%O This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third-party material in this article are included in the Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ %X 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 Ultraviolet/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 blackbody with an approximately constant temperature of T ∼ 2.5 × 104 K. At a redshift z = 0.5205, J221951 had a peak absolute magnitude of Mu,AB = −23 mag, peak bolometric luminosity Lmax = 1.1 × 1045 erg s−1 and a total radiated energy of E > 2.6 × 1052 erg. The archival Wide-field Infrared Survey Explorer 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 towards an outflow at coronal temperatures. The lack of emission in the higher H α 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 cannot be determined with certainty but has properties consistent with a tidal disruption event and the turn-on of an active galactic nucleus. %L discovery10192060 %K Black hole physics, gravitational waves, galaxies: nuclei, ultraviolet: general, transients: tidal disruption events %J Monthly Notices of the Royal Astronomical Society %I Oxford University Press (OUP) %D 2024 %P 1688-1710 %N 2 %A SR Oates %A NPM Kuin %A M Nicholl %A F Marshall %A E Ridley %A K Boutsia %A AA Breeveld %A DAH Buckley %A SB Cenko %A M De Pasquale %A PG Edwards %A M Gromadzki %A R Gupta %A S Laha %A N Morrell %A M Orio %A SB Pandey %A MJ Page %A KL Page %A T Parsotan %A A Rau %A P Schady %A J Stevens %A PJ Brown %A PA Evans %A C Gronwall %A JA Kennea %A NJ Klingler %A MH Siegel %A A Tohuvavohu %A E Ambrosi %A SD Barthelmy %A AP Beardmore %A MG Bernardini %A C Bonnerot %A S Campana %A R Caputo %A S Ciroi %A G Cusumano %A AD Aì %A PD Avanzo %A VD Elia %A P Giommi %A DH Hartmann %A HA Krimm %A DB Malesani %A A Melandri %A JA Nousek %A PTO Brien %A JP Osborne %A C Pagani %A DM Palmer %A M Perri %A JL Racusin %A T Sakamoto %A B Sbarufatti %A JE Schlieder %A G Tagliaferri %A E Troja %A D Xu %V 530 %T Swift/UVOT discovery of Swift J221951−484240: a UV luminous ambiguous nuclear transient