Constraints on active galactic nucleus accretion disc viscosity derived from continuum variability

We estimate a value of the viscosity parameter in active galactic nucleus (AGN) accretion discs for the Palomar-Green quasar sample. We assume that optical variability on time-scales of months to years is caused by local instabilities in the inner accretion disc. Comparing the observed variability time-scales to the thermal time-scales of alpha-disc models, we obtain constraints on the viscosity parameter for the sample. We find that, at a given L/L_Edd, the entire sample is consistent with a single value of the viscosity parameter, alpha. We obtain constraints of 0.01 <=alpha<= 0.03 for 0.01 <=L/L_Edd<= 1.0. This narrow range suggests that these AGNs are all seen in a single state, with a correspondingly narrow spread of black hole masses or accretion rates. The value of alpha we derive is consistent with predictions by current simulations in which magnetohydrodynamic (MHD) turbulence is the primary viscosity mechanism.