SN 2021aaev: A Hydrogen-rich Superluminous Supernova with Early Flash and Long-lived Circumstellar Interaction in an Unusual Host Environment

We present photometric and spectroscopic observations of SN 2021aaev, a hydrogen-rich, superluminous supernova with persistent (at least $\sim$100 days) narrow Balmer lines (SLSN-IIn) at redshift $z = 0.1557$. SN 2021aaev rose over $28.1 \pm 1.0$ rest-frame days after explosion, reaching a peak absolute magnitude of $-21.46 \pm 0.01$ in the \emph{ATLAS $o$ band}. The prepeak spectra resemble those of typical Type IIn supernovae with flash-ionization features arising from the interaction with a dense, confined circumstellar medium (CSM), albeit the flash timescale is longer than usual ($>20$ days). Postpeak, the narrow emission lines evolve slowly, and the absence of ejecta features indicates strong deceleration by the CSM. The total radiated energy ($1.41 \times 10^{51}$ erg) is possibly explained by a low-mass ejecta ($1$--$2,M_\odot$) ploughing into a massive ($9$--$19,M_\odot$), extended (outer radius $>1 \times 10^{16}$ cm) H-rich CSM, or alternatively by magnetar-powered models. Interestingly, the host environment consists of a spiral galaxy with a red substructure in the southeastern part, and the SN's exact location coincides with this quiescent red region (star formation rate $= 0.02_{-0.02}^{+0.13},M_\odot,\mathrm{yr}^{-1}$). Given the atypical environment and the obscuring effect of the massive CSM, a thermonuclear origin (Type Ia-CSM) cannot be ruled out. Altogether, SN 2021aaev is a compelling case to study the diversity of SLSN-IIn features and their host environments.