Space Telescope and Optical Reverberation Mapping Project. III. Optical
  Continuum Emission and Broad-Band Time Delays in NGC 5548
  We present ground-based optical photometric monitoring data for NGC 5548,
part of an extended multi-wavelength reverberation mapping campaign. The light
curves have nearly daily cadence from 2014 January to July in nine filters
(\emph{BVRI} and \emph{ugriz}). Combined with ultraviolet data from the
\emph{Hubble Space Telescope} and \emph{Swift}, we confirm significant time
delays between the continuum bands as a function of wavelength, extending the
wavelength coverage from 1158\,\AA\ to the $z$ band ($\sim\!9160$\,\AA). We
find that the lags at wavelengths longer than the {\it V} band are equal to or
greater than the lags of high-ionization-state emission lines (such as He\,{\sc
ii}\,$\lambda 1640$ and $\lambda 4686$), suggesting that the continuum-emitting
source is of a physical size comparable to the inner broad-line region (BLR).
The trend of lag with wavelength is broadly consistent with the prediction for
continuum reprocessing by an accretion disk with $\tau \propto \lambda^{4/3}$.
However, the lags also imply a disk radius that is 3 times larger than the
prediction from standard thin-disk theory, assuming that the bolometric
luminosity is 10\% of the Eddington luminosity ($L = 0.1L_{\rm Edd}$). Using
optical spectra from the Large Binocular Telescope, we estimate the bias of the
interband continuum lags due to BLR emission observed in the filters. We find
that the bias for filters with high levels of BLR contamination ($\sim\! 20\%$)
can be important for the shortest continuum lags, and likely has a significant
impact on the {\it u} and {\it U} bands owing to Balmer continuum emission.
