A Swift Fix II: Physical Parameters of Type I Superluminous Supernovae

In November 2020, the Swift team announced a major update to the calibration of the UltraViolet and Optical Telescope (UVOT) data to correct for the gradual loss of sensitivity over time. Beginning in roughly 2015, the correction affected observations in the three near ultraviolet (UV) filters, reaching levels of up to 0.3 mag immediately prior to the correction. Over the same time period, an increasing number of Type I superluminous supernovae (SLSNe-I) were discovered and studied. Many SLSNe-I are hot (T$_\textrm{eff}$ $\approx 10,000$ K) near peak, and therefore accurate UV data are imperative towards properly understanding their physical properties and energetics. We re-compute Swift UVOT photometry for SLSNe-I discovered between 2014 and 2021 with at least 5 Swift observations in 2015 or later. We calculate host-subtracted magnitudes for each SLSN and fit their spectral energy distributions with modified blackbodies to obtain the radius and temperature evolution. We also fit multi-band photometry using the Modular Open Source Fitter for Transients (MOSFiT) to obtain key parameters such as the spin period (P), magnetic field strength (B), ejecta mass (M$_\textrm{ej}$), and kinetic energy (E$_\textrm{kin}$). From our MOSFiT modeling, we also estimate the peak UV/optical luminosity (L$_\textrm{peak}$) and total radiative energy (E$_\textrm{rad}$). Under the assumption of magnetar-powered SLSNe we find several strong trends, including anti-correlations between P and both L$_\textrm{peak}$ and E$_\textrm{rad}$, a correlation between E$_\textrm{kin}$ and E$_\textrm{rad}$, and an anti-correlation between B and E$_\textrm{rad}$.