  We report the discovery of Swift J221951-484240 (hereafter: J221951), a
luminous slow-evolving blue transient that was detected by the Neil Gehrels
Swift Observatory Ultra-violet/Optical Telescope (Swift/UVOT) during the
follow-up of Gravitational Wave alert S190930t, to which it is unrelated.
Swift/UVOT photometry shows the UV spectral energy distribution of the
transient to be well modelled by a slowly shrinking black body with an
approximately constant temperature of T~2.5x10^4 K. At a redshift z=0.5205,
J221951 had a peak absolute magnitude of M_u,AB = -23 mag, peak bolometric
luminosity L_max=1.1x10^45 erg s^-1 and a total radiated energy of E>2.6x10^52
erg. The archival WISE IR photometry shows a slow rise prior to a peak near the
discovery date. Spectroscopic UV observations display broad absorption lines in
N V and O VI, pointing toward an outflow at coronal temperatures. The lack of
emission in the higher H~Lyman lines, N I and other neutral lines is consistent
with a viewing angle close to the plane of the accretion or debris disc. The
origin of J221951 can not be determined with certainty but has properties
consistent with a tidal disruption event and the turn-on of an active galactic
nucleus.
