Transformation Properties of External Radiation Fields, Energy‐Loss Rates and Scattered Spectra, and a Model for Blazar Variability

Abstract

We treat transformation properties of external radiation fields in the proper frame of a plasma moving with constant speed. The specific spectral energy densities of external isotropic and accretion disk radiation fields are derived in the comoving frame of relativistic outflows, such as those thought to be found near black hole jet and gamma-ray burst sources. Nonthermal electrons and positrons Compton-scatter this radiation field, and high-energy protons and ions interact with this field through photomeson and photopair production. We revisit the problem of the Compton-scattered spectrum associated with an external accretion disk radiation field and clarify a past treatment by the authors. Simple expressions for energy-loss rates and Thomson-scattered spectra are given for ambient soft photon fields consisting of either a surrounding external isotropic monochromatic radiation field or an azimuthally symmetric, geometrically thin accretion disk radiation field. A model for blazar emission is presented that displays a characteristic spectral and variability behavior due to the presence of a direct accretion disk component. The disk component and distinct flaring behavior can be bright enough to be detected from flat-spectrum radio quasars with GLAST. Spectral states of blazars are characterized by the relative importance of the accretion disk and scattered radiation fields and, in the extended jet, by the accretion disk, inner jet, and cosmic microwave background radiation fields.