SN 2021adxl: A luminous nearby interacting supernova in an extremely low metallicity environment

SN 2021adxl is a slowly evolving, luminous, Type IIn supernova with asymmetric emission line profiles, similar to the well-studied SN 2010jl. We present extensive optical, near-ultraviolet, and near-infrared photometry and spectroscopy covering ~1.5 years post discovery. SN 2021adxl occurred in an unusual environment, atop a vigorously star-forming region that is offset from its host galaxy core. The appearance of Ly-alpha, O II, as well as the compact core, would classify the host of SN 2021adxl as a Blueberry galaxy, analogous to the higher redshift Green Pea galaxies. Using several abundance indicators, we find a metallicity of the explosion environment of only 10% solar, the lowest reported metallicity for a Type IIn SN environment. SN 2021adxl reaches a peak magnitude of r ~ -20.2 mag and since discovery, SN 2021adxl has faded by only ~4 magnitudes in the r band with a cumulative radiated energy of ~1.5e50 erg over 18 months. SN 2021adxl shows strong signs of interaction with a complex circumstellar medium, seen by the detection of X-rays, revealed by the detection of coronal emission lines, and through multi-component hydrogen and helium profiles. In order to further understand this interaction, we model the H{\alpha} profile using a Monte-Carlo electron scattering code. The blueshifted high-velocity component is consistent with emission from a radially thin, spherical shell resulting in the broad emission components due to electron scattering. Using the velocity evolution of this emitting shell, we find that the SN ejecta collide with circumstellar material of at least 5 Msun, assuming a steady-state mass-loss rate of 4-6e-3 Msun per year for the first ~200 days of evolution. Continuing the observations of SN 2021adxl may reveal signatures of dust formation or an infrared excess, similar to that seen for SN 2010jl.