Electron Energization and Radiation in MRI-driven Accretion

Edison Liang, Rice University

Collaborators: G. Hilburn, Rice; S. Liu, H. Li, LANL; C. Gammie, UI; M. Boettcher, Ohio U.

DPP talk November 2008

(from S. Liu et al )

High-energy emission of LLBH such as SgrA* examplifiesaccretion which requires electron energization above the level predicted by

e-ion coulomb coupling

flare

quiescent

Motivation

In low-luminosity black holes (LLBH) such as Sgr A*, Coulomb heating by ions is too inefficient due to low density

Can MRI-driven turbulence directly heat relativistic electrons?

We find that wave turbulence alone provides only modest heating. But anomalous heating by thin current sheets, leads to a much hotter superthermal component.

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

thermal MRIdisk models

new approach

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512x512HARM

GRMHD Run

B2

512x512 HARM code runs 256x256

current sheets get thinner with increasing resolution but pattern maintains self-similarity

B2

B

Br

B

folded current sheets are a dominant feature of MRI-driven turbulence

| J |

ye/c

xe/c

Bz in

Bz out

Bz in

Jx

Jx

2.5 D PIC 1024x1024 doubly periodic grid, ~108 particles, mi=100me

Ti=0.25mec2

Te=0.25mec2

or 1.5mec2

Bx,y=Bosink(y,x)

single mode kL=4Te=1.5mec2 Bz=10Bo e=5 pe

current sheet thickens and bends due to wave perturbations

te=0 1000 4000

Bz

Bx

te=1000 4000

By

Jx

te=1000 4000

Jz

Magnetic energy is efficiently converted to hot electrons due to enhanced reconnection

Eem

Eparticle

EBz

Ee

Eion

EBxyEEte

te

Electrons are heated to form a 2-component Maxwellian:Lower-T component heated by Alfven wave cascadeHigher-T component heated by current sheet dissipation

fe()

te=4000

Te1~2MeV

Te2~10MeV

Results are in agreement with those of Zenitani and Hoshino (2005)

ions are also heated, likely by electrostatic modes

te=1000

4000

0

ion

fi()

Te=0.25mec2

Te=1.5mec2

fe()

fe()

fe()

fe()

No CS

No CS

CS

CS

MC photon transport

HARM to MC grid

The HARM code's grid is divided logarithmically in r and concentrated to the equatorial plane

The MC grid is divided equally in r and in z

Overlaying these two invariably leads to under- or oversampling in areas

synchrotron

Photon spectrum using HARM output as input for the 2D-MC code (95x95 grid) with density normalized by the Chandra

flare as due to bremsstrahlung. Note that our Compton humpis lower than the result of Ohsuga et al 2005.

bremsstrahlungCompton

Ohsuga et al 2005

Summary

1. Many LLBH exhibit superthermal/nonthermal spectra that require anomalous heating of electrons.

2. We explore such energization using MHD turbulenceself-generated in MRI - induced accretion flows.

3. PIC simulations suggest that current sheet dissipation is the dominant heating mechanism, producing a 2-componentelectron spectrum.

4. It would be very interesting to see if the x-ray spectra duringflares and quiescence can be explained by current sheet heating of electrons.

Above results seem to be insensitive to the initial electron temperature: Te=0.25mec2 gives roughly the same results: 2/3 electrons, 1/3 ions

te te

By

x y

Initial single mode cascades into higher and higher modes via parametric conversion

Bx