The structure and variability of dynamo driven accretion discs

A turbulent dynamo operating in an accretion disc around a black hole can produce fields strong enough so that the Maxwell stress due to the fluctuations dominates. In this dynamo driven limit, enormous localized fluctuations can be expected because the Kepler flow energy density is efficiently tapped. The detailed radial structure of this model is calculated, which for Cyg X-1, predicts a cool (Tmax ≾ 108 K), dense, thin (z0/r ≾ 10−2), and optically thick disc. A mean field B ≾ 108 G can be generated. Fluctuations of order $\tilde b \sim10^{12}$ G at the inner accretion disc radius (r⋆ ∼ 1), falling to $\tilde b \sim10^{10}$ G at r⋆ ∼ 30, provide an explanation for the Cyg X-l millisecond bursts and shot noise in terms of flares on the disc surface. This is established by means of model independent, scaled reconnection experiments. The optical variability of 3C 273 could be explained as arising from flares on an accretion disc around a 109M⊙ black hole, with flare fields of $\tilde b \sim10^7$ G at r⋆ ∼ 1.