Magnetic Reconnection in Low-Luminosity Accretion Flows: From Microphysical Simulations to Large-Scale Models

Abstract

Recent observations of low luminosity accretion flows, such as Sgr A*, reveal significant multiwavelength variability with properties that have not been fully explained. In the first part of this dissertation, I develop a model for understanding the origin of X-ray flares from Sgr A* using GRMHD simulations coupled to radiative transfer calculations that take into account the emission from non-thermal electrons, localized in regions where we expect acceleration processes to accelerate electrons to highly relativistic energies. I then explore whether magnetic reconnection may be a viable candidate for accelerating these high-energy electrons by quantifying the properties of current sheets that form self-consistently in GRMHD simulations. After finding that many current sheets are indeed present in the best-fit models for the accretion flow around Sgr A*, I perform a suite of PIC simulations of magnetic reconnection with the initial plasma conditions informed by those we found in the vicinity of current sheets in GRMHD simulations. In this study, I focus on understanding the electron acceleration efficiency and associated power-law index of the non-thermal distribution as a function of two key plasma parameters, the magnetization $\sigma$ and the plasma-$\beta$, and provide an empirical prescription for the non-thermal distribution to be used as subgrid physics in large-scale models of accretion flows that calculate observables. Finally, I explore in more detail the underlying physical mechanisms responsible for high-energy electron acceleration and develop a variety of diagnostics for probing the competing effects.