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(gzipped) PostScript## Relativistic electron beams in IDV blazars
The observed variability of BL Lac objects and Quasars on timescales day (intraday variability, IDV) have revealed radio brightness temperatures up to K. These values challenge the beaming model with isotropic comoving radio emission beyond its limits, requiring bulk relativistic motion with Lorentz factors . 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 reduces the intrinsic variability timescale to the observed value so that the intrinsic brightness temperature is reduced by a factor of order , easily below the Inverse Compton limit of K. When looking at a single event we expect the variability time scales 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 . 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.
Online publication: September 14, 1998 |