Kinetic and Two-Temperature Plasma Physics of Black Hole Accretion Disks and X-ray Coronae

Hankla, Amelia M.

Graduate Thesis Or Dissertation

Kinetic and Two-Temperature Plasma Physics of Black Hole Accretion Disks and X-ray Coronae Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/w3763815f

Abstract

The accretion disks and hot X-ray coronae surrounding black holes host plasmas spanning a wide range of parameter space. The plasma can be collisional or collisionless, depending on its location relative to the black hole and properties such as the accretion rate of surrounding material onto the black hole. In these plasmas, Coulomb collisions between electrons and protons can become inefficient, resulting in a two-temperature flow. In collisionless plasmas, magnetic turbulence and reconnection can accelerate particles to Lorentz factors of 1000 or more. Modeling these processes on scales of an entire disk/corona system is difficult computationally.

In this thesis, I examine the large and small scales of black hole accretion disks and their collisionless coronae. I first study the fundamental process of how turbulence in a collisionless, magnetized coronal plasma changes in the context of an accretion disk/corona system. By driving turbulence with asymmetric energy injection, I show that the timescales for nonthermal particle acceleration depend on the injected energy's imbalance. I also propose a relativistic momentum-coupling mechanism that efficiently converts the driven electromagnetic energy into bulk kinetic energy of the plasma. Then, I demonstrate that nonthermal electrons should exist in the plunging region of a black hole. I use prescriptions from particle-in-cell simulations to build the electron distribution function within the plunging region. By ray-tracing the emission from these electrons, I show that nonthermal electrons within the plunging region create an observable power-law compatible with observations of black hole binaries in the soft spectral state. Finally, I examine two-temperature effects on the accretion disk as a whole. I probe how Coulomb collisions between protons and electrons can alter accretion disk structure, either through efficient collisions leading to disk collapse or through inefficient collisions leading to disk inflation. I contextualize these results in the framework of the disk truncation model for black hole binaries and examine the thick-to-thin disk transition as a function of accretion rate.

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