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Bloody Stones

Bloody Stones. Towards an understanding of AGN engines. Mike J. Cai ASIAA, NTHU. April 4, 2003. What ’ s with the title?. Outline. Introduction to Active Galactic Nuclei Physics of accretion disks Black holes General Relativistic Magnetohydrodynsmics and jets. Basic Properties of AGN.

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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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