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 $0.5<{\tau }_T<2$.

submitted to ApJL, 6 pages, 1 figure, uses aas2pp4.sty

References in corpus (2)

• Influence of Ohmic Heating on Advection-Dominated Accretion Flows
• Big Blue Bump and Transient Active Regions in Seyfert Galaxies

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