Oates, S. R.Kuin, N. P. M.Nicholl, M.Marshall, F.Laha, SibasishParsotan, TylerKlingler, N.J.et al2023-07-182023-07-182023-07-03https://doi.org/10.48550/arXiv.2307.01044http://hdl.handle.net/11603/28756Authors: - E. Ridley, K. Boutsia, A. A. Breeveld, D. A. H. Buckley, S. B. Cenko, M. De Pasquale, P. G. Edwards, M. Gromadzki, R. Gupta, N. Morrell, M. Orio, S. B. Pandey, M. J. Page, K.L. Page, A. Rau, P. Schady, J. Stevens, P. J. Brown, P.A. Evans, C. Gronwall, J.A. Kennea, M. H. Siegel, A. Tohuvavohu, E. Ambrosi, S.D. Barthelmy, A.P. Beardmore, M.G. Bernardini, C. Bonnerot, S. Campana, R. Caputo, S. Ciroi, G. Cusumano, A. D'Ai, P. D'Avanzo, V. D'Elia, P. Giommi, D.H. Hartmann, H.A. Krimm, D. B. Malesani, A. Melandri, J. A. Nousek, P.T. O'Brien, J.P. Osborne, C. Pagani, D.M. Palmer, M. Perri, J. L. Racusin, T. Sakamoto, B. Sbarufatti, J. E. Schlieder, G. Tagliaferri, E. Troja, D. XuWe 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.37 pagesen-USThis work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.Public Domain Mark 1.0http://creativecommons.org/publicdomain/mark/1.0/Text