Date of Award

May 2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Philip Chang

Committee Members

Dawn Erb, Jolien Creighton, Sarah Vigeland, Ionel Popa

Abstract

The detection of supermassive black holes (SMBH) in our universe has proven to be a challenge. One way to find an SMBH in a quiescent, inactive galaxy is through a tidal disruption event, or a TDE. A TDE occurs when a star is slightly perturbed and is subsequently disrupted by the SMBH. When this happens, part of the debris from the disrupted star remains bound to the SMBH while the rest is unbound. The SMBH accretes the bound matter and reveals their presence by a temporary X-ray flare, lasting a few months. Using conservation of energy, the fallback rate of the material to the SMBH can be shown to follow a timescale of t to the -5/3. However, the complete picture of TDEs is not well understood due to the diverse physics involved, and therefore TDEs necessitates 3-D numerical simulations to dive deeper into the physics that are at play in a TDE. Here two methodologies are commonly employed, each of which comes with its own set of advantages: smoothed-particle hydrodynamics and Eulerian grid codes. A hybrid of these methods known as the moving-mesh code has been developed in an attempt to capture the best characteristics of each. We use the moving-mesh solver MANGA to study characteristics of TDEs.

In this work, we begin with an introduction to TDEs in Chapter 1. We describe their observational characteristics, the mechanics of the event and the dynamics of the disruption. Next, we present two different simulation techniques in Chapter 2 and how the best aspects of each are combined in the technique used for our work, moving-mesh. In Chapter 3, we introduce a parameter study of the effect of impact parameter on the energy distribution of the debris. We show how this influences the fallback rate of the unbound debris, the time of peak accretion and the evolution of the accretion rate. In Chapter 4 we further investigate how this result determines the expected radio emission from the TDE. With observed radio emission from TDEs, we show that it is possible to determine the density profile of the surrounding medium.

TDE simulations to date mostly ignore the influence of an initial magnetic field on the surface of the star. In Chapter 5 we describe a recently implemented magnetohydrodynamics scheme in MANGA. When we apply an initial magnetic field to the star, we notice an outflow from the TDE that is not present in our simulations without an initial magnetic field. We model the outflow as an expanding photosphere and relate our results to observational signatures of several TDEs.

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