidence for the value of the dimensionless accretion disc viscosity parameter $\alpha$, defined by Shakura & Sunyaev (1973, 1976).
• We find that, for fully ionized discs, the value of $\alpha$ is readily amenable to reliable estimation and that the observations are consistent with the hypothesis that $\alpha \sim 0.2 - 0.3$.
• In contrast in discs that are not fully ionized, estimates of the value of $\alpha$ are generally less direct and the values obtained are generally $ < 0.01$ and often $ \ll 0.01$.