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We perform a systematic search for long-term extreme variability quasars(EVQs) in the overlapping Sloan Digital Sky Survey (SDSS) and 3-Year DarkEnergy Survey (DES) imaging, which provide light curves spanning more than 15years. We identified ~1000 EVQs with a maximum g band magnitude change of morethan 1 mag over this period, about 10% of all quasars searched. The EVQs haveL_bol~10^45-10^47 erg/s and L/L_Edd~0.01-1. Accounting for selection effects,we estimate an intrinsic EVQ fraction of ~30-50% among all g<~22 quasars over abaseline of ~15 years. These EVQs are good candidates for so-called"changing-look quasars", where a spectral transition between the two types ofquasars (broad-line and narrow-line) is observed between the dim and brightstates. We performed detailed multi-wavelength, spectral and variabilityanalyses for the EVQs and compared to their parent quasar sample. We found thatEVQs are distinct from a control sample of quasars matched in redshift andoptical luminosity: (1) their UV broad emission lines have larger equivalentwidths; (2) their Eddington ratios are systematically lower; and (3) they aremore variable on all timescales. The intrinsic difference in quasar propertiesfor EVQs suggest that internal processes associated with accretion are the maindriver for the observed extreme long-term variability. However, despite theirdifferent properties, EVQs seem to be in the tail of a continuous distributionof quasar properties, rather than standing out as a distinct population. Wespeculate that EVQs are normal quasars accreting at relatively low accretionrates, where the accretion flow is more likely to experience instabilities thatdrive the factor of few changes in flux on multi-year timescales.

Extreme variability quasars from the Sloan Digital Sky Survey and the Dark Energy Survey