What powers hydrogen-poor superluminous supernovae?
In plain words
A rare kind of supernova shines 10 to 100 times brighter than usual, more than its radioactive nickel can supply, perhaps powered by a newborn spinning magnetar or by crashing into gas shed earlier.
Precise statement
Type I superluminous supernovae peak at $L \sim 1e44\,\mathrm{erg}/\mathrm{s}$ and radiate $\sim 1e51\,\mathrm{erg}$. Decide among spin-down of a millisecond magnetar $(P \sim 1\ \text{to}\ 5\,\mathrm{ms}, B \sim 1e14\,\mathrm{G})$, interaction with hydrogen-poor circumstellar shells, and pair-instability explosions, using late-time light-curve slopes, light-curve bumps, nebular spectra and late radio or X-ray emission from a central nebula. Answer: the power source for each subclass with the evidence.
What would settle it
Detection or exclusion of late-time radio and X-ray emission from an embedded magnetar nebula in nearby events, together with nebular spectra that fix the ejecta mass.