%0 Journal Article %A Oates, SR %A Kuin, NPM %A Nicholl, M %A Marshall, F %A Ridley, E %A Boutsia, K %A Breeveld, AA %A Buckley, DAH %A Cenko, SB %A De Pasquale, M %A Edwards, PG %A Gromadzki, M %A Gupta, R %A Laha, S %A Morrell, N %A Orio, M %A Pandey, SB %A Page, MJ %A Page, KL %A Parsotan, T %A Rau, A %A Schady, P %A Stevens, J %A Brown, PJ %A Evans, PA %A Gronwall, C %A Kennea, JA %A Klingler, NJ %A Siegel, MH %A Tohuvavohu, A %A Ambrosi, E %A Barthelmy, SD %A Beardmore, AP %A Bernardini, MG %A Bonnerot, C %A Campana, S %A Caputo, R %A Ciroi, S %A Cusumano, G %A Aì, AD %A Avanzo, PD %A Elia, VD %A Giommi, P %A Hartmann, DH %A Krimm, HA %A Malesani, DB %A Melandri, A %A Nousek, JA %A Brien, PTO %A Osborne, JP %A Pagani, C %A Palmer, DM %A Perri, M %A Racusin, JL %A Sakamoto, T %A Sbarufatti, B %A Schlieder, JE %A Tagliaferri, G %A Troja, E %A Xu, D %D 2024 %F discovery:10192060 %I Oxford University Press (OUP) %J Monthly Notices of the Royal Astronomical Society %K Black hole physics, gravitational waves, galaxies: nuclei, ultraviolet: general, transients: tidal disruption events %N 2 %P 1688-1710 %T Swift/UVOT discovery of Swift J221951−484240: a UV luminous ambiguous nuclear transient %U https://discovery.ucl.ac.uk/id/eprint/10192060/ %V 530 %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. %Z 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/