High-Redshift Extreme Variability Quasars From Sloan Digital Sky Survey Multiepoch Spectroscopy

We perform a systematic search for high-redshift (z > 1.5) extreme variability quasars (EVQs) using repeat spectra from the Sixteenth Data Release of the Sloan Digital Sky Survey, which provides a baseline spanning up to similar to 18 yr in the observed frame. We compile a sample of 348 EVQs with a maximum continuum variability at rest frame 1450 A of more than 100% (i.e., delta V equivalent to (Max - Min)/Mean > 1). The EVQs show a range of emission-line variability, including 23 where at least one line in our redshift range disappears below detectability, which can then be seen as analogous to low-redshift changing-look quasars (CLQs). Importantly, spurious CLQs caused by problematic SDSS spectral flux calibration, e.g., fiber-drop issue, have been rejected. The similar properties (e.g., continuum/line, difference-composite spectra and Eddington ratio) of normal EVQs and CLQs imply that they are basically the same physical population with analogous intrinsic variability mechanisms, as a tail of a continuous distribution of normal quasar properties. In addition, we find no reliable evidence (less than or similar to 1 sigma) to support that CLQs are a subset of EVQs with less efficient accretion. Finally, we also confirm the antibreathing of C IV (i.e., the line width increases as luminosity increases) in EVQs and find that in addition to the similar to 0.4 dex systematic uncertainty in single-epoch C IV virial black hole mass estimates, an extra scatter of similar to 0.3 dex will be introduced by extreme variability.