High Energy Gamma-Ray Emission from Blazars: Egret Observations

Abstract

We will present a summary of the observations of blazars by the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO). EGRET has detected high energy γ-ray emission at energies greater than 100 MeV from more that 50 blazars. These sources show inferred isotropic luminosities as large as 3 × 10⁴⁹ ergs s^-1. One of the most remarkable characteristics of the EGRET observations is that the γ-ray luminosity often dominates the bolometric power of the blazar. A few of the blazars are seen to exhibit variability on very short time-scales of one day or less. The combination of high luminosities and time variations seen in the γ-ray data indicate that γ-rays are an important component of the relativistic jet thought to characterize blazars. Currently most models for blazars involve a beaming scenario. In leptonic models, where electrons are the primary accelerated particles, γ-ray emission is believed to be due to inverse Compton scattering of low energy photons, although opinions differ as to the source of the soft photons. Hardronic models involve secondary production or photomeson production followed by pair cascades, and predict associated neutrino production.

Invited review papter to appear in Observational Evidence for Black Holes in the Universe, ed. S.K. Chakarabarti (Dordrecht: Kluwer)