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Bloody Stones

This document explores the complex nature of Active Galactic Nuclei (AGN), focusing on the physics of accretion disks, black hole dynamics, and the mechanisms behind AGN jets. It discusses the characteristics and behavior of Seyfert galaxies, quasars, and blazars, while introducing concepts of general relativistic magnetohydrodynamics (GRMHD). Key topics include angular momentum transport, the role of black holes in AGN, and ongoing questions in the field regarding galaxy evolution and supermassive black hole formation.

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Bloody Stones

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  1. Bloody Stones Towards an understanding of AGN engines Mike J. Cai ASIAA, NTHU April 4, 2003

  2. What’s with the title?

  3. Outline • Introduction to Active Galactic Nuclei • Physics of accretion disks • Black holes • General Relativistic Magnetohydrodynsmics and jets

  4. Basic Properties of AGN • High luminosity (1043~48erg s-1) • Lnucleus~Lgalaxy Seyfert galaxy • Lnucleus~100 Lgalaxy Quasar • Very small angular size • Short variability time scale • Apparent superluminal motion • A lot more AGN’s at z>2.5

  5. Unified Model of AGN Seyfert 1

  6. Unified Model of AGN Seyfert 2

  7. Unified Model of AGN Blazar

  8. Accretion Disk • Disk geometry • Matter needs to lose angular momentum to reach central black hole. • Interaction of different orbits will mix angular momentum. • Scale height is roughly h~r cs/vorb. • The inner region is well approximated by a perfect plasma. • Unstable to rotation if d(r2W)/dr<0.

  9. Angular Momentum Transport Viscosity? • Friction between adjacent rings can transport angular momentum out • a disk – hide our ignorance MHD Winds • Magneto-centrifugal acceleration (bead on a wire) Magnetic Turbulence (Balbus-Hawley instability) Gravitational Radiation

  10. Schwarzschild Black Holes • Static and spherically symmetric metric. • grr=∞ defines horizon (rSch=2M). • Circular photon orbit at rph=3M (independent of l). • Last stable orbit at rms=6M (l2=12M2). • Maximal accretion efficiency ~ 5.7%.

  11. Kerr Black Holes • Stationary and axisymmetric metric • Dragging of inertial frames (gtf≠0). • gtt=0 defines ergosphere. • grr=∞ defines horizon (M<rH<2M). • Circular photon orbit • rph=M (prograde), 4M (retrograde) for a=M • Last stable orbit • rms=M (prograde), 9M (retrograde) for a=M • Maximal accretion efficiency ~ 42%.

  12. An Ant’s Impression of a Kerr Black Hole

  13. How to Power AGN Jets • Accretion onto a supermassive Kerr black hole that is near maximum rotation • Extraction of the rotational energy of the black hole via Penrose or Blandford-Znajek process • Magnetocentrifugal acceleration • Collimation of outflow by magnetic fields (through hoop stress)

  14. Extracting Rotational Energy of a Black Hole • A rotating black hole has an ergosphere where all particles have to corotate with the black hole. • Penrose process: explosion puts fragments into negative energy and angular momentum orbits. • Blandford-Znajek process: magnetic field pulls particles into negative energy and angular momentum orbits.

  15. GRMHD • MHD assumption Fu=0, F=dA LuF = 0 Field freezing T = Tfluid+TEM • Stationarity and axisymmetry LxF = 0, x = ∂t or ∂f 2pAf= invariant flux • Isothermal equation of state, p = gr • Conservation of stress energy, Tmn;n = 0

  16. GRMHD • Conserved quantities • w = - A0,m/Af,m (isorotation, no sum) • E, L (energy & angular momentum) • h n uP (injection parameter) • umTmn;n= 0 1st law of thermodynamics • BaPamTmn;n= 0 u2 = -1 (algebraic wind equation) • QabPbmTmn;n = 0 Scalar Grad-Shafranov equation, determines field geometry (ugly)

  17. Open Questions • Do all galaxies go through an AGN phase? • How are AGNs fueled from their environment? • Bar driven inflow? • Interacting galaxies? • Where do supermassive black holes come from? • Is GRMHD the ultimate answer to jets? • Can stones actually bleed?

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