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\nresult from magnetic dissipation in an optically thin magnetized corona above\nan optically thick accretion disk. If coronal magnetic field originates in the\ndisk and the disk is turbulent, then only magnetic structures large enough for\ntheir turbulent shredding time to exceed their buoyant rise time survive the\njourney to the corona. We use this concept and a physical model to constrain\nthe minimum fraction of magnetic energy above the critical scale for buoyancy\nas a function of the observed coronal to bolometric emission. Our results\nsuggest that a significant fraction of the magnetic energy in accretion disks\nresides in large scale fields, which in turn provides circumstantial evidence\nfor significant non-local transport phenomena and the need for large scale\nmagnetic field generation. For the example of Seyfert AGN, for which of order\n30 per cent of the bolometric flux is in the X-ray band, we find that more than\n20 per cent of the magnetic energy must be of large enough scale to rise and\ndissipate in the corona.\n