The Hard X‐Ray to Gamma‐Ray Spectrum in the EGRET Active Galactic Nuclei

EGRET (20 MeV to 30 GeV) on board the Compton GRO has observed high-energyemission from about 40-50 Active Galactic Nuclei. Theoretical models of thisemission based on the upscattering of thermal disk photons by cooling,relativistic electrons can successfully account for the EGRET observations, butthey predict a considerably greater X-ray flux than that actually observed in amajority of these sources. This inconsistency may be an indication that theparticles are energized during the Compton scattering process, since the X-rayemission is produced by the lowest energy electrons, whose density may berelatively small due to the acceleration. Such a situation may arise as aresult of resistive field generation in electromagnetic acceleration schemes,which we here explore. A key feature of this model is the assumed existence ofa current associated with the azimuthal component B_phi of the underlyingmagnetic field by a slight imbalance in the energy distributions of outwardlymoving, relativistic electrons and protons produced at the disk surface viashock acceleration. The generation of an electric field (via magnetic fieldline reconnection) is thus required to maintain the current in the presence ofa resistivity induced by the radiative drag on the relativistic electrons. Weshow that the resulting spectrum can exhibit a significant deficit of X-rayscompared with gamma-rays. In addition, due to the uni-directional flow of thecurrent associated with B_phi, this model would predict that the electrons areenergized 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 anticipatethat any given blazar should have a ~50% probability of being gamma-bright,which appears to be consistent with the observed ratio.Comment: 21 pages (Latex), 4 figures (ps), accepted for publication in Ap