1. Electron thermalization and emission from compact magnetized sources
Indrek Vurm and Juri Poutanen
University of Oulu, Finland

2. Spectra of accreting black holes
Hard state
Thermal Comptonization
Weak
non-thermal tail
Soft state
Dominant disk blackbody
Non-thermal tail extending to a few MeV
Zdziarski et al. 2002

3. Spectra of accreting black holes
Cygnus X-1
Hard state
Thermal Comptonization
Weak non-thermal tail
Soft state
Dominant disk blackbody
Non-thermal tail extending to a few MeV
keV
Zdziarski & Gierlinski 2004

4. Electron distribution
Why electrons are (mostly) thermal in the hard state?
Why electrons are (mostly) non-thermal in the soft state?
Spectral transitions can be explained if electrons are heated in HS, and accelerated in SS (Poutanen & Coppi 1998).
What is the thermalization?
Coulomb - not efficient
synchrotron self-absorption?

5. Cooling vs. escape lrad > 1 and/or lB > 1 Compton scattering:
Synchrotron radiation:
Luminosity
compactness:
Magnetic compactness: R
Cooling is always faster than escape if
lrad > 1 and/or lB > 1
Vesc

6. Thermalization by Coulomb collisions
Cooling
Rate of energy exchange with a low energy thermal pool of electrons by Coulomb collisions:
Thermalization happens only at very low energies:
In compact sources, Coulomb thermalization is not efficient!

7. Synchrotron self-absorption
Katarzynski et al., 2006
Assume power-law e– distribution:
Electron heating in self-absorption (SA) regime:
Nonrelativistic limit
Relativistic limit
Electron cooling
Ratio of heating and cooling in SA relativistic regime:
At low energies heating always dominates

8. Synchrotron self-absorption
Efficient thermalizing mechanism.
Time-scale = synchrotron cooling time
Ghisellini, Haardt, Svensson 1998

9. Numerical simulations
Synchrotron boiler (Ghisellini, Guilbert, Svensson 1988):
synchrotron emission and thermalization by synchrotron self-absorption (SSA), electron equation only, self-consistent
Ghisellini, Haardt, Svensson (1998)
synchrotron and Compton cooling, SSA thermalization
not fully self-consistent (only electron equation solved)
EQPAIR (Coppi):
Compton scattering, pair production, bremsstrahlung, Coulomb thermalization; self-consistent, but electron thermal pool at low energies
Large Particle Monte Carlo (Stern):
Compton scattering, pair production, SSA thermalization; self-consistent, but numerical problems because of SSA

10. Our code One-zone, isotropic particle distributions, tangled B-field
Processes:
Compton scattering:
exact Klein-Nishina scattering cross-sections for all particles
diffusion limit at low energies
synchrotron radiation: exact emissivity/absorption for photons and heating/cooling (thermalization) for pairs.
pair-production, exact rates
Time-dependent, coupled kinetic equations for electrons, positrons and photons.
Contain both integral and differential terms
Discretized on energy and time grids and solved iteratively as a set of coupled systems of linear algebraic equations
Exact energy conservation.

11. Variable injection slope3 4
inj=2
ELECTRONS 3 4
PHOTONS

12. Variable luminosity
ELECTRONS
L=1036 erg/s
PHOTONS
L=1036 erg/s

13. Variable luminosity PHOTONS L=1036 erg/s XTE J1550–564 GRS 1915+105
GX 339-4
GRS
XTE J1550–564
Cyg X-3

14. Role of magnetic field
ELECTRONS
PHOTONS 5 10

15. Role of the external disk photons
ELECTRONS
Ldisk/Linj=10
PHOTONS
0.1 1

16. Role of the external disk photons
0.1 1

17. Conclusions
Hard injection produces too soft spectra (due to strong synchrotron emission) inconsistent with hard state of GBHs.
Hard state spectra of GBHs = synchrotron self-Compton, no feedback or contribution from the disk is needed.
At high L, the spectrum is close to saturated Comptonization peaking at ~5 keV, similar to thermal bump in the very high state.
Spectral state transitions can be a result of variation of the ratio of disk luminosity and power dissipated in the hot flow. Our self-consistent simulations show that the electron distribution in this case changes from nearly thermal in the hard state to nearly non-thermal in the soft state.