Magnetic Flares and the Observed Optical Depth in Seyfert Galaxies

Abstract

We here consider the pressure equilibrium during an intense magnetic flare above the surface of a cold accretion disk. Under the assumption that the heating source for the plasma trapped within the flaring region is an influx of energy transported inwards with a group velocity close to $c$, e.g., by magnetohydrodynamic waves, this pressure equilibrium can constrain the Thomson optical depth $τ_T$ to be of order unity. We suggest that this may be the reason why $τ_T\sim 1$ in Seyfert Galaxies. We also consider whether current data can distinguish between the spectrum produced by a single X-ray emitting region with $τ_T\sim 1$ and that formed by many different flares spanning a range of $τ_T$. We find that the current observations do not yet have the required energy resolution to permit such a differentiation. Thus, it is possible that the entire X-ray/$γ$-ray spectrum of Seyfert Galaxies is produced by many independent magnetic flares with an optical depth $0.5<τ_T<2$.