2021ApJ...911L..10D

The peak frequencies of the two broad humps evident in the spectral energy density of blazars (SED) are time dependent and vary a lot between different blazars. However, their ratio in most blazars appears to be almost universal and equal to m_ec²/4(1 + z)ε_p to a good approximation, where m_e is the electron mass, ε_p is the peak energy of the cosmic microwave background radiation, and z is the redshift of the blazar. We discuss a possible origin of such a universal ratio in blazars: gamma-ray bursts (GRBs). We point out a possible connection between the knee in the energy spectrum of cosmic-ray electrons and the maximal peak energies of the two broad humps in the SED of high energy peaked blazars and GRBs. We also point out that a universal peak ratio in double hump blazars that belong to different classes in the BL Lac sequence may simply reflect different viewing angles of otherwise similar blazars.