Extreme variability in an active galactic nucleus: Gaia16aax

• Published in: Monthly notices of the Royal Astronomical Society Vol. 493; no. 1; pp. 477 - 495
• Main Authors: Cannizzaro, G, Fraser, M, Jonker, P G, Pringle, J E, Mattila, S, Hewett, P C, Wevers, T, Kankare, E, Kostrzewa-Rutkowska, Z, Wyrzykowski, Ł, Onori, F, Harmanen, J, Ford, K E S, McKernan, B, Nixon, C J
• Format: Journal Article
• Language: English
• Published: Oxford University Press 01.03.2020
• Subjects: quasars: supermassive black holes; galaxies: nuclei; transients: tidal disruption events; quasars: individual: Gaia16aax; galaxies: active; accretion, accretion discs
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/staa186

ABSTRACT 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 ${\sim }6\times 10^8\, \mathrm{M}_\odot$. 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 pre-outburst spectrum. During the outburst, the H α and H β 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 a microlensing 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 ${\ge } 1 \, \mathrm{M}_{\odot }$ 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.