Radiation Processes in Blazars

We present an overview of the current theoretical models attempting to describe the structure and radiative processes operating in blazars and discuss the observational constraints that these models must confront. Many of these objects are found by CGRO to be strong high energy γ–ray sources. Their spectra consist of two broad components: the low-energy component, peaking between the infrared and soft X–ray band, and the high-energy component, peaking in the high energy γ–rays. The overall energetics is often dominated by the latter, by as much as an order of magnitude over the former. Both components show rapid large-amplitude variability, which indicates a very compact emission region. Most current models for the structure of blazars, to avoid the problems with excessive opacity of γ–rays to pair production, invoke beaming of electromagnetic emission from the radiating matter moving in a jet pointed close to the line of sight towards the observer, in an analogy to the jet-like structure inferred from other observations. The non-thermal shape of the spectrum of the lowenergy component as well as its polarization suggest synchrotron emission. For the high-energy component, most current models invoke Comptonization of the lower energy photons, either those internal to the jet, as in synchrotron selfCompton (SSC) models, or external to the jet (UV radiation from the accretion disk or from the emission-line region, or IR radiation from dust), as in External Radiation Compton (ERC) models. Comparison of the energy densities of the external radiation fields with the energy density of the synchrotron radiation field (both as measured in the jet comoving frame) suggests that while the SSC model may be adequate to explain the γ–ray emission for BL Lac objects, the ERC models are probably more applicable for flat spectrum radio-quasars (FSRQ).