Diagnosing Magnetar GW Emission From Supernova Light Curves: UV –Optical Colors and Direct Detectability

Newborn millisecond magnetars can power supernovae via electromagnetic spin‐down, while non‐axisymmetric deformation drains rotational energy through gravitational waves (GWs). We quantify how these competing losses affect ultraviolet light curves, early shock dynamics and GW detectability. The calculation combines birth‐initialized spin‐down, a one‐zone supernova diffusion model, a thin‐shell shock calculation, and a three‐interferometer Einstein Telescope (ET) response with ET‐D sensitivity. For ms, G and , GW losses lower the bolometric peak by about and dim the NUV maximum by mag. For 3500 population draws at 10 Mpc, the 7‐day evolving‐signal calculation gives SNR for (sky/inclination‐averaged ET‐D response) and (explicit orientations); an O5c proxy gives for a single interferometer. These are ideal coherent‐SNR benchmarks, not pipeline efficiencies. Using approximate ULTRASAT sensitivity, the idealized cumulative rate to 100 Mpc is year −1 without GW losses and year −1 with a broad ellipticity prior. Hence substantial emission changes in individual highly deformed sources need not produce comparably large changes in population‐averaged flux‐limited counts. These conclusions remain conditional on simplified assumptions.