Extreme variability QSOs from SDSS DR16

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.