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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.

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