The Time Domain Spectroscopic Survey: Changing-Look Quasar Candidates
  from Multi-Epoch Spectroscopy in SDSS-IV
  Active galactic nuclei (AGN) can vary significantly in their rest-frame
optical/UV emission, and with strong associated changes in broad line emission,
on much shorter timescales than predicted by standard models of accretion disks
around supermassive black holes. Most such "changing-look" AGN - and at higher
luminosities, quasars (CLQs) have been found via spectroscopic follow-up of
known quasars showing strong photometric variability. The Time Domain
Spectroscopic Survey of SDSS-IV has instead obtained repeat spectroscopy of
large numbers of previously-known quasars, with spectral epochs separated by
months to decades. Our visual examination of these repeat spectra for strong
broad line variability yielded 61 newly-discovered CLQ candidates. We
quantitatively compare spectral epochs to measure changes in continuum and
Hbeta broad line emission. Defining a CLQ by a S/N threshold of at least 3sigma
on variability of broad Hbeta emission, we find 19 CLQs, of which 15 are
newly-recognized. Since the parent sample includes only broad line quasars, our
study tends to find objects that later dim, i.e., turn-off CLQs. However, we
nevertheless find 4 turn-on CLQs. We study the response of both Hbeta and Mg II
emission lines to continuum changes. The Eddington ratios of CLQs is low,
and/or their Hbeta broad line width is large relative to the overall quasar
population. Repeat quasar spectroscopy in the upcoming SDSS-V Black Hole Mapper
program promises to reveal significant numbers of CLQs, helping to further our
understanding of the frequency and duty-cycle of such strong variability, and
constrain the physics and dynamics of the phenomenon.
