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Thermodynamics and Spectra of Optically Thick Accretion Disks. Omer Blaes, UCSB. With Shane Davis, Shigenobu Hirose and Julian Krolik. Standard Disks are Observed to be Simple And Stable. E.g. Cyg X-1 (Churazov et al. 2001):. Plenty of X-ray Binaries Get to High Eddington Ratios,
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Thermodynamics and Spectra of Optically Thick Accretion Disks Omer Blaes, UCSB With Shane Davis, Shigenobu Hirose and Julian Krolik
Standard Disks are Observed to be Simple And Stable E.g. Cyg X-1 (Churazov et al. 2001):
Plenty of X-ray Binaries Get to High Eddington Ratios, And Do NOT Show Signs of Putative Thermal Instability
Except Perhaps GRS 1915+105? -Belloni et al. (1997)
Black HoleDisk Models AGNSPEC & BHSPEC -Hubeny & Hubeny 1997, 1998; Hubeny et al. (2000, 2001), Davis & Hubeny (2006), Hui & Krolik (2008)
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.)
The Bad --- Ad Hoc Assumptions: • Stationary, with no torque inner boundary condition. • RPtot 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.
BHSPEC Does a Pretty Good Job With Black Hole X-ray Binaries -McClintock, Narayan & Shafee (2007)
LMC X-3 in the thermal dominant state - there is NO significant corona! BeppoSAX RXTE -Davis, Done, & Blaes (2005)
Thermodynamically consistent, radiation MHD simulations in vertically stratified shearing boxes:
Convergence??? (But magnetic Prandtl number ~ 1)
Does the stress prescription matter? Disk-integrated spectrum for Schwarzschild, M=10 M, L/Ledd=0.1, i=70and =0.1 and 0.01. -Davis et al. 2005
Azimuthal Flux Reversals Prad<<Pgas
3D visualization of tension/density fluctuation correlation due to Parker instability.
Time Averaged Vertical Energy Transport Radiation Diffusion Advection of radiation Poynting Flux Advection of gas internal energy Prad>>Pgas
The (Numerical!) Dissipation Profile is Very Robust Across All Simulations Prad>>Pgas Prad~Pgas, Prad<<Pgas, Turner (2004)
CVI K-edge i=55 -Blaes et al. (2006)
Time and Horizontally Averaged Acceleration Profiles g/Total Magnetic Radiation Pressure Gas Pressure Prad>>Pgas
CVI K-edge With magnetic fields No magnetic fields ~18% increase in color temperature -Blaes et al. (2006)
Large Density Fluctuations at Effective and Scattering Photospheres -upper effective photosphere at t=200 orbits in Prad>>Pgas simulation.
Photospheric Density Fluctuations Strong density fluctuations, at both scattering and effective photospheres. Strong fluctuations also seen at effective photosphere in previous simulations with Pgas>>Prad and Prad~Pgas.
Prad<<Pgas (60 orbits) Prad~Pgas (90 orbits) Prad>>Pgas (200 orbits) Effects of Inhomogeneities: 3D vs. Horizontally Averaged Atmospheres Flux enhancements in 3D imply decreases in color temperatures compared to 1D atmosphere models: 9% 6% 11%
Faraday Depolarization Magnetic fields in disk atmospheres might be strong enough to cause significant Faraday rotation of polarized photons (Gnedin & Silant’ev 1978):
Prad<<Pgas (60 orbits) Prad~Pgas (90 orbits) Prad>>Pgas (200 orbits) Effects ofFaraday Depolarization
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!
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???
Vertical Hydrostatic Balance t = 200 orbits
Time-Averaged Vertical Dissipation Profile Most of the dissipation is concentrated near midplane.
Turbulence near Midplane is Incompressible -----Silk Damping is Negligible