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 ~18yr in the observed frame. We compile a sample of 348 EVQs with a maximum continuum variability at rest frame 1450{AA} of more than 100% (i.e., {delta}V=(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 (<=1{sigma}) to support that CLQs are a subset of EVQs with less efficient accretion. Finally, we also confirm the antibreathing of CIV (i.e., the line width increases as luminosity increases) in EVQs and find that in addition to the ~0.4dex systematic uncertainty in single-epoch CIV virial black hole mass estimates, an extra scatter of ~0.3dex will be introduced by extreme variability.

Cone search capability for table J/ApJ/905/52/table3 (Catalog)