GAMMA-RAY EMISSION AND SPECTRAL EVOLUTION OF PAIR PLASMAS IN AGN JETS.1. GENERAL-THEORY AND A PREDICTION FOR THE GEV-TEV EMISSION FROM ULTRARELATIVISTIC JETS

The evolution of the particle distribution functions inside a relativi stic jet containing an e(-)-e(+) pair plasma and of the resulting gamm a-ray and X-ray spectra is investigated. The first phase of this evolu tion is governed by heavy radiative energy losses. For this phase, app roximative expressions for the energy-loss rates due to inverse-Compto n scattering, using the full Klein-Nishina cross section, are given as one-dimensional integrals for both cooling by inverse-Compton scatter ing of synchrotron photons (SSC) and of accretion disk photons (EIC). We calculate instantaneous and time-integrated gamma-ray spectra emitt ed by such a jet for various sets of parameters, deducing some general implications on the observable broadband radiation. Finally, we prese nt model fits to the broadband spectrum of Mrk 421. We find that the m ost plausible way to explain both the quiescent and the flaring state of Mrk 421 consists of a model where EIC and SSC dominate the observed spectrum in different frequency bands. In our model the flaring state is primarily related to an increase of the maximum Lorentz factor of the injected pairs.