A Systematic Search for Changing-Look Quasars in SDSS
  We present a systematic search for changing-look quasars based on repeat
photometry from SDSS and Pan-STARRS1, along with repeat spectra from SDSS and
SDSS-III BOSS. Objects with large, |\Delta g|>1 mag photometric variations in
their light curves are selected as candidates to look for changes in broad
emission line (BEL) features. Out of a sample of 1011 objects that satisfy our
selection criteria and have more than one epoch of spectroscopy, we find 10
examples of quasars that have variable and/or "changing-look" BEL features.
Four of our objects have emerging BELs; five have disappearing BELs, and one
object shows tentative evidence for having both emerging and disappearing BELs.
With redshifts in the range 0.20 < z < 0.63, this sample includes the
highest-redshift changing-look quasars discovered to date. We highlight the
quasar J102152.34+464515.6 at z = 0.204. Here, not only have the Balmer
emission lines strongly diminished in prominence, including H$\beta$ all but
disappearing, but the blue continuum $f_{\nu} \propto \nu^{1/3}$ typical of an
AGN is also significantly diminished in the second epoch of spectroscopy. Using
our selection criteria, we estimate that >15% of strongly variable luminous
quasars display changing-look BEL features on rest-frame timescales of 8 to 10
years. Plausible timescales for variable dust extinction are factors of 2-10
too long to explain the dimming and brightening in these sources, and simple
dust reddening models cannot reproduce the BEL changes. On the other hand, an
advancement such as disk reprocessing is needed if the observed variations are
due to accretion rate changes.
