THE VARIABILITY OF OPTICAL Fe ii EMISSION IN PG QSO 1700+518

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 fλ(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+100−147 days, 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+72−104 days. 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.

研究发现，铁II发射对大多数活动星系核的光学和紫外光谱有显著贡献。光学/紫外铁II发射的起源仍存在争议。铁II的可变性将为其起源提供线索。利用一个具有强烈光学铁II发射的帕洛玛 - 格林（PG）类星体PG 1700 + 518长达7.5年的光谱监测数据，我们通过光谱分解得到了连续谱$f_{\lambda}(5100\mathring{A})$、铁II、Hβ宽线成分以及Hβ窄线成分的光变曲线。通过插值互相关方法，我们计算了铁II、总Hβ、Hβ宽线成分以及Hβ窄线成分的光变曲线相对于连续谱光变曲线的时间延迟。我们发现PG 1700 + 518中铁II的时间延迟为$209^{+100}_{-147}$天，而Hβ的时间延迟无法确定。假设铁II和Hβ发射区域遵循Hβ和铁II发射线的时间延迟与半峰全宽之间的维里关系，我们可以推导出Hβ的时间延迟为$148^{+72}_{-104}$天。根据经验光度 - 尺度关系计算出的Hβ时间延迟为222天，这与我们测量的铁II时间延迟相符。考虑到光学铁II的贡献，PG 1700 + 518与我们之前工作中的其他15个PG类星体具有相同的光谱斜率可变性特征，即在更亮阶段光谱更硬。