Electron thermalization and emission from compact magnetized sources

1. Electron thermalizationand 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 • 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. Katarzynski et al., 2006 Synchrotron self-absorption • 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. PHOTONS ELECTRONS inj=2 3 4 4 3 Variable injection slope

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

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

14. PHOTONS ELECTRONS 10 5 Role of magnetic field

15. PHOTONS ELECTRONS 1 0 0.1 0 Ldisk/Linj=10 Role of the external disk photons

16. PHOTONS 1 0.1 0 Role of the external disk photons 0

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.