@article{discovery10192060,
           title = {Swift/UVOT discovery of Swift J221951?484240: a UV luminous ambiguous nuclear transient},
            year = {2024},
         journal = {Monthly Notices of the Royal Astronomical Society},
          number = {2},
            note = {This work is licensed under a Creative Commons Attribution 4.0 International License. The images
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           pages = {1688--1710},
           month = {May},
          volume = {530},
       publisher = {Oxford University Press (OUP)},
          author = {Oates, SR and Kuin, NPM and Nicholl, M and Marshall, F and Ridley, E and Boutsia, K and Breeveld, AA and Buckley, DAH and Cenko, SB and De Pasquale, M and Edwards, PG and Gromadzki, M and Gupta, R and Laha, S and Morrell, N and Orio, M and Pandey, SB and Page, MJ and Page, KL and Parsotan, T and Rau, A and Schady, P and Stevens, J and Brown, PJ and Evans, PA and Gronwall, C and Kennea, JA and Klingler, NJ and Siegel, MH and Tohuvavohu, A and Ambrosi, E and Barthelmy, SD and Beardmore, AP and Bernardini, MG and Bonnerot, C and Campana, S and Caputo, R and Ciroi, S and Cusumano, G and A{\`i}, AD and Avanzo, PD and Elia, VD and Giommi, P and Hartmann, DH and Krimm, HA and Malesani, DB and Melandri, A and Nousek, JA and Brien, PTO and Osborne, JP and Pagani, C and Palmer, DM and Perri, M and Racusin, JL and Sakamoto, T and Sbarufatti, B and Schlieder, JE and Tagliaferri, G and Troja, E and Xu, D},
             url = {http://dx.doi.org/10.1093/mnras/stae795},
        abstract = {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 {$\sim$} 2.5 {$\times$} 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 {$\times$} 1045 erg s?1 and a total radiated energy of E {\ensuremath{>}} 2.6 {$\times$} 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 {\ensuremath{\alpha}} 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.},
        keywords = {Black hole physics, gravitational waves, galaxies: nuclei, ultraviolet: general, transients: tidal disruption events}
}