• radiation mechanisms: non-thermal;
  • galaxies: active;
  • galaxies: jets


Many blazars detected by the Fermi satellite, observed spectroscopically in the optical, are line-less, and have been classified as BL Lac objects. Optical–ultraviolet (UV) photometry of nearly 100 of them allowed us to determine the redshift for a handful of objects and redshift upper limits in the great majority. A few of these are candidates to be ‘blue quasars’, namely flat spectrum radio quasars whose broad emission lines are hidden by an overwhelming synchrotron emission peaking in the UV. This implies that the emitting electrons have high energies. In turn, this requires relatively weak radiative cooling, a condition that can be met if the main radiative dissipation of the jet power occurs outside the broad-line region. We confirm this hypothesis by studying and modelling the spectral energy distributions of the four ‘blue quasars’ recently discovered. Furthermore, we discuss the distribution of Fermi blazars in the γ-ray spectral index–γ-ray luminosity plane, and argue that ‘blue quasars’ objects are a minority within the blazar populations.