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

We treat transformation properties of external radiation fields in the properframe of a plasma moving with constant speed. The specific spectral energydensities of external isotropic and accretion-disk radiation fields are derivedin the comoving frame of relativistic outflows, such as those thought to befound near black-hole jet and gamma-ray burst sources. Nonthermal electrons andpositrons Compton-scatter this radiation field, and high-energy protons andions interact with this field through photomeson and photopair production. Werevisit the problem of the Compton-scattered spectrum associated with anexternal accretion-disk radiation field, and clarify a past treatment by theauthors. Simple expressions for energy-loss rates and Thomson-scattered spectraare given for ambient soft photon fields consisting either of a surroundingexternal isotropic monochromatic radiation field, or of an azimuthallysymmetric, geometrically thin accretion-disk radiation field. A model forblazar emission is presented that displays a characteristic spectral andvariability behavior due to the presence of a direct accretion-disk component.The disk component and distinct flaring behavior can be bright enough to bedetected from flat spectrum radio quasars with {\it GLAST}. Spectral states ofblazars are characterized by the relative importance of the accretion-disk andscattered radiation fields and, in the extended jet, by the accretion disk,inner jet, and cosmic microwave background radiation fields.