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

Abstract We present photometric and spectroscopic observations of SN 2021aaev, a hydrogen-rich, superluminous supernova with persistent (at least ∼100 days) narrow Balmer lines (SLSN-IIn) at redshift z = 0.1557. SN 2021aaev rose over 28.1 ± 1.0 rest-frame days after explosion, reaching a peak absolute magnitude of −21.46 ± 0.01 in the ATLAS o band. The prepeak spectra resemble those of typical Type IIn SNe 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 (about 1.41 × 10 51 erg) is possibly explained by a low-mass (1–2 M ⊙ ) ejecta ploughing into a massive (9–19 M ⊙ ), extended (outer radius >1 × 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 = 0.0 2 − 0.02 + 0.13 M ⊙ yr −1 ). Given the atypical environment and the obscuring effect of the massive CSM, a thermonuclear (Type Ia-CSM) origin cannot be ruled out. Altogether, SN 2021aaev is a compelling case to study the diversity of SLSN-IIn features and their host environments.