M. Böttcher,
Astrophysical Journal Letters, 515, L21 (1999)
A prediction for the energy shift of the synchrotron spectrum of flat-spectrum radio quasars (FSRQs) during high-energy flares is presented. If the gamma-ray emission of FSRQs is produced by Comptonization of external radiation, then the peak of the synchrotron spectrum is predicted to move to lower energies in the flare state. This is opposite to the well-known broadband spectral behavior of high-frequency peaked BL Lac objects where the external radiation field is believed to be weak and synchrotron-self Compton scattering might be the dominant gamma-ray radiation mechanism. The synchrotron peak shift, if observed in FSRQs, can thus be used as a diagnostic to determine the dominant radiation mechanism in these objects. I suggest a few FSRQs as promising candidates to test the prediction of the external-Comptonization model.