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 $\tau_T$ to be of order unity. We suggest that this may be the reason why $\tau_T\sim 1$ in Seyfert Galaxies. We also consider whether current data can distinguish between the spectrum produced by a single X-ray emitting region with $\tau_T\sim 1$ and that formed by many different flares spanning a range of $\tau_T$. 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/$\gamma$-ray spectrum of Seyfert Galaxies is produced by many independent magnetic flares with an optical depth $0.5<\tau_T<2$.