Coronae as a consequence of large-scale magnetic fields in turbulent accretion disks

Abstract

Non-thermal X-ray emission in compact accretion engines can be interpreted to result from magnetic dissipation in an optically thin magnetized corona above an optically thick accretion disk. If coronal magnetic field originates in the disk and the disk is turbulent, then only magnetic structures large enough for their turbulent shredding time to exceed their buoyant rise time survive the journey to the corona. We use this concept and a physical model to constrain the minimum fraction of magnetic energy above the critical scale for buoyancy as a function of the observed coronal to bolometric emission. Our results suggest that a significant fraction of the magnetic energy in accretion disks resides in large-scale fields, which in turn provides circumstantial evidence for significant non-local transport phenomena and the need for large-scale magnetic field generation. For the example of Seyfert active galactic nuclei, for which 30% of the bolometric flux is in the X-ray band, we find that more than 20% of the magnetic energy must be of large enough scale to rise and dissipate in the corona. © 2009 The American Astronomical Society. All rights reserved.