%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.
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