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

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.</p

Near- and Extended-Edge X-Ray-Absorption Fine-Structure Spectroscopy Using Ultrafast Coherent High-Order Harmonic Supercontinua

Recent advances in high-order harmonic generation have made it possible to use a tabletop-scale setup to produce spatially and temporally coherent beams of light with bandwidth spanning 12 octaves, from the ultraviolet up to x-ray photon energies >1.6  keV. Here we demonstrate the use of this light for x-ray-absorption spectroscopy at the K- and L-absorption edges of solids at photon energies near 1 keV. We also report x-ray-absorption spectroscopy in the water window spectral region (284-543 eV) using a high flux high-order harmonic generation x-ray supercontinuum with 10^{9}  photons/s in 1% bandwidth, 3 orders of magnitude larger than has previously been possible using tabletop sources. Since this x-ray radiation emerges as a single attosecond-to-femtosecond pulse with peak brightness exceeding 10^{26}  photons/s/mrad^{2}/mm^{2}/1% bandwidth, these novel coherent x-ray sources are ideal for probing the fastest molecular and materials processes on femtosecond-to-attosecond time scales and picometer length scales.093002

Modified Fluid Closure of Weakly-Collisional Plasmas in Radiatively-Inefficient Accretion Flows

The diversity of plasmas in nature divides their study into many different regimes which are valid only within certain approximations. This paper attempts to extend the validity of one such regime (a fluids framework) to another (kinetic theory, for collisionless plasmas). The focus is more narrowly on astrophysical systems, where studies of weakly collisional plasmas very often use the fluid model which should theoretically not apply. Recent kinetic simulations of black hole accretion flows make radiatively-inefficient accretion flows an ideal starting point to investigate the possibility of using a modified fluids closure to model weakly collisional plasmas. If the fluid regime is found to be an appropriate model, then the door is opened for future work on global simulations and other weakly collisional plasmas. Study of these accretion flows is done through three-dimensional local shearing box magnetohydrodynamic simulations with anisotropic viscosity and a maximum pressure anisotropy, a choice motivated by the aforementioned kinetic simulations

Nonthermal emission from the plunging region: a model for the high-energy tail of black hole X-ray binary soft states

X-ray binaries exhibit a soft spectral state comprising thermal blackbody emission at 1 keV and a power-law tail above 10 keV. Empirical models fit the high-energy power-law tail to radiation from a nonthermal electron distribution, but the physical location of the nonthermal electrons and the reason for their power-law index and high-energy cut-off are still largely unknown. Here, we propose that the nonthermal electrons originate from within the black hole's innermost stable circular orbit (the ''plunging region''). Using an analytic model for the plunging region dynamics and electron distribution function properties from particle-in-cell simulations, we outline a steady-state model that can reproduce the observed spectral features. In particular, our model reproduces photon indices of $\Gamma\gtrsim2$ and power-law luminosities on the order of a few percent of the disk luminosity for strong magnetic fields, consistent with observations of the soft state. Because the emission originates so close to the black hole, we predict that the power-law luminosity should strongly depend on the system inclination angle and black hole spin. This model could be extended to the power-law tails observed above 400 keV in the hard state of X-ray binaries.Comment: 10 pages, 8 figures. Accepted in MNRA

Near- and Extended-Edge X-Ray-Absorption Fine-Structure Spectroscopy Using Ultrafast Coherent High-Order Harmonic Supercontinua.

Recent advances in high-order harmonic generation have made it possible to use a tabletop-scale setup to produce spatially and temporally coherent beams of light with bandwidth spanning 12 octaves, from the ultraviolet up to x-ray photon energies &gt;1.6  keV. Here we demonstrate the use of this light for x-ray-absorption spectroscopy at the K- and L-absorption edges of solids at photon energies near 1&nbsp;keV. We also report x-ray-absorption spectroscopy in the water window spectral region (284-543&nbsp;eV) using a high flux high-order harmonic generation x-ray supercontinuum with 10^{9}  photons/s in 1% bandwidth, 3 orders of magnitude larger than has previously been possible using tabletop sources. Since this x-ray radiation emerges as a single attosecond-to-femtosecond pulse with peak brightness exceeding 10^{26}  photons/s/mrad^{2}/mm^{2}/1% bandwidth, these novel coherent x-ray sources are ideal for probing the fastest molecular and materials processes on femtosecond-to-attosecond time scales and picometer length scales

Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas.

High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function, the emission from each species is highest when a short-wavelength ultraviolet driving laser is used. However, phase matching--the constructive addition of x-ray waves from a large number of atoms--favors longer-wavelength mid-infrared lasers. We identified a regime of high-harmonic generation driven by 40-cycle ultraviolet lasers in waveguides that can generate bright beams in the soft x-ray region of the spectrum, up to photon energies of 280 electron volts. Surprisingly, the high ultraviolet refractive indices of both neutral atoms and ions enabled effective phase matching, even in a multiply ionized plasma. We observed harmonics with very narrow linewidths, while calculations show that the x-rays emerge as nearly time-bandwidth-limited pulse trains of ~100 attoseconds