SIMBAD references

The observed variability of BL Lac objects and Quasars on timescales ≲1 day (intraday variability, IDV) have revealed radio brightness temperatures up to T_b∼10¹⁶-10²⁰K. These values challenge the beaming model with isotropic comoving radio emission beyond its limits, requiring bulk relativistic motion with Lorentz factors Γ>100. We argue in favor of a model where an anisotropic distribution of relativistic electrons streams out along the field lines. When this relativistic beam is scattered in pitch angle and/or hits a magnetic field with components perpendicular to the beam velocity it starts to emit synchrotron radiation and redistribute in momentum space. The propagation of relativistic electrons with Lorentz factor γ₀∼10²-10⁴ reduces the intrinsic variability timescale Δt' to the observed value Δt~Δt'/γ₀ so that the intrinsic brightness temperature is reduced by a factor of order ∼1/γ₀², easily below the Inverse Compton limit of T_b≲10¹²K. When looking at a single event we expect the variability time scales Δt to be independent of frequency for a monoenergetic electron beam, whereas for a beam with a spread out distribution of energies (e.g. power-law) parallel to the magnetic field the timescales are shortened towards higher frequencies according to Δt∝ν^–0.5. The observations seem to favor monoenergetic relativistic electrons which explain several properties of variable blazar spectra. The production of variable X- and gamma-ray flux is briefly discussed.