A Large Systematic Search for Close Supermassive Binary and Rapidly
  Recoiling Black Holes - II. Continued Spectroscopic Monitoring and Optical
  Flux Variability
  We present new spectroscopic observations that are part of our continuing
monitoring campaign of 88 quasars at z<0.7 whose broad H$\beta$ lines are
offset from their systemic redshifts by a few thousand km/s. These quasars have
been considered candidates for hosting supermassive black hole binaries (SBHBs)
by analogy with single-lined spectroscopic binary stars. We present the data
and describe our improved analysis techniques, which include an extensive
evaluation of uncertainties. We also present a variety of measurements from the
spectra that are of general interest and will be useful in later stages of our
analysis. Additionally, we take this opportunity to study the variability of
the optical continuum and integrated flux of the broad H$\beta$ line. We
compare the variability properties of the SBHB candidates to those of a sample
of typical quasars with similar redshifts and luminosities observed multiple
times during the Sloan Digital Sky Survey. We find that the variability
properties of the two samples are similar (variability amplitudes of 10-30% on
time scales of approximately 1-7 years) and that their structure functions can
be described by a common model with parameters characteristic of typical
quasars. These results suggest that the broad-line regions of SBHB candidates
have a similar extent as those of typical quasars. We discuss the implications
of this result for the SBHB scenario and ensuing constraints on the orbital
parameters.
