1. 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

2. High-energy emission of LLBH such as SgrA* examplifies
accretion which requires electron energization above the level predicted by
e-ion coulomb coupling
flare
quiescent
(from S. Liu et al )

3. 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.

4. weakly magnetized initial torus
MRI-induced accretion flow with
saturated MHD turbulence
thermal MRI
disk models
new approach
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

5. 512x512
HARM
GRMHD
Run
B 

6. current sheets get thinner with increasing resolution
but pattern maintains self-similarity B2
512x512 HARM code runs
256x256

7. folded current sheets are a dominant feature of MRI-driven turbulenceBr B
| J |

8. 2.5 D PIC 1024x1024 doubly periodic grid, ~108 particles, mi=100me
Bx,y=Bosink(y,x)
Bz in Jx
Ti=0.25mec2
Te=0.25mec2
or 1.5mec2
ywe/c
Bz out Jx
Bz in
xwe/c

9. current sheet thickens and bends due to wave perturbations
single mode kL=4p Te=1.5mec2 Bz=10Bo We=5w pe
current sheet thickens and bends due to wave perturbations
twe= Bz

10. Bx
twe= By

11. Jx
twe= Jz

12. Magnetic energy is efficiently converted to hot electrons due to
enhanced reconnection
Eem
EBz
Eparticle Ee
Eion EE
twe
twe
EBxy

13. Electrons are heated to form a 2-component Maxwellian:
Lower-T component heated by Alfven wave cascade
Higher-T component heated by current sheet dissipation
fe(g)
Te1~2MeV
Te2~10MeV g g
twe=4000
Results are in agreement with those of Zenitani and Hoshino (2005)

14. ions are also heated, likely by electrostatic modes
fi(g)
4000
twe=1000
ion

15. No CS
fe(g) CS
fe(g)
Te=0.25mec2 g g
fe(g)
No CS CS
fe(g)
Te=1.5mec2 g g

16. MC photon transport

17. 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

18. 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 hump
is lower than the result of Ohsuga et al 2005.
synchrotron
Compton
bremsstrahlung
Ohsuga et al 2005

19. Summary
Many LLBH exhibit superthermal/nonthermal spectra that require anomalous heating of electrons.
We explore such energization using MHD turbulence
self-generated in MRI - induced accretion flows.
PIC simulations suggest that current sheet dissipation is the
dominant heating mechanism, producing a 2-component
electron spectrum.
It would be very interesting to see if the x-ray spectra during
flares and quiescence can be explained by current sheet
heating of electrons.

20. Te=0.25mec2 gives roughly the same results: 2/3 electrons, 1/3 ions
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
twe twe

21. Initial single mode cascades into higher and higher modes
via parametric conversion By Bx x y