An approach to blazar spectral modeling is presented where the energy distribution of jet electrons is described by a log-parabola form, and near-equipartition relations are used to define the energy densities of the electrons, magnetic field, and radiation fields. By adjusting the parameters defining near-equipartition conditions, reasonable fits to contemporaneous multiwavelength blazar data are obtained that yield definite values for the Doppler factor, the jet magnetic field, and the energy densities of the external radiation fields that are Compton-scattered to make the gamma rays observed from FSRQ blazars.

Our technique is applied to different epochs of 3C 279 and 3C 454.3. The inferred external radiation field energy densities are compared with values expected from observations of the broad line region (BLR) and IR torus. Our results indicate that the GeV cutoffs discovered with Fermi-LAT from FSRQs and low- and intermediate-synchrotron peaked blazars are formed when BLR radiation dominated by Ly alpha photons is scattered by jet electrons. During epochs where GeV cutoffs are found, therefore, the gamma-ray emitting region is within the BLR.