CRA Seminar: Chris Bambic Princeton UN

Title: Insights from Local Simulations of Accretion Disk Coronae: Two-temperature effects, thermal conduction, and radiation transport

Abstract: I will present a series of local accretion disk models, i.e., stratified shearing-box magnetohydrodynamic (MHD) simulations, for radiatively efficient accretion flows (REFs), which for the first time, include the effects of field-aligned free-streaming thermal conduction and radiation transport. The goal of my work is to understand the formation and evolution of hot, magnetically dominated coronae in active galactic nuclei (AGN) and X-ray binary (XRB) accretion flows, relevant for black hole systems accreting from about 1 to 10 percent of the Eddington rate. First, I will show that coronal plasmas should be in the two-temperature regime, where ion and electron/ positron (lepton) temperatures are only weakly coupled such that ions cool primarily through Coulomb collisions with rapidly Compton-cooled leptons. This realization allows me to introduce a simple, two-temperature model for the coronal ions that captures their thermodynamic evolution. The models naturally form temperature inversions, with a hot corona “sandwiching” a colder, thin disk.

Within this framework, I examine the evaporation and truncation of disks through field-aligned ion thermal conduction from the hot corona into the colder disk, which has been invoked as a mechanism to explain soft-to-hard state transitions in XRBs. I will show that, independent of the chosen magnetic field structure in the flow, thermal conduction is unable to evaporate disks and form a radiatively inefficient accretion flow (RIAF) near the black hole. Since thermal conduction alone cannot account for the observed truncation of disks in XRB accretion flows, an alternative physical mechanism is necessary to account for the soft-to-hard state transitions observed in XRBs. The introduction of net-vertical magnetic flux (NF) may provide such a mechanism. NF fields launch magnetocentrifugal outflows, which can deplete the disk’s surface layers near the black hole, allowing the plasma there to become two-temperature and evaporate into a RIAF. To study how NF fields heat coronae and mediate state transitions, I will discuss ongoing work to assemble a suite of local radiation MHD simulations with varying NF that self-consistently solve the radiation transport equation. These calculations will allow me to quantify the dissipation of energy in the optically thin surface layers of our model disks as a function of disk magnetization and surface density. Informed by these local models, I will discuss under what conditions we should expect hard X-ray coronae to form in luminous accretion flows.

Author: Christopher J. Bambic, with Eliot Quataert, Matthew W. Kunz, and Yan-Fei Jiang