Friday, October 06, 2023 at 12:15 pm

JILA Foothills Room

Christopher Bambic, Princeton

"Insights from Local Simulations of Two-temperature Accretion Disc Coronae: Structure, Outflows, Thermal Conduction, and Energetics"

Abstract:

Hard X-ray observations of luminous active galactic nuclei (AGN) accreting around a few percent of the Eddington rate display a cut-off power law component associated with a hot, magnetically-dominated "corona," named in analogy with the solar corona. I will present a series of local accretion disc models, i.e. stratified shearing-box magnetohydrodynamic (MHD) simulations, for radiatively efficient accretion flows and the insights they provide into the structure and energetics of accretion disc coronae and their outflows/ winds. First, I will introduce a "two-temperature" model for the thermodynamics of coronae and demonstrate that two-temperature models naturally form temperature inversions, with a hot, magnetically dominated corona surrounding a cold disc. Through varying the initial magnetic field strengths and geometries, namely the amount of the conserved net vertical magnetic flux (NF) threading the disc, I will show that simulations with NF magnetic fields launch powerful magnetocentrifugal winds that would enhance accretion in a global system. Winds into two- temperature coronae may be sufficiently strong to evaporate a thin disc and form a radiatively inefficient accretion flow---a mechanism that could be responsible for mediating state transitions in X-ray binaries. Next, I will discuss an alternative means of disc evaporation: field aligned thermal conduction from the hot corona into the cold disc. By including free-streaming thermal conduction in our simulations, I will show that conduction is unable to evaporate the innermost regions of AGN or X-ray binary discs. I will close by discussing how NF magnetic fields may power the coronae of luminous Seyfert galaxies and quasars, what uncertainties are introduced by NF fields, and our ongoing work to tackle these questions with self- consistent radiation transport simulations. These simulations allow us to directly compare our theoretical findings with observations to understand the decades-old problem of the origin of high energy radiation in accreting black hole systems.

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