Relativistic electron beams in IDV blazars

Details Relativistic electron beams in IDV blazars Relativistic electron beams in IDV blazars

Oct 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998a%26a...338..399c&link_type=abstract

Astronomy and Astrophysics, v.338, p.399-404 (1998)

Astronomy and Astrophysics

Astrophysics

5

Acceleration Of Particles, Plasmas, Radiation Mechanisms: Non-Thermal, Bl Lacertae Objects: General, Galaxies: Jets, Galaxies: Quasars: General

Scientific paper

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\sim 1016-1020 K. These values challenge the beaming model with isotropic comoving radio emission beyond its limits, requiring bulk relativistic motion with Lorentz factors \Gamma > 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 scatte= red 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 \gamma_0\sim 10^2-10^4 reduces the intrinsic variability timescale \Delta t' to the observed value \Delta t\sim \Delta t'/\gamma_0 so that the intrinsic brightness temperature is reduced by a factor of order \sim 1/\gamma_0^2, easily below the Inverse Compton limit of T_b < 1012 K. When looking at a single event we expect the variability time scales \Delta t 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 \Delta t\propto \nu^{-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.

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