Extreme variability in an active galactic nucleus: Gaia16aax
: Cannizzaro G, Fraser M, Jonker PG, Pringle JE, Mattila S, Hewett PC, Wevers T, Kankare E, Kostrzewa-Rutkowska Z, Wyrzykowski L, Onori F, Harmanen J, Ford KES, McKernan B, Nixon CJ
Publisher: OXFORD UNIV PRESS
: 2020
: Monthly Notices of the Royal Astronomical Society
: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
: MON NOT R ASTRON SOC
: 493
: 1
: 477
: 495
: 19
: 0035-8711
: 1365-2966
DOI: https://doi.org/10.1093/mnras/staa186
: https://research.utu.fi/converis/portal/detail/Publication/46520489
We present the results of a multiwavelength follow-up campaign for the luminous nuclear transient Gaia16aax, which was first identified in 2016 January. The transient is spatially consistent with the nucleus of an active galaxy at z = 0.25, hosting a black hole of mass similar to 6 x 10(8) M-circle dot. The nucleus brightened by more than 1 mag in the Gaia G band over a time-scale of less than 1 yr, before fading back to its pre-outburst state over the following 3 yr. The optical spectra of the source show broad Balmer lines similar to the ones present in a preoutburst spectrum. During the outburst, the H alpha and H beta emission lines develop a secondary peak. We also report on the discovery of two transients with similar light-curve evolution and spectra: Gaia16aka and Gaia16ajq. We consider possible scenarios to explain the observed outbursts. We exclude that the transient event could be caused by amicrolensing event, variable dust absorption or a tidal encounter between a neutron star and a stellar mass black hole in the accretion disc. We consider variability in the accretion flow in the inner part of the disc, or a tidal disruption event of a star >= 1 M-circle dot by a rapidly spinning supermassive black hole as the most plausible scenarios. We note that the similarity between the light curves of the three Gaia transients may be a function of the Gaia alerts selection criteria.
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