Characterizing Quasar C iv Emission-line Measurements from Time-resolved Spectroscopy

We use multiepoch quasar spectroscopy to determine how accurately single-epoch spectroscopy can locate quasars in emission-line parameter space in order to inform investigations where time-resolved spectroscopy is not available. We explore the improvements in emission-line characterization that result from using nonparametric information from many lines as opposed to a small number of parameters for a single line, utilizing reconstructions based on an independent component analysis applied to the data from the Sloan Digital Sky Survey Reverberation Mapping project. We find that most of the quasars are well described by just two components, while more components signal a quasar likely to yield a successful reverberation mapping analysis. In single-epoch spectroscopy the apparent variability of equivalent width is exaggerated because it is dependent on the continuum. Multiepoch spectroscopy reveals that single-epoch results do not significantly change where quasars are located in C iv parameter space and do not have a significant impact on investigations of the global Baldwin effect. Quasars with emission-line properties indicative of higher L/LEdd are less variable, consistent with models with enhanced accretion disk density. Narrow absorption features at the systemic redshift may be indicative of orientation (including radio-quiet quasars) and may appear in as much as 20% of the quasar sample. Future work applying these techniques to lower-luminosity quasars will be important for understanding the nature of accretion disk winds.