Mg II LINE VARIABILITY OF HIGH-LUMINOSITY QUASARS

Abstract

We monitored five high-luminosity quasars with λL 3000 A > 1045 erg s–1 at 0.4 z 0.6 to measure the flux variability of the Mg II λ2798 line and explore the feasibility of reverberation mapping using Mg II. Over the two-year monitoring program, imaging data were obtained with the A Noble Double-Imaging Camera on the 1.3 m telescope at the Cerro Tololo Inter-American Observatory (CTIO), while spectroscopic data were obtained on the same nights with the R-C spectrograph on the 1.5 m telescope at the CTIO. By performing differential photometry using available field stars in each quasar image, we measured variability—10-24% peak-to-peak changes and 3-8% rms variations—in the B band, which includes flux changes in the rest-frame UV continuum (~2500 A-3600 A) as well as the Mg II line. Utilizing photometric measurements for spectroscopic flux calibration, we measured the Mg II line flux and the continuum flux at 3000 A from each single-epoch spectrum. Four objects showed Mg II line flux variability with 23-50% peak-to-peak changes and 8-17% rms variations over 1-1.5 year rest-frame timescales, while one object showed no Mg II flux variability within the measurement error (<5%). We also detected 4-15% rms variations of the Mg II line width for all five objects. With synchronous observations for photometry and spectroscopy, we demonstrated the feasibility of the Mg II line reverberation mapping for high-luminosity quasars at intermediate redshift.