Dynamo Activity in Strongly Magnetized Accretion Disks

Mon, Dec 7, 2015 at 12:00 pm

131A Campbell Hall

Greg Salvesen (JILA)

Strongly magnetized accretion disks around black holes have attractive features that may explain enigmatic aspects of X-ray binary behavior.  The structure and evolution of these disks are governed by a dynamo-like mechanism, which channels part of the accretion power liberated by the magnetorotational instability (MRI) into an ordered toroidal magnetic field.  To study dynamo activity, we performed three-dimensional, stratified, isothermal, ideal magnetohydrodynamic shearing box simulations.  The strength of the self-sustained toroidal magnetic field depends on the net vertical magnetic flux, which we vary across almost the entire range over which the MRI is linearly unstable.  We find that the entire disk develops into a magnetic pressure-dominated state for a sufficiently strong net vertical magnetic flux and dynamo activity persists.  Still stronger fields result in a highly inhomogeneous disk structure, with large density fluctuations. We show that the magnetized state of the disk in our simulations is well-matched by an analytic model describing the creation and buoyant escape of toroidal field.  Finally, I will discuss the implications of these results for observed properties of X-ray binaries.