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We present new Gemini/GMOS optical spectroscopy of 16 extreme variabilityquasars (EVQs) that dimmed by more than 1.5 mag in the $g$ band between theSloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) epochs(separated by a few years in the quasar rest frame). The quasar sample covers aredshift range of $0.5 < z < 2.1$. Nearly half of these EVQs brightenedsignificantly (by more than 0.5 mag in the $g$ band) in a few years afterreaching their previous faintest state, and some EVQs showed rapid (non-blazar)variations of greater than 1-2 mag on timescales of only months. Leveraging onthe large dynamic range in continuum variability between the earlier SDSS andthe new GMOS spectra, we explore the associated variations in the broad MgII,$\lambda2798$ line, whose variability properties have not been well studiedbefore. The broad Mg II flux varies in the same direction as the continuumflux, albeit with a smaller amplitude, which indicates at least some portion ofMg II is reverberating to continuum changes. However, the width (FWHM) of Mg IIdoes not vary accordingly as continuum changes for most objects in the sample,in contrast to the case of the broad Balmer lines. Using the width of broad MgII to estimate the black hole mass therefore introduces a luminosity-dependentbias.

Spectral Variability of a Sample of Extreme Variability Quasars and Implications for the MgII Broad-line Region