Swift/UVOT discovery of Swift J221951-484240: a UV luminous ambiguous nuclear transient
Troja, E.; Cenko, S. B.; Sakamoto, T.; D'Avanzo, P.; Campana, S.; Tagliaferri, G.; Melandri, A.; Cusumano, G.; D'Elia, V.; Sbarufatti, B.; Gromadzki, M.; Brown, P. J.; Page, M. J.; Hartmann, D. H.; Pandey, S. B.; Giommi, P.; Schady, P.; Boutsia, K.; Rau, A.; Orio, M.; Pagani, C.; O'Brien, P. T.; Ciroi, S.; D'Aì, A.; Kennea, J. A.; Buckley, D. A. H.; Morrell, N.; Edwards, P. G.; Stevens, J.; De Pasquale, M.; Perri, M.; Osborne, J. P.; Page, K. L.; Beardmore, A. P.; Oates, S. R.; Evans, P. A.; Kuin, N. P. M.; Caputo, R.; Racusin, J. L.; Marshall, F.; Nicholl, M.; Gupta, R.; Palmer, D. M.; Breeveld, A. A.; Nousek, J. A.; Barthelmy, S. D.; Bernardini, M. G.; Malesani, D. B.; Krimm, H. A.; Gronwall, C.; Klingler, N. J.; Siegel, M. H.; Tohuvavohu, A.; Ambrosi, E.; Laha, S.; Schlieder, J. E.; Xu, D.; Bonnerot, C.; Ridley, E.; Parsotan, T.
United Kingdom, United States, Chile, South Africa, Italy, Australia, Poland, India, Germany, Canada, Netherlands, Denmark, Japan, China
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 > 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.