Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994ap.....37..247n&link_type=abstract
Astrophysics, Volume 37, Issue 3, pp.247-255
Astronomy and Astrophysics
Astrophysics
Scientific paper
In a series of papers of You, Cheng, et al. (cf., for example, [2 4]) the Cherenkov mechanism of radiation was proposed as the explanation of the wide emission lines in the spectra of quasars. This mechanism acts because the refractive index of the plasma in a certain frequency range near a spectral line can exceed one. It has been found that the role of Cherenkov radiation becomes significant for an optically thick medium when the density of the packet of relativistic electrons ñe is 104 106 cm-3. However, the question of the formation of a spectral line as the result of ordinary (recombination and collision) processes under the same physical conditions has not been considered, and hence no comparison has been made between the intensities of lines formed by different mechanisms. Moreover, a large number of effects that may have significant influence on the profile of a line (multiple scattering in the medium, redistribution of radiation over frequencies within a line, redistribution of energy between a line and the continuous spectrum) have remained unexamined by these authors. The present work aims at sharpening some of the formulas used in [2 4] and filling up some of these gaps. We pose the classical problem of formation of a spectral line taking account of the action of the Cherenkov radiation mechanism and give an exact solution of it. The computations are carried out for the line Lα. It is shown that under the values presently accepted for the electron temperature for an average quasar (Te ˜ 1.5 · 104 K) the influence of the Cherenkov mechanism becomes noticeable only for implausibly high densities of relativistic electrons (≳1011 cm-3, The geometric model proposed in [4] for quasar radiation in a line encounters insuperable difficulties involving the energy of the quasar and the sizes of the radiating regions.
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