Turbulence Heating and Nonthermal Radiation
From MRI-induced Accretion onto Low-Luminosity
Black Holes
E.Liang, G.Hilburn, S.M.Liu, H. Li, C. Gammie, M. Boettcher
Presentation at the 2007 APS/DPP Meeting in Orlando
Work partially supported by NSF, NASA, LANL
High-energy emission of black hole SgrA* examplifies low-luminosity
accretion which requires
energization
above shape
the leveland
predicted
by conventional
high-energy
spectral
help to
discriminate
thermal SSC model
models.
Integral
(from S. Liu et al )
weakly magnetized initial torus
MRI-induced accretion flow with
saturated MHD turbulence
new approach
thermal
disk paradigm
compressional
heating of ions
turbulence energization of
nonthermal electrons and ions
coulomb heating of
electrons by virial ions
synchrotron emission by
nonthermal electrons
thermal cyclotron
emission at low energy
SSC+EC of nonthermal
electrons
SSC + EC emission at
high energy
pion decay emission of
Nonthermal ions
MRI-induced flow from global GRMHD simulations
B2
density
256x256
t=2002
Extend turbulence spectrum by increasing resolution
256x256
512x512
B2
t=914
256x256
512x512
density
t=914
Based on current parallelism, it is difficult to make long
GRMHD runs using much larger than 1000x1000 grid.
This still leaves each MHD zone > 106 Debye length.
How can we tackle the subgrid microphysics?
Impractical to simulate dissipation with explicit PIC code
with zones ≤ Debye length. ( >1012 zones in 2D).
Two approaches:
1.Extrapolate turbulence spectrum to subgrid scales as power
law and solve Fokker-Planck equation for wave-particle
interaction
2. Use implicit PIC code with large zones (>> Debye length)
and large time steps.
We will employ both methods and compare their results
Once the electron spectrum for each zone is
obtained, we can couple it to our 2D Monte Carlo
(MC) photon transport code via implicit schemes.
This part of computation is easily parallelized since
MC photon time steps >> electron evolution time
and MC is fully parallel by itself.
MC photon transport
Sample output of MC-FP code with wave spectrum ~ d2k-5/3
electron spectra
(from
Boettcher
and Liang
2002)
photon spectra
o discriminate between leptonic and hadronic
Polar grid of General Relativistic MHD simulation output is mapped onto
the cylindrical grid of Monte Carlo photon transport
B2
density
Sample spectrum from 2D MC code with GRMHD results as input
(at high density so that bremsstrahlung dominates over Compton
and without turbulence heating)
synchrotron
peak
bremsstrahlung
peak
Hard tail
would require
nonthermal
acceleration of
electrons/ions
by MHD
turbulence
above thermal
heating
PIC simulation of turbulence cascade converts EM energy into particle
energy and formation of power-law in both e+e- and e-ion plasmas.
e+eEem
f()
f()
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Graphics decompressor
are needed to see this picture.
ion
QuickTime™ anda
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are needed to seethis picture.
Eparticle
tpe/3

sample input: magnetosonic waves with l=1024c/pe
and dB2/4prc2 = 100

Development of current instability is key to the cascade
of EM turbulence to smaller and smaller scales
Summary
1. Many BH exhibit nonthermal hard spectra that strongly
suggest nonthermal energization of electrons/ions by
EM turbulence.
2. We propose to study such energization using turbulence
self-generated in MRI - induced accretion flows.
3. We will use both FP and implicit PIC codes to study
dissipation of EM turbulence at the sub-grid scale.
4. We propose to couple the resultant electron spectra to MC
photon transport.