We investigate the wavelength-dependent intensity and reverberation properties of the UV-optical diffuse continuum emission expected from broad emission line gas. The "locally optimally emitting clouds" picture is adopted, with the cloud distribution functions in gas density and distance from the ionizing source determined by a fit to the mean UV emission-line spectrum from the 1993 HST campaign of NGC 5548 in our previous paper. The model Balmer continuum's strength and variability characteristics are in agreement with those derived from the observations of NGC 5548. A key prediction is a wavelength-dependent lag across the UV-optical spectrum that can broadly mimic the signature from X-ray reprocessing in an accretion disk, calling into question claims of discovery of the latter in NGC 7469. The influence of the diffuse continuum on the optical continuum may result in a small yet significant underestimation of the characteristic sizes of the regions emitting the optical emission lines. Its contribution can also alter the inferred spectral energy distribution of the UV-optical continuum, even well outside the spectral region near the well-known Balmer jump. The reverberation of the diffuse continuum emitted by the broad-line region may account for perhaps one-third of the observed effect that the λ1350-λ5100 continuum becomes bluer as it becomes brighter (even after accounting for nonvariable optical starlight). And as in other models of the broad emission line gas, a significant yet unobserved Lyman jump is predicted. We highlight the importance of careful studies of the UV-optical continuum, especially the Balmer continuum.
展开▼
