Hydrogen-poor Superluminous Supernovae in the Nebular Phase: Spectral Diversity Due to Ejecta Ionization as a Probe of the Power Source

We present a large sample of 39 nebular-phase optical spectra of 25 hydrogen-poor superluminous supernovae (SLSNe-I) and jointly analyze them with previously published spectra of 12 events. We measure the properties of key emission features, namely, those at 6300, 7300, and 7774 Å (associated with [O i ], [Ca ii ]/[O ii ], and O i , respectively), and find that SLSNe exhibit much wider spectral diversity than normal SNe Ic, primarily in the line ratio L 7300 / L 6300 , which is highly sensitive to ejecta ionization. Some events exhibit weak [O i ] and a clear [O ii ] contribution to the 7300 Å feature, enhancing the ratio, along with [O iii ] lines at 4363 and 5007 Å. Other SLSNe show weak or no lines of ionized oxygen. Moreover, we find that the population exhibits decreasing L 7300 / L 6300 over time, while a few outliers instead display sustained high or increasing ratios for extended periods. The ratio L 7300 / L 6300 is also correlated with the rise and decline times of the light curves, with slower events exhibiting higher ionization, the first robust connection between early light-curve and late-time spectral properties, likely due to the magnetar’s impact: slower-evolving SLSNe are generally powered by engines with longer spin-down timescales, which deposit more energy at later phases. Among the events with decreasing L 7300 / L 6300 , SLSNe with high ionization are on average powered by magnetars with higher thermalized spin-down power, a correlation that is most significant for events with M ej ≲ 12 M ⊙ . The ionization in the outliers with increasing L 7300 / L 6300 may be due to late circumstellar medium interaction. L 7300 / L 6300 and its evolution are therefore key diagnostics of SLSN engines and progenitor mass loss.