Reverberation mapping of the emission line regions of Seyfert galaxies and quasars

An indirect method is outlined for mapping the emission line regions of Seyfert galaxies and quasars having variable line intensities and profiles, provided that (1) the galactic nuclei in question comprise a compact, central continuum source surrounded by an extended distribution of line-emitting gas, and (2) that the covering factor of the gas be small and the entire region optically thin to Thomson scattering so that most continuum and line photons may escape directly from the nucleus. The results obtained are demonstrated by means of models in which the line luminosity of an element of gas is directly proportional to the incident observed continuum flux. Additional assumptions justified on physical grounds are rapid reprocessing and the stability of the gas distribution over several light crossing times.