Space Telescope and Optical Reverberation Mapping Project. III. Optical Continuum Emission and Broadband Time Delays in NGC 5548

We present ground-based optical photometric monitoring data for NGC 5548, part of an extended multiwavelength reverberation mapping campaign. The light curves have nearly daily cadence from 2014 January to July in ninefilters (BVRI and ugriz). Combined with ultraviolet data from the Hubble Space Telescope and Swift, we confirm significant time delays between the continuum bands as a function of wavelength, extending the wavelength coverage from 1158 Angstrom to the z band (approximately 9160 angstrom). We find that the lags at wavelengths longer than the V band are equal to or greater than the lags of high-ionization-state emission lines (such as He pi lambdal1640 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 varies as lambda(exp 4/3)). However, the lags also imply a disk radius that is 3 times larger than the prediction from standardthin-disk theory, assuming that the bolometric luminosity is 10 percent of the Eddington luminosity (L 0.1L(sub Edd)).Using optical spectra from the Large Binocular Telescope, we estimate the bias of the interband continuum lagsdue to BLR emission observed in the filters. We find that the bias for filters with high levels of BLR contamination(20 percent) can be important for the shortest continuum lags and likely has a significant impact on the u and U bandsowing to Balmer continuum emission.