<i>Swift</i>/UVOT discovery of <i>Swift</i> J221951-484240: a UV luminous ambiguous nuclear transient
Creators
- 1. Univ Coll London, Mullard Space Sci Lab, Dorking RH5 6NT, Surrey, England
- 2. NASA, Goddard Space Flight Ctr, Astrophys Sci Div, Greenbelt, MD 20771 USA
- 3. Univ Messina, MIFT Dept, Viale FS DAlcontres 31, I-98166 Messina, Italy
- 4. CSIRO Space & Astron, Australia Telescope Natl Facil, POB 76, Epping, NSW 1710, Australia
- 5. Univ Warsaw, Astron Observ, Al Ujazdowskie 4, PL-00478 Warsaw, Poland
- 6. Aryabhatta Res Inst Observat Sci ARIES, Manora Peak 263002, Nainital, India
- 7. Univ Leicester, Sch Phys & Astron, Leicester LE1 7RH, England
Description
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 similar to 2.5 x10(4) K. At a redshift z = 0.5205, J221951 had a peak absolute magnitude of M-u,M-AB = -23 mag, peak bolometric luminosity L-max = 1.1 x10(45) erg s(-1) and a total radiated energy of E > 2.6 x1052 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 alpha 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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