Hybrid thermal-nonthermal synchrotron emission from hot accretion flows

Research output: Contribution to journalArticle

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Abstract

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
Volume541
Issue number1 PART 1
StatePublished - Sep 20 2000
Externally publishedYes

Fingerprint

synchrotrons
accretion
electron
electron energy
electrons
shoulders
energy distribution
limb brightening
radio interferometers
slopes
low frequencies
Coulomb collisions
radio spectra
energy
self absorption
radio
thermal plasmas
brightness temperature
electron distribution
power law

Keywords

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

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

Hybrid thermal-nonthermal synchrotron emission from hot accretion flows. / Ozel, Feryal; Psaltis, Dimitrios; Narayan, Ramesh.

In: Astrophysical Journal, Vol. 541, No. 1 PART 1, 20.09.2000, p. 234-249.

Research output: Contribution to journalArticle

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N2 - 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.

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