SN 2024afav: A Superluminous Supernova with Multiple Light-curve Bumps and Spectroscopic Signatures of Circumstellar Interaction

Abstract We present a comprehensive optical and near-infrared (NIR) spectroscopic study of SN 2024afav, a hydrogen-poor superluminous supernova (SLSN-I) that peaks at ≈−20.7 mag and exhibits an unusual multibumped light curve. Our spectroscopic observations, spanning phases of −14 to +160 days, reveal several unusual features: (i) a narrow (1800 km s −1 ) and blueshifted (11,000 km s −1 ) absorption from H α starting at +20 days; (ii) persistent optical and NIR He i lines at all available phases, showing double absorption structure in NIR spectra at +23 days, with a high-velocity component at a similar velocity to H α ; (iii) early appearance of nebular [O iii ] emission starting at ≈+50 days; and (iv) a strong [O ii ] + [Ca ii ] 7300 Å emission complex starting at ≈+110 days. These unusual features, and their onset at the time of the light-curve bumps, provide compelling evidence of circumstellar interaction between the SN ejecta and a nearby hydrogen-rich shell, as well as the presence of helium in both the outer layers of the progenitor star and the circumstellar medium. A comparison of SN 2024afav to other SLSNe-I showing bumpy light curves and similar spectral properties (PTF 10hgi, SN 2017egm, SN 2019hge) points to a rare subgroup of SLSNe-I in which circumstellar medium interaction provides an important modulation to the energy input.