A Mid-IR Selected Changing-Look Quasar and Physical Scenarios for Abrupt
  AGN Fading
  We report a new changing-look quasar, WISE~J105203.55+151929.5 at $z=0.303$,
found by identifying highly mid-IR variable quasars in the WISE/NEOWISE data
stream. Compared to multi-epoch mid-IR photometry of a large sample of
SDSS-confirmed quasars, WISE J1052+1519 is an extreme photometric outlier,
fading by more than a factor of two at $3.4$ and $4.6 \mu$m since 2009. Swift
target-of-opportunity observations in 2017 show even stronger fading in the
soft X-rays compared to the ROSAT detection of this source in 1995, with at
least a factor of fifteen decrease. We obtained second-epoch spectroscopy with
the Palomar telescope in 2017 which, when compared with the 2006 archival SDSS
spectrum, reveals that the broad H$\beta$ emission has vanished and that the
quasar has become significantly redder. The two most likely interpretations for
this dramatic change are source fading or obscuration, where the latter is
strongly disfavored by the mid-IR data. We discuss various physical scenarios
that could cause such changes in the quasar luminosity over this timescale, and
favor changes in the innermost regions of the accretion disk that occur on the
thermal and heating/cooling front timescales. We discuss possible physical
triggers that could cause these changes, and predict the multiwavelength
signatures that could distinguish these physical scenarios.
