Space Telescope and Optical Reverberation Mapping Project. V. Optical
  Spectroscopic Campaign and Emission-Line Analysis for NGC 5548
  We present the results of an optical spectroscopic monitoring program
targeting NGC 5548 as part of a larger multi-wavelength reverberation mapping
campaign. The campaign spanned six months and achieved an almost daily cadence
with observations from five ground-based telescopes. The H$\beta$ and He II
$\lambda$4686 broad emission-line light curves lag that of the 5100 $\AA$
optical continuum by $4.17^{+0.36}_{-0.36}$ days and $0.79^{+0.35}_{-0.34}$
days, respectively. The H$\beta$ lag relative to the 1158 $\AA$ ultraviolet
continuum light curve measured by the Hubble Space Telescope is roughly
$\sim$50% longer than that measured against the optical continuum, and the lag
difference is consistent with the observed lag between the optical and
ultraviolet continua. This suggests that the characteristic radius of the
broad-line region is $\sim$50% larger than the value inferred from optical data
alone. We also measured velocity-resolved emission-line lags for H$\beta$ and
found a complex velocity-lag structure with shorter lags in the line wings,
indicative of a broad-line region dominated by Keplerian motion. The responses
of both the H$\beta$ and He II $\lambda$4686 emission lines to the driving
continuum changed significantly halfway through the campaign, a phenomenon also
observed for C IV, Ly $\alpha$, He II(+O III]), and Si IV(+O IV]) during the
same monitoring period. Finally, given the optical luminosity of NGC 5548
during our campaign, the measured H$\beta$ lag is a factor of five shorter than
the expected value implied by the $R_\mathrm{BLR} - L_\mathrm{AGN}$ relation
based on the past behavior of NGC 5548.
