The variability of optical \feii emission in PG QSO 1700+518

TL;DR

This study investigates the variability of optical eii emission in PG 1700+518 over 7.5 years, revealing a time lag of approximately 209 days, which helps understand the emission region and its relation to other spectral features.

Contribution

First measurement of eii emission variability and time lag in PG 1700+518, providing insights into the emission region and its relation to broad-line region dynamics.

Findings

eii emission lag is approximately 209 days.

eii and hemission regions follow virial relation.

Spectral slope becomes harder during brighter phases.

Abstract

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.