The Story of Thin Accretion Disks

1
The Story ofThin Accretion Disks

• Niayesh Afshordi
• Princeton Observatory
• 6 November, 2002

2
An Outline

• Why Thin Disks?
• Steady State Accretion
• And About Viscosity
• Coronae of Accretion Disks
• Characteristic Time Scales
• Instabilities
• The Moral of the Story

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Why Thin Disks?
L1 L2 L3 L4 L5

• In order to
• Minimize the Energy
• Conserve the Total Angular Momentum
• the orbits should become
• circular and
• lie on a disk.
• Then, viscosity leads to a
• diffusion equation for Surface Density
• So, matter spreads in the disk and is sunk into
  the center.

L1 L2 L3 L4 L5
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Steady State Accretion

• Equations
• Mass
• Angular Momentum
• Radial Momentum
• Vertical Momentum
• Energy
• Sakura Sunyaevs
• (1973) Turbulence Scale H so
• Then Far from the
• BH Horizon

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Steady State Accretion

• Spectrum
• Big Blue Bump (not observed)
• Emission Lines (not observed)

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And About Viscosity

• Molecular Viscosity is too small
• Keplerian disks are Rayleigh stable, so no
  hydrodynamic turbulence
• Magnetic disturbances grow due to
• differential rotation, MHD Turbulence (Balbus
  Hawley 1991)
• 0.1 gt gt 0.01 and in general varies with time
  and R in simulations.
• But all simulations ignore cooling which is
  essential for having disks.

density and a volumetric rendering of the
magnetic pressure (Stone et al. 1996)
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Coronae of Accretion Disks

• Observational Evidence
• Polarization (presumably caused by scattering)
• Reprocessed Emission Lines in X-ray
• Theoretical Picture
• Parker Instability and evaporation of magnetic
  field lines/loops
• Heating due to magnetic reconnection

Parker Instability, http//www.hao.ucar.edu/yfan
/fluxemg.html
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Coronae of Accretion Disks
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Characteristic Time Scales

• Rotational
• Thermal
• Infall/Viscous
• Magneto-rotational

Far From the BH Horizon
This is responsible for the formation of thin disk
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Instabilities

• Thermal
• when heating/cooling equilibrium is not
  stable

Stable
Ionized
Unstable
Recombination
Neutral
Stable
Theoretical duty cycle of a dwarf nova
Light curve observation vs. theory
Lasota, J.-P. 2001, New Astr. Rev, 45, 449
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Instabilities

• Viscous
• When the momentum transfer equations are not
  stable
• Depends crucially on how changes with other
  physical parameters

Unstable
Stable
R/R_S
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Instabilities

• MHD
• Parker, Magneto-rotational

Deviation of angular momentum from the Keplerian
one in magneto-rotational instability (Stone et
al. 1996)
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Instabilities

• Rayleigh (inside marginally stable orbit)
• Papaloizou-Pringle (azimuthally asymmetric)
• Jets
• Is it some kind of instability?
• ??

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The Moral of the Story

• Thin Accretion Disks are inevitable in the
  presence of Cooling and Gravity
• Almost non of the predictions of the simple
  theory of steady state thin accretion is
  confirmed with observations
• Most observations can be explained by adding
  extra components, e.g. corona, instabilities,
  MHD, outflows etc.

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The Moral of the Story
Pringle, J. 1981, ARAA, 19, 137