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
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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
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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?

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Cooling vs. escape

• Compton scattering
• Synchrotron radiation

Luminosity compactness
Magnetic compactness
R
Cooling is always faster than escape if lrad
gt 1 and/or lB gt 1
Vesc
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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!

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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
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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

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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.

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Variable injection slope
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Variable luminosity
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Variable luminosity
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Role of magnetic field
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Role of the external disk photons
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Role of the external disk photons
0
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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.