Constraints on AGN accretion disc viscosity derived from continuum variability

We estimate a value of the viscosity parameter in AGN accretion discs for the PG 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 α-disc models we obtain constraints on the viscosity parameter for the sample. We find that, at a given L/LEdd, the entire sample is consistent with a single value of the viscosity parameter, α. We obtain constraints of 0.01 ≤ α ≤ 0.03 for 0.01 ≤ L/LEdd ≤ 1.0. This narrow range suggests that these AGN are all seen in a single state, with a correspondingly narrow spread of black hole masses or accretion rates. The value of α we derive is consistent with predictions by current simulations in which MHD turbulence is the primary viscosity mechanism.