Extreme Stellar Death and Galaxy Feedback at z = 2: A Rest-frame Ultraviolet Characterization of the Strongly Lensed Superluminous Supernova 2025wny

We present photometric and spectroscopic follow-up observations of the strongly-lensed H-poor superluminous supernova (SLSN) 2025wny at z = 2.0155. We use integral-field Keck-II/KCWI spectroscopy to obtain rest-frame ultraviolet spectra spanning 11 to 61 days after maximum light. We use Pan-STARRS photometry to constrain the brightest image's magnification, finding μ A=19.4 plus or minus1.6, a factor of about 2-4 higher than mass-modeling estimates. We fit the Pan-STARRS photometry with a magnetar energy-injection model including 56 Ni decay using the Modular Open Source Fitter for Transients (MOSFiT); the model can reproduce the near-maximum light curve with parameters typical of local universe SLSNe-I, including a magnetar spin period P spin=4.1+0.7-1.0, ms and magnetic field B=2.4+1.1-0.8 times10 14, G; however, the late-time plateau suggests an additional power source. Spectroscopically, iron-group element line blanketing is virtually absent in the rest-frame UV spectra, and the absorption features match previously identified SLSN-I features from abundance tomography. Based on narrow ISM line metallicity estimates, SN 2025wny exploded in a sub-solar-metallicity galaxy. From host-galaxy lines in our rest-frame UV-to-optical spectrum we estimate high-velocity outflows of about 575 km s-1 and an exceptionally high Ly α escape fraction of f esc Lyα=0.23 plus or minus0.03. SN 2025wny matches lower-redshift SLSNe reasonably well. Upcoming surveys like LSST and Roman will discover additional high-redshift SLSNe, further testing whether SLSNe-I explosions remain consistent at the population level in the early universe.