Time dependence of emission-line strengths from a photoionized nebula

Discussions are given for the time dependence of emission-line intensities from nebulae that are photoionized by a central source with a variable continuum flux. Explicit calculations are carried out for some cases (especially when the nebula can be considered a thin shell) for a temporary increase of continuum flux of simple "square-wave hump" shape. In some cases, changes in line intensities are predicted over times that are small compared with the light travel time across the nebula. The observational situation is described and further measurements are suggested for a number of objects that are relatively easy to study and whose further investigations seem likely to lead to important conclusions regarding the nebular parameters. These promising objects include QSOs, and N-type, Seyfert, and compact galaxies, as well as some X-ray sources and old novae.