Pair cascades in extragalactic jets. 1: Gamma rays

A model of the 0.1-10 GeV γ-ray jets observed by the EGRET instrument on the Compton Gamma Ray Observatory is developed. It is shown that the soft X-ray background in an AGN contributes an opacity to pair production and that a γ-ray photosphere or "γ-sphere" can be defined whose radius increases with γ-ray energy Egamma_. It is proposed that the observed γ-ray emission is due to inverse Compton scattering of the ambient soft X-rays by relativistic pairs accelerated in situ by shock fronts in a relativistic jet. For a wide range of assumed physical conditions, the emission at a given Egamma_ originates from near the associated γ-spheres; emission from below the γ-sphere initiates a cascade down to the energy where the γ-rays can escape freely. In this model, the slope of the emergent γ-ray spectrum is determined by the scattered, soft X-ray spectrum and the variation of the particle acceleration rate with jet radius. In general it is expected that the variation in the γ-ray flux will be either slower or later at higher energy. It is also shown that the efficiency of conversion of energy from injected high-energy pairs to 0.1-10 GeV γ-rays is typically high so that the models are radiatively efficient. It is argued that the observed γ-ray jets are likely to be particle-dominated, though magnetically confined. The γ-ray spectrum should continue down to an energy ~5 MeV emitted from an annihilation radius within which the pair content of the jet is limited by annihilation. This is probably the site of the beamed hard X- ray emission. It is speculated that the relativistic jets associated with radio-loud AGNs are powered electromagnetically by a spinning black hole and that they are collimated by an encircling MHD wind leaving the accretion disk at a slower speed. Powerful FR2 radio sources are formed when the hole spins rapidly and the relativistic core accelerates the MHD sheath; low-power FRI sources ensue when the opposite occurs. Finally, it is suggested that the key factor which determines whether or not a given active nucleus can form a jet and a radio to γ-ray nonthermal continuum is the central density of mass-losing stars which, when large, precludes the formation of a super-Alfvenic, collimating wind.