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Astron. Astrophys. 338, 399-404 (1998)

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Relativistic electron beams in IDV blazars

André R. Crusius-Wätzel and Harald Lesch

Institut für Astronomie und Astrophysik der Universität München, Scheinerstrasse 1, D-81679 München, Germany

Received 17 June 1998 / Accepted 21 July 1998

Abstract

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

Key words: acceleration of particles – plasmas – radiation mechanisms: non-thermal – BL Lacertae objects: general – galaxies: jets – galaxies: quasars: general

Send offprint requests to: A.R. Crusius-Wätzel

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998
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