understanding thermal stability of radiation-dominated disks and radiative efficiency of global...
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![Page 1: Understanding Thermal Stability of Radiation-Dominated Disks and Radiative Efficiency of Global Relativistic Disks with Omer Blaes, Shigenobu Hirose Scott](https://reader030.vdocuments.us/reader030/viewer/2022032704/56649d445503460f94a21581/html5/thumbnails/1.jpg)
Understanding Thermal Stability of Radiation-Dominated Disks
and
Radiative Efficiency of Global Relativistic Disks
withOmer Blaes, Shigenobu Hirose
Scott Noble, John Hawley, Kris Beckwith
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Understanding Thermal Stability of Radiation-Dominated Disks
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Radiation-Dominance Is the Natural State of the Interesting Portions of Bright Disks
r=rg < 170(L=LE )16=21(M=M ¯ )2=21
Radiation pressure exceeds gas pressure for
That is, for the most interesting parts of all bright accretion disks around black holes
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Yet – Model Predicts Thermal Instability When pr > pg
$\int dz Q \propto p_r h$$
Shakura & Sunyaev 1976
When radiation pressure dominates, h / F =
ZdzQ
And pr » Qtcool » Q(h=c)¿ » (¿=c)ZdzQ
Thermal Instability
Energy conservation gives
ZdzQ »
ZdzTrÁ
The model asserts
ZdzTr Á »
Zdzp
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Dissipation and Pressure Are Correlated
So why doesn’t the thermal instability take place ?
![Page 6: Understanding Thermal Stability of Radiation-Dominated Disks and Radiative Efficiency of Global Relativistic Disks with Omer Blaes, Shigenobu Hirose Scott](https://reader030.vdocuments.us/reader030/viewer/2022032704/56649d445503460f94a21581/html5/thumbnails/6.jpg)
What Does Dimensional Analysis Really Imply?
Orbital shear does work on magnetic field, magnetic field dissipates, heat becomes radiation----so magnetic energy and stress drive the pressure, not the other way around!
Pressure and stress are comparable, but does that mean pressure controls stress?
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Evidence from Simulation Data
Magnetic Energy vs. Radiation Energy
Magnetic leads Radiation leads
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Explore with Toy Model
In dimensionless form,
dEBd¿ = R(t)EnR ¡ EB
dERd¿ =
EB 0ER0
¡EB ¡ E1¡ sR
¢
dEBdt =R(t)
EB 0tgrowth
µERER0
¶n¡EBtdiss
dERdt =
EBtdiss
¡ERtcool
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Results Strongly Resemble Simulations
Without any intrinsic pressure-stress correlation: n=s=0
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Including a Pressure/Magnetic Energy Correlation --- After the Fact
EB / E1¡ sR
Thermal balance means
Independent of n
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Which Variables Really Control the Stress?
As suggested by the structure of shearing-box simulations, and are the truly fundamental variables wherever the inflow time is the longest timescale.
Magnetic field intensity, and secondarily, the pressure, follow, with dissipation of magnetic energy driving the pressure, as regulated by the radiative loss rate.
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Radiative Efficiency of Global Relativistic Disks
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Origin of Traditional Efficiency Numbers:the Novikov-Thorne model
• Full GR• Time-steady, axisymmetric, vertically-integrated• Energy and angular momentum conservation• Boundary conditions—
energy: prompt radiation carries off dissipation
angular momentum: zero-stress at ISCO
= ut(ISCO)
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MHD Stresses Don’t Know to Stop at the ISCO
(Thorne 1974): “In the words of my referee, James M. Bardeen (which echo verbal warnings that I have received from Ya. B. Zel’dovich and V.F. Schwartzman), ‘It seems quite possible that magnetic stresses could cause large deviations from circular orbits in the very inner part of the accretion disk….’”
It follows that the Novikov-Thorne radiative efficiency numbers may not be the last word when magnetic stresses are important.
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Numerical Procedure
• Extend HARM (GR/MHD, total-energy, conservative) from 2-d axisymmetric to 3-d
• Introduce toy-model optically thin cooling function: (1) rapidly radiates (almost) all the heat generated (2) allows aspect ratio regulation
L = ½²·
²(H )2
¡ 1+ j²
(H )2¡ 1j
¸1=2
r ºT º¹ = ¡ Lu¹
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A First Result
a/M = 0.9
H/r = 0.1
T = 15000 GM/c3
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Surface Brightness in the Fluid Frameaveraged over 10000—12000M
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Preliminary Summary
• There is noticeable radiation beyond N-T• Dependence on H/r, a/M to be explored