Thermodynamics and Spectra of

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Thermodynamics and Spectra of Optically Thick
Accretion Disks
Omer Blaes, UCSB
With Shane Davis, Shigenobu Hirose and Julian
Krolik
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Standard Disks are Observed to be Simple And
Stable
E.g. Cyg X-1 (Churazov et al. 2001)
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Plenty of X-ray Binaries Get to High Eddington
Ratios, And Do NOT Show Signs of Putative Thermal
Instability
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Except Perhaps GRS 1915105?
-Belloni et al. (1997)
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Black Hole???Disk Models? AGNSPEC BHSPEC
-Hubeny Hubeny 1997, 1998 Hubeny et al. (2000,
2001), Davis Hubeny (2006), Hui Krolik (2008)
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• The Good
• Models account for relativistic disk structure
  and relativistic
• Doppler shifts, gravitational redshifts, and
  light bending in
• a Kerr spacetime.
• Models include a detailed non-LTE treatment of
  abundant
• elements.
• Models include continuum opacities due to
  bound-free and
• free-free transitions, as well as
  Comptonization. (No lines
• at this stage, though.)

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• The Bad --- Ad Hoc Assumptions
• Stationary, with no torque inner boundary
  condition.
• ?R???Ptot with ? constant with radius -
  determines surface
• density.
• Vertical structure at each radius depends only
  on height
• and is symmetric about midplane.
• Vertical distribution of dissipation per unit
  mass assumed
• constant.
• Heat is transported radiatively (and not, say,
  by bulk
• motions, e.g. convection).
• Disk is supported vertically against tidal field
  of black
• hole by gas and radiation pressure only.

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BHSPEC Does a Pretty Good Job With Black Hole
X-ray Binaries
-McClintock, Narayan Shafee (2007)
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LMC X-3 in the thermal dominant state - there is
NO significant corona!
RXTE
BeppoSAX
-Davis, Done, Blaes (2005)
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Thermodynamically consistent, radiation
MHD simulations in vertically stratified shearing
boxes
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Convergence???
(But magnetic Prandtl number 1)
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Does the stress prescription matter?
Disk-integrated spectrum for Schwarzschild, M10
M?, L/Ledd0.1, i70???and ?0.1 and 0.01.
-Davis et al. 2005
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Azimuthal Flux Reversals
PradltltPgas
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3D visualization of tension/density fluctuation co
rrelation due to Parker instability.
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Time Averaged Vertical Energy Transport
Radiation Diffusion
Advection of radiation
Poynting Flux
Advection of gas internal energy
PradgtgtPgas
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The (Numerical!) Dissipation Profile is Very
Robust Across All Simulations
PradgtgtPgas
PradPgas, PradltltPgas, Turner (2004)
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CVI K-edge
i55?
-Blaes et al. (2006)
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Time and Horizontally Averaged Acceleration
Profiles
g/Total Magnetic Radiation Pressure Gas Pressure
PradgtgtPgas
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CVI K-edge
With magnetic fields
No magnetic fields
18 increase in color temperature
-Blaes et al. (2006)
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Large Density Fluctuations at Effective and
Scattering Photospheres
-upper effective photosphere at t200 orbits in
PradgtgtPgas simulation.
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Photospheric Density Fluctuations
Strong density fluctuations, at both scattering
and effective photospheres.
Strong fluctuations also seen at
effective photosphere in previous simulations
with PgasgtgtPrad and PradPgas.
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Effects of Inhomogeneities 3D vs. Horizontally
Averaged Atmospheres
Flux enhancements in 3D imply decreases in color
temperatures compared to 1D atmosphere models
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6
11
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Faraday Depolarization
Magnetic fields in disk atmospheres might be
strong enough to cause significant Faraday
rotation of polarized photons (Gnedin Silantev
1978)
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Effects of Faraday Depolarization
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Summary The Vertical Structure of Disks

• Hydrostatic balance Disks are supported by
  thermal
• pressure near the midplane, but by magnetic
  forces in
• the outer (but still subphotospheric layers).
• Thermal balance Dissipation (numerical) occurs
  at great
• depth, and accretion power is transported
  outward largel
• by radiative diffusion. There is no locally
  generated corona,
• in agreement with observations!
• Stability There is no radiation pressure
  driven thermal
• instability, in agreement with observations!

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Implications of Simulation Data on Spectra

• Actual stress (alpha) and vertical dissipation
  profiles
• are irrelevant, provided disk remains
  effectively thick.
• Magnetically supported upper layers decrease
  density at
• effective photosphere, producing a (20)
  hardening of
• the spectrum.
• Strong density inhomogeneities at photosphere
  produce
• a (10) softening of the spectrum.
• Polarization is reduced only slightly by
  photospheric
• inhomogeneities, and is Faraday depolarized
  only below
• the peak - a possible diagnostic for accretion
  disk B-fields
• with X-ray polarimeters???

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Vertical Hydrostatic Balance
t 200 orbits
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Time-Averaged Vertical Dissipation Profile
Most of the dissipation is concentrated near
midplane.
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Turbulence near Midplane is Incompressible -----Si
lk Damping is Negligible