Quantifying the Diffuse Continuum Contribution of BLR Clouds to AGN
  Continuum Inter-band Delays
  Disk reverberation mapping of a handful of nearby AGN suggest accretion disk
sizes which are a factor few too large for their luminosities, apparently at
odds with the standard model. Here, we investigate the likely contribution to
the measured delay signature of diffuse continuum emission arising from broad
line region gas. We start by constructing spherically symmetric pressure-law
BLR models (i.e., $P(r)\propto r^{-s}$) that approximately reproduce the
observed emission line fluxes of the strong UV--optical emission-lines in the
best-studied source, NGC~5548. We then determine the contribution of the
diffuse continuum to the measured continuum flux and inter-band delays,
accounting for the observed variability behaviour of the ionizing nuclear
continuum. Those pressure-law models that approximately reproduce the observed
emission-line luminosities unavoidably produce substantial diffuse continuum
emission. This causes a significant contamination of the disk reverberation
signature (i.e., wavelength-dependent continuum delays). Qualitatively, the
diffuse continuum delay signatures produced by our models resemble that
observed for NGC~5548, including the deviation of the lag spectrum above that
of a simple power-law in wavelength, short-ward of the Balmer and Paschen
jumps. Furthermore, for reasonable estimates of the BLR covering fraction, the
delay induced by diffuse continuum emission causes elevated inter-band delays
over the entire UV--optical regime; for these pressure-law models, there are no
`disk-dominated' wavelength intervals. Thus, the diffuse continuum contribution
must be taken into account in order to correctly infer AGN accretion disk sizes
based on inter-band continuum delays.
