The 'Big Dipper': The nature of the extreme variability of the AGN SDSS
  J2232-0806
  SDSS J2232-0806 (the 'Big Dipper') has been identified as a 'slow-blue
nuclear hypervariable': a galaxy with no previously known active nucleus, blue
colours and large-amplitude brightness evolution occurring on a timescale of
years. Subsequent observations have shown that this source does indeed contain
an active galactic nucleus (AGN). Our optical photometric and spectroscopic
monitoring campaign has recorded one major dimming event (and subsequent rise)
over a period of around four years; there is also evidence of previous events
consistent with this in archival data recorded over the last twenty years. Here
we report an analysis of the eleven optical spectra obtained to date and we
assemble a multiwavelength data set including infrared, ultraviolet and X-ray
observations. We find that an intrinsic change in the luminosity is the most
favoured explanation of the observations, based on a comparison of continuum
and line variability and the apparent lagged response of the hot dust. This
source, along with several other recently-discovered 'changing-look' objects,
demonstrate that AGN can exhibit large-amplitude luminosity changes on
timescales much shorter than those predicted by standard thin accretion disc
models.
