Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-09-10
Mon.Not.Roy.Astron.Soc.347:67,2004
Astronomy and Astrophysics
Astrophysics
7 pages, 5 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society
Scientific paper
10.1111/j.1365-2966.2004.07167.x
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 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 AGN 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 simulation s in which MHD turbulence is the primary viscosity mechanism.
Siemiginowska Aneta
Soria Roberto
Starling Rhaana L. C.
Uttley Phil
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