The variability of optical \feii emission in PG QSO 1700+518
  It is found that \feii emission contributes significantly to the optical and
ultraviolet spectra of most active galactic nuclei. The origin of the
optical/UV \feii emission is still a question open to debate. The variability
of \feii would give clues to this origin. Using 7.5 yr spectroscopic monitoring
data of one Palomer-Green (PG) quasi-stellar object (QSO), PG 1700+518, with
strong optical \feii emission, we obtain the light curves of the continuum \lv,
\feii, the broad component of \hb, and the narrow component of \hb by the
spectral decomposition. Through the interpolation cross-correlation method, we
calculate the time lags for light curves of \feii, the total \hb, the broad
component of \hb, and the narrow component of \hb with respect to the continuum
light curve. We find that the \feii time lag in PG1700+518 is
$209^{+100}_{-147}$ days, and the \hb time lag cannot be determined. Assuming
that \feii and \hb emission regions follow the virial relation between the time
lag and the FWHM for the \hb and \feii emission lines, we can derive that the
\hb time lag is $148^{+72}_{-104}$ days. The \hb time lag calculated from the
empirical luminosity--size relation is 222 days, which is consistent with our
measured \feii time lag. Considering the optical \feii contribution, PG
1700+518 shares the same characteristic on the spectral slope variability as
other 15 PG QSOs in our previous work, i.e., harder spectrum during brighter
phase.
