Hybrid thermal-nonthermal synchrotron emission from hot accretion flows

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132 Scopus citations


We investigate the effect of a hybrid electron population, consisting of both thermal and nonthermal particles, on the synchrotron spectrum, image size, and image shape of a hot accretion flow onto a supermassive black hole. We find two universal features in the emitted synchrotron spectrum: (1) a prominent shoulder at low (≲1011 Hz) frequencies that is weakly dependent on the shape of the electron energy distribution, and (2) an extended tail of emission at high (≳1013 Hz) frequencies whose spectral slope depends on the slope of the power-law energy distribution of the electrons. In the low-frequency shoulder, the luminosity can be up to 2 orders of magnitude greater than with a purely thermal plasma even if only a small fraction (<1%) of the steady state electron energy is in the nonthermal electrons. We apply the hybrid model to the Galactic center source, Sgr A*. The observed radio and IR spectra imply that at most 1% of the steady state electron energy is present in a power-law tail in this source. This corresponds to no more than 10% of the electron energy injected into the nonthermal electrons and hence 90% into the thermal electrons. We show that such a hybrid distribution can be sustained in the flow because thermalization via Coulomb collisions and synchrotron self-absorption are both inefficient. The presence of nonthermal electrons enlarges the size of the radio image at low frequencies and alters the frequency dependence of the brightness temperature. A purely thermal electron distributions produces a sharp-edged image, while a hybrid distribution causes strong limb brightening. These effects can be seen up to frequencies ∼1011 Hz and are accessible to radio interferometers.

Original languageEnglish (US)
Pages (from-to)234-249
Number of pages16
JournalAstrophysical Journal
Issue number1 PART 1
StatePublished - Sep 20 2000


  • Accretion, accretion disks
  • Black hole physics
  • Galaxy: center
  • Radiation mechanisms: nonthermal
  • Radiation mechanisms: thermal

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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