The hard X-ray to gamma-ray spectrum in the egret active galactic nuclei

Marco Fatuzzo, Fulvio Melia

Research output: Contribution to journalArticlepeer-review


EGRET (20 MeV-30 GeV), on board the Compton Gamma Ray Observatory, has observed high-energy emission from about ∼ 40-50 active galactic nuclei. Theoretical models of this emission based on the upscattering of thermal disk photons by cooling, relativistic electrons can successfully account for the EGRET observations, but they predict a considerably greater X-ray flux than that actually observed in a majority of these sources. This inconsistency may be an indication that the particles are energized during the Compton scattering process, since the X-ray emission is produced by the lowest energy electrons, whose density may be relatively small because of the acceleration. Such a situation may arise as a result of resistive field generation in electromagnetic acceleration schemes, which we here explore. A key feature of this model is the assumed existence of a current associated with the azimuthal component Bψ of the underlying magnetic field by a slight imbalance in the energy distributions of outwardly moving, relativistic electrons and protons produced at the disk surface via shock acceleration. The generation of an electric field (via magnetic field line reconnection) is thus required to maintain the current in the presence of a resistivity induced by the radiative drag on the relativistic electrons. We show that the resulting spectrum can exhibit a significant deficit of X-rays compared with gamma rays. In addition, because of the unidirectional flow of the current associated with Bψ, this model would predict that the electrons are energized relative to the protons in the outflow only on one side of the disk. They should be decelerated on the reverse side. As such, we would anticipate that any given blazar should have a ∼50% probability of being gamma-ray-bright, which appears to be consistent with the observed ratio.

Original languageEnglish (US)
Pages (from-to)601-607
Number of pages7
JournalAstrophysical Journal
Issue number2 PART II
StatePublished - 1998


  • Acceleration of particles
  • Galaxies: active
  • Gamma rays: observations
  • MHD
  • Radiation mechanisms: nonthermal
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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