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

We 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. Our goal 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, we show that coronal plasmas should be in the two-temperature regime, where ion and electron/ positron (lepton) temperatures are only weakly coupled, and ions cool primarily through Coulomb collisions with rapidly Compton-cooled leptons. This realization allows us to introduce a simple, two-temperature model for the coronal ions that captures their thermodynamic evolution. Our models naturally form temperature inversions, with a hot corona “sandwiching” a colder, thin disk. Within this framework, we study 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. We 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, we present a suite of local radiation MHD simulations with varying NF that self-consistently solve the radiation transport equation. We quantify the dissipation of energy in the optically thin surface layers of our model disks as a function of disk magnetization and surface density, and we discuss under what conditions we should expect hard X-ray coronae to form in REFs.