A Detection of Helium in the Bright Superluminous Supernova SN 2024rmj

Abstract We present extensive ultraviolet, optical, and near-infrared (NIR) photometric and spectroscopic observations of the nearby hydrogen-poor superluminous supernova (SLSN-I) SN 2024rmj at z = 0.1189. SN 2024rmj reached a peak absolute magnitude of M g ≈ −21.9, placing it at the luminous end of the SLSN-I distribution. The light curve exhibits a pronounced prepeak bump (≈60 days before the main peak) and a postpeak bump (≈55 days after the main peak). The bulk of the light curve is otherwise well fit by a magnetar spin-down model, with typical values (spin: ≈2.1 ms; magnetic field: ≈6 × 10 13 G; ejecta mass: ≈12 M ⊙ ). The optical spectra exhibit characteristic SLSN-I features and evolution, but with a relatively high velocity of ≈8000 km s −1 postpeak. Most significantly, we find a clear detection of helium in the NIR spectra at He i λ 1.083 μ m and λ 2.058 μ m, blueshifted by ≈15,000 km s −1 (13 days before peak) and ≈13,000 km s −1 (40 days after peak), indicating that helium is confined to the outermost ejecta; based on these NIR detections, we also identify likely contribution from He i λ 5876 in the optical spectra on a similar range of timescales. This represents the most definitive detection of helium in a bright SLSN-I to date, and indicates that progenitors with a thin helium layer can still explode as SLSNe.