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 angstroms (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 angstrom feature, enhancing the ratio, along with [O III] lines at 4363 and 5007 angstroms. 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 $M_{ \rm ej}\lesssim12$ M$_{ \odot}$. The ionization in the outliers with increasing $L_{ 7300}/L_{ 6300}$ may be due to late CSM interaction. $L_{ 7300}/L_{ 6300}$ and its evolution are therefore key diagnostics of SLSN engines and progenitor mass loss.
Categories: astro-ph.HE