2010ApJ...718..460B

It is found that Fe II emission contributes significantly to the optical and ultraviolet spectra of most active galactic nuclei. The origin of the optical/UV Fe II emission is still open to debate. The variability of Fe II would give clues to this origin. Using 7.5 yr spectroscopic monitoring data of one Palomar-Green (PG) quasi-stellar object (QSO), PG 1700+518, with strong optical Fe II emission, we obtain the light curves of the continuum f_λ(5100 Å), Fe II, the broad component of Hβ, and the narrow component of Hβ by spectral decomposition. Through the interpolation cross-correlation method, we calculate the time lags for the light curves of Fe II, the total Hβ, the broad component of Hβ, and the narrow component of Hβ with respect to the continuum light curve. We find that the Fe II time lag in PG 1700+518 is 209⁺¹⁰⁰_–147days, while the Hβ time lag cannot be determined. Assuming that the Fe II and Hβ emission regions follow the virial relation between the time lag and the FWHM for the Hβ and Fe II emission lines, we can derive the Hβ time lag to be 148⁺⁷²_–104days. The Hβ time lag calculated from the empirical luminosity-size relation is 222 days, which is consistent with our measured Fe II time lag. Considering the optical Fe II contribution, PG 1700+518 shares the same spectral slope variability characteristic, i.e., harder spectrum during brighter phase, as the other 15 PG QSOs in our previous work.