Slow blue nuclear hypervariables in PanSTARRS-1
  We discuss 76 large amplitude transients (Delta-m>1.5) occurring in the
nuclei of galaxies, nearly all with no previously known Active Galactic Nucleus
(AGN). They have been discovered as part of the Pan-STARRS1 (PS1) 3pi survey,
by comparison with SDSS photometry a decade earlier, and then monitored with
the Liverpool Telescope, and studied spectroscopically with the William
Herschel Telescope (WHT). Based on colours, light curve shape, and spectra,
these transients fall into four groups. A few are misclassified stars or
objects of unknown type. Some are red/fast transients and are known or likely
nuclear supernovae. A few are either radio sources or erratic variables and so
likely blazars. However the majority (~66%) are blue and evolve slowly, on a
timescale of years. Spectroscopy shows them to be AGN at z~ 0.3 - 1.4, which
must have brightened since the SDSS photometry by around an order of magnitude.
It is likely that these objects were in fact AGN a decade ago, but too weak to
be recognised by SDSS; they could then be classed as "hypervariable" AGN. By
searching the SDSS Stripe 82 quasar database, we find 15 similar objects. We
discuss several possible explanations for these slow blue hypervariables - (i)
unusually luminous tidal disruption events; (ii) extinction events; (iii)
changes in accretion state; and (iv) large amplitude microlensing by stars in
foreground galaxies. A mixture of explanations (iii) and (iv) seems most
likely. Both hold promise of considerable new insight into the AGN phenomenon.
