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Towards More Predictive Models of Galactic Center Accretion UC Berkeley UC Berkeley Electronic Theses and Dissertations Title Towards More Predictive Models of Galactic Center Accretion Permalink https://escholarship.org/uc/item/31d79442 Author Ressler, Sean Michael Publication Date 2019 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California Towards More Predictive Models of Galactic Center Accretion By Sean Michael Ressler A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Physics in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Eliot Quataert, Chair Professor Dan Kasen Professor Joshua Bloom Summer 2019 Towards More Predictive Models of Galactic Center Accretion Copyright 2019 by Sean Michael Ressler 1 Abstract Towards More Predictive Models of Galactic Center Accretion by Sean Michael Ressler Doctor of Philosophy in Physics University of California, Berkeley Professor Eliot Quataert, Chair Sagittarius A* (Sgr A*), the roughly 4 million M black hole at the center of our galaxy, is ar- guably the best natural test-bed for supermassive black hole accretion models. Its close proxim- ity allows for detailed observations to be made across the electromagnetic spectrum that provide strong, multi-scale constraints on analytic and numerical calculations. These include exciting new event horizon scale results that directly probe the strong field regime for the first time. Spanning roughly 7 orders of magnitude in radius, the accretion system begins at ∼ parsec scales where a large population of Wolf-Rayet (WR) stars interact via powerful stellar winds. A fraction of the wind material accretes to the event horizon scales, heating up and radiating on the way down to provide the observed multi-wavelength emission. Detailed modeling of this process requires 3D numerical simulations spanning as large of a dynamical range as possible. The goal of this the- sis has been to improve the predictive power of such simulations applied to Sgr A* by limiting the number of free parameters that they contain. This is done by incorporating more theoretical and observational knowledge into the calculations, for example, what collisionless physics tells us about how electrons/ions are heated in a turbulent medium and the properties of the winds of the WR stars. The resulting simulations have shown excellent agreement with a wide range of obser- vational constraints and could have major implications for how the accretion flow around Sgr A* is modeled in the future. The techniques presented here are also applicable to other low luminosity AGN and X-ray binaries. i I dedicate this dissertation to my parents, Ron and Denise Ressler. I hope this makes up for the time when I wrote about “Nintendo” as my hero in that infamous 3rd grade essay. ii Contents List of Figures vi List of Tables ix Acknowledgmentsx 1 Introduction1 1.1 Background......................................1 1.2 Thesis Summary and Guide To Chapters.......................6 1.3 Applications To Other Systems............................7 1.4 Other Work......................................8 2 Electron Thermodynamics in GRMHD Simulations of Low-Luminosity Black Hole Accretion 9 2.1 Abstract........................................9 2.2 Introduction...................................... 10 2.3 Electron Thermodynamics.............................. 11 2.3.1 Basic Model................................. 12 2.3.2 Electron Heating............................... 15 2.3.3 Anisotropic Electron Conduction...................... 16 2.4 Numerical Implementation of Electron Heating and Conduction.......... 17 2.4.1 Heating in Conservative Codes........................ 18 2.4.2 Calculating the Total Heating Rate...................... 19 2.4.3 Electron Heating Update........................... 20 2.4.4 Electron Conduction Update......................... 20 2.4.5 Treatment of the Floors............................ 22 2.5 Tests of Numerical implementation.......................... 23 2.5.1 Tests of Electron Heating........................... 23 2.5.2 Tests of Electron Conduction......................... 31 2.6 Application to an Accreting Black Hole in 2D GRMHD Simulations........ 32 2.6.1 Electron Parameter Choices......................... 34 2.6.2 Electron Heating Only............................ 38 Contents iii 2.6.3 Conduction and Electron Heating...................... 39 2.7 Conclusions...................................... 42 2.A Tests of Electron Conduction............................. 49 2.A.1 Conduction Along Field Lines........................ 49 2.A.2 Linear Modes Test.............................. 49 2.A.3 1D Atmosphere in a Schwarzschild Metric................. 51 2.A.4 Relativistic Bondi Accretion......................... 51 2.B Derivations...................................... 52 2.B.1 Total Heating Rate.............................. 52 2.B.2 Whistler Instability Limit on Conduction.................. 54 2.B.3 Electron Conduction Numerical Stability.................. 55 2.B.4 Electron Heating in a 1D Shock....................... 57 2.C Electron Heating in a Viscous Shock......................... 59 2.D Torus Initial Conditions................................ 62 3 The Disc-Jet Symbiosis Emerges: Modeling the Emission of Sagittarius A* with Elec- tron Thermodynamics 63 3.1 Abstract........................................ 63 3.2 Introduction...................................... 63 3.3 Fluid Model and Electron Thermodynamics..................... 65 3.4 Radiation Transport.................................. 67 3.5 Accretion Disc Model................................. 68 3.6 Results......................................... 69 3.6.1 Basic Flow Properties............................ 69 3.6.2 Spectra and Images.............................. 71 3.6.3 Time Variability............................... 75 3.6.4 Dependence of Observables on Disc Parameters............... 75 3.6.5 Convergence of Spectra........................... 77 3.7 Thermodynamics in the Polar Outflow........................ 77 3.7.1 Low Frequency Radio Slope......................... 78 3.8 Comparison to Previous Models of Sgr A*...................... 79 3.9 Conclusions...................................... 82 3.A Observational Data.................................. 84 3.B Initial conditions, Coordinate System and Floors.................. 84 3.B.1 Initial Conditions............................... 84 3.B.2 Coordinate System.............................. 86 3.B.3 Density Floors................................ 88 3.C Motivation For the Choice of Magnetization Cut-off ................. 91 3.D Steep Entropy Wave Test............................... 91 Contents iv 4 Hydrodynamic Simulations of the Inner Accretion Flow of Sagittarius A* Fueled By Stellar Winds 97 4.1 Abstract........................................ 97 4.2 Introduction...................................... 98 4.3 Model and Computational Methods......................... 100 4.3.1 Equations Solved............................... 100 4.3.2 Calculating The Cooling Function...................... 101 4.3.3 Computational Grid and Boundary/Initial Conditions............ 102 4.3.4 Floors and Ceilings.............................. 104 4.4 Tests of Implementation............................... 104 4.4.1 Stationary Stellar Wind............................ 104 4.4.2 Isotropic Stars on Circular Orbits...................... 105 4.5 3D Simulation of Accreting Stellar Winds Onto Sgr A*............... 106 4.5.1 Stellar Winds and Orbits........................... 108 4.5.2 Parameters and Initialization......................... 109 4.5.3 Results.................................... 110 4.6 Constraining Stellar Wind Mass-loss Rates and Wind Speeds with X-ray Observa- tions.......................................... 124 4.7 Implications for Horizon-Scale Accretion Modeling................. 126 4.8 Comparison to Previous Work............................ 128 4.9 Conclusions...................................... 130 4.A Angular Momentum Distribution of a Single Accreting Star............. 133 5 Accretion of Magnetized Stellar Winds in the Galactic Centre: Implications for Sgr A* and PSR J1745-2900 135 5.1 Abstract........................................ 135 5.2 Introduction...................................... 135 5.3 Magnetized Wind Model............................... 136 5.4 Analytic Model For The RM of PSR J1745-2900.................. 138 5.5 3D MHD Simulations................................. 139 5.5.1 Parameter Choices and Computational Grid................. 139 5.5.2 Rotation Measure of PSR J1745-2900.................... 140 5.5.3 Rotation Measure of Sgr A*......................... 143 5.6 Discussion And Conclusions............................. 145 6 The Surprisingly Small Impact of Magnetic Fields On The Inner Accretion Flow of Sagittarius A* Fueled By Stellar Winds 147 6.1 Abstract........................................ 147 6.2 Introduction...................................... 148 6.3 Computational Methods................................ 150 6.4 Isolated, Magnetized Stellar Wind Test........................ 152 6.5 3D Simulation of MHD Wind Accretion Onto Sgr A*................ 155 Contents v 6.5.1 Computational Grid and Boundary/Initial Conditions............ 155 6.5.2 Overview................................... 156 6.5.3 Dynamics of The Inner Accretion Flow................... 159 6.5.4 Stresses.................................... 168 6.5.5 MRI...................................... 170 6.5.6 Magnetic
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