Gleeok's Fire-breathing: Triple Flares of AT 2021aeuk within Five Years from the Active Galaxy SDSS J161259.83+421940.3

Abstract

We present a noteworthy transient AT 2021aeuk exhibiting three distinct optical flares between 2018 and 2023. It is hosted in a radio-loud narrow-line Seyfert 1 (NLSy1) galaxy, with an optical image showing a minor tidal morphology and a red mid-infrared color W1-W2=1.1. Flares II and III exhibit rapid rises, and long-term decays (around 1000 days) with recurring after-peak bumps. The g-r color after subtracting the reference magnitude exhibited a rapid drop and recovery during Flare II, followed by the minor after-peak evolution in blue colors. We applied a canonical tidal disruption event (TDE) fitting on the light curves which gives a decay index p of $-2.99_{-0.14}^{+0.13}$ for Flare II and $-1.61_{-0.65}^{+0.34}$ for Flare III. The blackbody fitting shows lower temperatures ($\sim 10^{3.8}$K) with minor after-peak evolution. The blackbody radius ($\gtrsim 10^{16}\ \rm cm$) and luminosity ($\sim 10^{45}\rm erg\ s^{-1}$) are larger than the typical TDE sample's. The time lag (in rest frame) between ZTF g- and r-band ($\rm \tau_{g,r}=3.4^{+1.0}_{-0.9}$ days) significantly exceeds the prediction from the standard accretion disk. Pre-burst spectra reveal prominent Bowen fluorescence lines, indicating a vigorous or potentially long-lasting process that enriches the local metallicity. Additionally, we derived black hole masses of $\log\it{M}_{\bullet}\rm=7.09^{+0.18}_{-0.31}\ \it{M}_{\odot}$ and $\log\it{M}_{\bullet}\rm=7.52^{+0.08}_{-0.10}\ \it{M}_{\odot}$ using $\rm H\beta$ and $\rm H\alpha$ emission lines. The variation and recurring features of AT 2021aeuk are not likely induced by the radio-beaming effect or Type-II superluminous supernova (SLSN-II), however, we cannot rule out the possibility of TDE or enhanced active galactic nuclei (AGN) accretion process. The unusually high occurrence of three flares within five years may also induced by the complex local environment.