MgII Line Variability of High Luminosity Quasars

Abstract

We monitored five high-luminosity quasars with lambda L_{3000A} > 10^45 erg s^-1 at 0.4 < z < 0.6 to measure flux variability of the MgII 2798 line and explore feasibility of reverberation mapping using MgII. 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 at the same night with the R-C spectrograph on the 1.5-m telescope at the CTIO. By performing differencial 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 (~2500A -- ~3600A) as well as the MgII line. Utilizing photometric measurements for spectroscopic flux calibration, we measured the MgII line flux and the continuum flux at 3000\AA from each single-epoch spectrum. Four objects showed MgII line flux variability with 23%-50% peak-to-peak changes and 8%-17% rms variations over 1-1.5 year rest-frame time scales, while one object showed no MgII flux variability within the measurement error (<5%). We also detected 4%-15% rms variations of the MgII line width for all five objects. With synchronous observations for photometry and spectroscopy, we demonstrated the feasibility of the MgII line reverberation mapping for high-luminosity quasars at intermediate redshift.