%I Oxford University Press (OUP)
%L discovery10192060
%K Black hole physics, gravitational waves, galaxies: nuclei, ultraviolet: general, transients: tidal disruption events
%J Monthly Notices of the Royal Astronomical Society
%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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%A SR Oates
%A NPM Kuin
%A M Nicholl
%A F Marshall
%A E Ridley
%A K Boutsia
%A AA Breeveld
%A DAH Buckley
%A SB Cenko
%A M De Pasquale
%A PG Edwards
%A M Gromadzki
%A R Gupta
%A S Laha
%A N Morrell
%A M Orio
%A SB Pandey
%A MJ Page
%A KL Page
%A T Parsotan
%A A Rau
%A P Schady
%A J Stevens
%A PJ Brown
%A PA Evans
%A C Gronwall
%A JA Kennea
%A NJ Klingler
%A MH Siegel
%A A Tohuvavohu
%A E Ambrosi
%A SD Barthelmy
%A AP Beardmore
%A MG Bernardini
%A C Bonnerot
%A S Campana
%A R Caputo
%A S Ciroi
%A G Cusumano
%A AD Aì
%A PD Avanzo
%A VD Elia
%A P Giommi
%A DH Hartmann
%A HA Krimm
%A DB Malesani
%A A Melandri
%A JA Nousek
%A PTO Brien
%A JP Osborne
%A C Pagani
%A DM Palmer
%A M Perri
%A JL Racusin
%A T Sakamoto
%A B Sbarufatti
%A JE Schlieder
%A G Tagliaferri
%A E Troja
%A D Xu
%V 530
%T Swift/UVOT discovery of Swift J221951−484240: a UV luminous ambiguous nuclear transient
%N 2
%P 1688-1710
%D 2024