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Title:Dissipative heating and nonthermal distributions in black hole accretion flaring events
Author(s):Petersen, Eric
Director of Research:Gammie, Charles F
Doctoral Committee Chair(s):Holder, Gilbert
Doctoral Committee Member(s):Filippini, Jeffrey P; Cooper, Lance
Department / Program:Physics
Discipline:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):Black Hole
Accretion
plasma
reconnection
nonthermal
Abstract:The center of the Milky Way galaxy contains a supermassive black hole called Sgr A*, which has been observed at radio, mm, X-ray, and near infrared (NIR) wavelengths. The NIR emission flares about once per day with the flaring state being about an order of magnitude brighter than the non-flaring state. These flares have a flat spectrum that drops off at high frequency much slower than would be expected from thermal emission alone. This thesis describes work to model these flares using general relativistic magnetohydrodynamics (GRMHD) and radiative transfer calculations with a nonthermal kappa distribution function (which effectively adds a power-law tail to the thermal distribution) for electrons accelerated by resistive heating in reconnecting current sheets. This approach is well supported by the literature on the acceleration of electrons in magnetized plasma, which shows that current sheets do accelerate electrons and those electrons can have a distribution function similar to a kappa distribution. In axisymmetric (two dimensional) simulations presented here, a model with a constant fraction of electrons in the kappa distribution is able to enhance NIR emission but is unable to produce any substantial variability in the NIR flux density. Similar models that heat electrons through resistive dissipation are able to produce flares. In three-dimensional standard and normal evolution (SANE) and magnetically arrested disk (MAD) models, the total current in the simulation showed only small variability. This resulted in some small-scale variability in the light curve, but no flares are observed. In all cases, nonthermal models were able to reproduce the observed spectral slope of Sgr A* in the NIR region.
Issue Date:2020-12-04
Type:Thesis
URI:http://hdl.handle.net/2142/109367
Rights Information:Copyright 2020 Eric Petersen
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12


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