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

Basic Properties of AGN High luminosity (10 43~48 erg s -1 ) High luminosity (10 43~48 erg s -1 ) L nucleus ~L galaxy Seyfert galaxy L nucleus ~L galaxy Seyfert galaxy L nucleus ~100 L galaxy Quasar L nucleus ~100 L galaxy Quasar Very small angular size Very small angular size Short variability time scale Short variability time scale Apparent superluminal motion Apparent superluminal motion A lot more AGNs at z>2.5 A lot more AGNs at z>2.5

Unified Model of AGN Seyfert 1

Unified Model of AGN Seyfert 2

Unified Model of AGN Blazar

Accretion Disk Disk geometry Disk geometry Matter needs to lose angular momentum to reach central black hole. Matter needs to lose angular momentum to reach central black hole. Interaction of different orbits will mix angular momentum. Interaction of different orbits will mix angular momentum. Scale height is roughly h~r c s /v orb. Scale height is roughly h~r c s /v orb. The inner region is well approximated by a perfect plasma. The inner region is well approximated by a perfect plasma. Unstable to rotation if d(r 2 )/dr<0. Unstable to rotation if d(r 2 )/dr<0.

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

Schwarzschild Black Holes Static and spherically symmetric metric. Static and spherically symmetric metric. g rr = defines horizon (r Sch =2M). g rr = defines horizon (r Sch =2M). Circular photon orbit at r ph =3M (independent of l). Circular photon orbit at r ph =3M (independent of l). Last stable orbit at r ms =6M (l 2 =12M 2 ). Last stable orbit at r ms =6M (l 2 =12M 2 ). Maximal accretion efficiency ~ 5.7%. Maximal accretion efficiency ~ 5.7%.

Kerr Black Holes Stationary and axisymmetric metric Stationary and axisymmetric metric Dragging of inertial frames (g t 0). Dragging of inertial frames (g t 0). g tt =0 defines ergosphere. g tt =0 defines ergosphere. g rr = defines horizon (M<r H <2M). g rr = defines horizon (M<r H <2M). Circular photon orbit Circular photon orbit r ph =M (prograde), 4M (retrograde) for a=M r ph =M (prograde), 4M (retrograde) for a=M Last stable orbit Last stable orbit r ms =M (prograde), 9M (retrograde) for a=M r ms =M (prograde), 9M (retrograde) for a=M Maximal accretion efficiency ~ 42%. Maximal accretion efficiency ~ 42%.

An Ant s Impression of a Kerr Black Hole

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

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

GRMHD MHD assumption F u=0, F=dA MHD assumption F u=0, F=dA L u F = 0Field freezing T = T fluid +T EM Stationarity and axisymmetry Stationarity and axisymmetry L = t or L = t or = invariant flux = invariant flux Isothermal equation of state, p = Isothermal equation of state, p = Conservation of stress energy, T 0 Conservation of stress energy, T 0

GRMHD Conserved quantities Conserved quantities = - A 0, /A, (isorotation, no sum) = - A 0, /A, (isorotation, no sum) E, L (energy & angular momentum) E, L (energy & angular momentum) n u P (injection parameter) n u P (injection parameter) u T = 01 st law of thermodynamics u T = 01 st law of thermodynamics B P T = 0u 2 = -1 (algebraic wind equation) B P T = 0u 2 = -1 (algebraic wind equation) Q P T = 0Scalar Grad-Shafranov equation, determines field geometry (ugly) Q P T = 0Scalar Grad-Shafranov equation, determines field geometry (ugly)

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?