<i>Swift</i>/UVOT discovery of <i>Swift</i> J221951−484240: a UV luminous ambiguous nuclear transient

Oates, S R and Kuin, N P M and Nicholl, M and Marshall, F and Ridley, E and Boutsia, K and Breeveld, A A and Buckley, D A H and Cenko, S B and De Pasquale, M and Edwards, P G and Gromadzki, M and Gupta, R and Laha, S and Morrell, N and Orio, M and Pandey, S B and Page, M J and Page, K L and Parsotan, T and Rau, A and Schady, P and Stevens, J and Brown, P J and Evans, P A and Gronwall, C and Kennea, J A and Klingler, N J and Siegel, M H and Tohuvavohu, A and Ambrosi, E and Barthelmy, S D and Beardmore, A P and Bernardini, M G and Bonnerot, C and Campana, S and Caputo, R and Ciroi, S and Cusumano, G and D’Aì, A and D’Avanzo, P and D’Elia, V and Giommi, P and Hartmann, D H and Krimm, H A and Malesani, D B and Melandri, A and Nousek, J A and O’Brien, P T and Osborne, J P and Pagani, C and Palmer, D M and Perri, M and Racusin, J L and Sakamoto, T and Sbarufatti, B and Schlieder, J E and Tagliaferri, G and Troja, E and Xu, D (2024) <i>Swift</i>/UVOT discovery of <i>Swift</i> J221951−484240: a UV luminous ambiguous nuclear transient. Monthly Notices of the Royal Astronomical Society, 530 (2). pp. 1688-1710. ISSN 0035-8711

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 ∼ 2.5 × 104 K. At a redshift z = 0.5205, J221951 had a peak absolute magnitude of Mu,AB = −23 mag, peak bolometric luminosity $L_{max}=1.1\times 10^{45}~{\rm erg\, s}^{-1}$ and a total radiated energy of E &gt; 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.