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

(from S. Liu et al )

High-energy emission of black hole SgrA* examplifies low-luminosity

accretion which requires energization above the level predicted by conventional thermal SSC model

weakly magnetized initial torus

MRI-induced accretion flow withsaturated MHD turbulence

compressionalheating of ions

coulomb heating of electrons by virial ions

thermal cyclotronemission at low energy

SSC + EC emission at high energy

turbulence energization ofnonthermal electrons and ions

synchrotron emission bynonthermal electrons

pion decay emission of Nonthermal ions

SSC+EC of nonthermal electrons

thermaldisk paradigm

new approach

B2 density

256x256

t=2002

MRI-induced flow from global GRMHD simulations

256x256 512x512

B2

t=914

Extend turbulence spectrum by increasing resolution

256x256 512x512

density

t=914

Based on current parallelism, it is difficult to make longGRMHD 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 codewith zones ≤ Debye length. ( >1012 zones in 2D).

Two approaches:1.Extrapolate turbulence spectrum to subgrid scales as powerlaw and solve Fokker-Planck equation for wave-particleinteraction

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 sinceMC photon time steps >> electron evolution timeand MC is fully parallel by itself.

MC photon transport

Sample output of MC-FP code with wave spectrum ~ k-5/3

electron spectra photon spectra

(from Boettcherand Liang2002)

Polar grid of General Relativistic MHD simulation output is mapped ontothe cylindrical grid of Monte Carlo photon transport

B2density

Hard tailwould requirenonthermal

acceleration ofelectrons/ions

by MHDturbulence

above thermalheating

synchrotron peak

bremsstrahlung peak

Sample spectrum from 2D MC code with GRMHD results as input (at high density so that bremsstrahlung dominates over Compton

and without turbulence 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.

sample input: magnetosonic waves with =1024c/pe

and B2/4c2 = 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 byEM turbulence.

2. We propose to study such energization using turbulenceself-generated in MRI - induced accretion flows.

3. We will use both FP and implicit PIC codes to studydissipation of EM turbulence at the sub-grid scale.

4. We propose to couple the resultant electron spectra to MCphoton transport.