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

S. R. Oates, N. P.M. Kuin, M. Nicholl, F. Marshall, E. Ridley, K. Boutsia, A. A. Breeveld, D. A.H. Buckley, S. B. Cenko, M. De Pasquale, P. G. Edwards, M. Gromadzki, R. Gupta, S. Laha, N. Morrell, M. Orio, S. B. Pandey, M. J. Page, K. L. Page, T. ParsotanA. Rau, P. Schady, J. Stevens, P. J. Brown, P. A. Evans, C. Gronwall, J. A. Kennea, N. J. Klingler, 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. Aì, 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. Xu

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Pages (from-to)1688-1710
Number of pages23
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date19 Mar 2024
Publication statusPublished - 1 May 2024

Data Availability Statement

The Swift data underlying this article are available in the Swift archives at https://www.swift.ac.uk/swift_live/, https://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/swift.pl, https://www.ssdc.asi.it/mmia/index.php?mission = swiftmastr. The photometry of J221951 is available in the online supplementary material. The ATCA data are available from the Australia Telescope Online Archive – https://atoa.atnf.csiro.au/. The HST and GALEX observations are available from the MAST Portal – https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html. The X-shooter spectrum and GROND data are available from the ESO main archive – http://archive.eso.org/eso/eso_archive_main.html.


  • black hole physics
  • galaxies: nuclei
  • gravitational waves
  • transients: tidal disruption events
  • ultraviolet: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this