Gaia transients in galactic nuclei
  The high spatial resolution and precise astrometry and photometry of the Gaia
mission should make it particularly apt at discovering and resolving transients
occurring in, or near, the centres of galaxies. Indeed, some nuclear transients
are reported by the Gaia Science Alerts (GSA) team, but not a single confirmed
Tidal Disruption Event has been published. In order to explore the sensitivity
of GSA, we performed an independent and systematic search for nuclear
transients using Gaia observations. Our transient search is driven from an
input galaxy catalogue (derived from the Sloan Digital Sky Survey Release 12).
We present a candidate detection metric which is independent from the existing
GSA methodology, to see if Gaia Alerts are biased against the discovery of
nuclear transients, and in particular which steps may have an impact. Our
technique does require significant manual vetting of candidates, making
implementation in the GSA system impractical for daily operations, although it
could be run weekly, which for month-to-year long transients would make a
scientifically valuable addition. Our search yielded ~480 nuclear transients, 5
of which were alerted and published by GSA. The list of (in some cases ongoing)
transients includes candidates for events related to enhanced accretion onto a
super-massive black hole and TDEs. An implementation of the detection
methodology and criteria used in this paper as an extension of GSA could open
up the possibility for Gaia to fulfil the role as a main tool to find transient
nuclear activity as predicted in the literature.
