Extreme variability in an active galactic nucleus: Gaia16aax
  We present the results of a multi-wavelength follow up campaign for the
luminous nuclear transient Gaia16aax, which was first identified in January
2016. The transient is spatially consistent with the nucleus of an active
galaxy at z=0.25, hosting a black hole of mass $\rm \sim6\times10^8M_\odot$.
The nucleus brightened by more than 1 magnitude in the Gaia G-band over a
timescale of less than one year, before fading back to its pre-outburst state
over the following three years. The optical spectra of the source show broad
Balmer lines similar to the ones present in a pre-outburst spectrum. During the
outburst, the $\rm H\alpha$ and $\rm H\beta$ emission lines develop a secondary
peak. We also report on the discovery of two transients with similar light
curve evolution and spectra: Gaia16aka and Gaia16ajq. We consider possible
scenarios to explain the observed outbursts. We exclude that the transient
event could be caused by a microlensing event, variable dust absorption or a
tidal encounter between a neutron star and a stellar mass black hole in the
accretion disk. We consider variability in the accretion flow in the inner part
of the disk, or a tidal disruption event of a star $\geq 1 M_{\odot}$ by a
rapidly spinning supermassive black hole as the most plausible scenarios. We
note that the similarity between the light curves of the three Gaia transients
may be a function of the Gaia alerts selection criteria.
