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4.2. Other plasma-type processes

Even though it is entirely possible that the continuum from quasars, in all wavebands, is synchrotron-radiation, this is by no means the only tenable option. Colgate and collaborators [6, 37] have attributed the continuum to photons with v appeq 2vp produced from plasma oscillations which are then "upscattered", first by the oscillations themselves and then by so-called "Comptonization" (i.e., Compton scattering by hot though non-relativistic - electrons). Other authors [38, 39] have shown explicitly how Comptonization can mould a power-law continuum spectrum in the UV and X-ray band, provided only that there is some (unspecified) source of "soft" photons, and the optical depth taues is of order (kTe / mec2)-1/2. The resulting brightness temperature cannot exceed Te, except at low frequencies where the predominant emission or scattering processes are coherent. There is no evidence for any coherent emission in quasars, except possibly at frequencies ltapprox 1 GHz in low-frequency radio variables, where the inferred brightness temperatures are embarassingly high. On the other hand there is no reason why coherent processes at frequencies ~ vp should not manifest themselves in a thermal plasma. Moreover, the magnetic field strengths near a massive accreting black hole may be up to ~ 106 G. Conditions for coherent radiation by relativistic particles would be at least as favorable as (for instance) near the light cylinder of pulsars. Intensive searches for rapid low frequency variability in compact radio sources would seem very worthwhile.