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Title: Extreme variability in an active galactic nucleus: Gaia16aax
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.  more » « less
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Date Published:
Journal Name:
Monthly Notices of the Royal Astronomical Society
Page Range / eLocation ID:
477 to 495
Medium: X
Sponsoring Org:
National Science Foundation
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