Annu. Rev. Astron. Astrophys. 1988. 36: 539-598
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6.2. Comments: Beaming Effects in Relativistic Outflows

VLBI observations of proper motions in the cores of radio galaxies and quasars have shown that compact radio jets possess bulk Lorentz factors up to gammabulk ~ 10 (Zensus 1996). However, intraday variability (IDV) of some compact radio sources may also be claimed as indirect evidence of highly relativistic flows (Qian et al 1991, Quirrenbach et al 1989, 1991, Witzel 1992). Begelman et al (1994) show that, in order to interpret these rapid variabilities in terms of an incoherent synchrotron-emitting jet with non-self-absorbed convected disturbances, the required Lorentz factors are very high, up to 103. In addition, the emitting disturbance must be close to the self-absorption limit, and this reduces the emission efficiency. Even relatively "normal" sources would correspond to jet powers Lj geq 1047 erg s-1 above the Compton catastrophe limit. Such Lorentz factors would rule out any kind of radiative or thermal jet acceleration, favoring hydromagnetic acceleration (Begelman et al 1994). For highly beamed radiation in the AGN cores in the framework of unification models, we refer to the review by Ulrich et al (1997).

A possible resolution of the serious limitations of the incoherent synchrotron model is to refer to coherent emission from plasma oscillations. Benford (1992) has discussed this solution with reference to laboratory experiments that show how very high brightness luminosities can be achieved. Stimulated Compton scattering in the deep cores of AGNs could, however, prevent this radiation from escaping (Coppi et al 1993).

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