Some superluminous supernovae show a bright oxygen imprint in their light and expand rapidly, while others do not. It seemed like different phenomena. But supernova PTF12dam broke the rules: as its brightness grew, its oxygen signature changed, shifting from one type to another.
The solution is simple: all the diversity is just a matter of the temperature of the ejected material at the moment of maximum brightness. The mechanism works like a blowing bubble. At the center is a newborn neutron star the size of a city, spinning hundreds of times per second, with a magnetic field a quadrillion times stronger than Earth's. It pumps energy into the cloud of expelled matter. If the shell is massive, the bubble inflates slowly, cools off quickly, and oxygen in the glow is almost invisible, while the expansion speed drops off gently. Conversely, a light shell yields a rapid rise, a hot peak, and a clear oxygen signal. A simple model, where explosions differ only by the mass of the ejecta, fit the observations perfectly. Moreover, the distribution of material turned out to be uniform, like the film of a soap bubble. Thus, one supernova united what were thought to be different types.
as written by the authors arXiv:2601.05363v1
Type I superluminous supernovae (SLSNe) are a diverse class of exceptionally bright massive star explosions, which typically exhibit absorption from ionised oxygen in their early spectra. While their photometric properties (luminosity and duration) both span an order of magnitude, population studies suggest that these distributions are continuous. However, spectroscopic samples have shown some indications of distinct sub-types, either through similarity to certain prototype objects, or in terms of their velocity evolution. Here we show that a well-observed SLSN, PTF12dam, completely changes its O II absorption profile as it rises to maximum light, moving from one proposed sub-type to another. This supports an interpretation where spectroscopic diversity is driven by the ejecta temperature at maximum light, rather than fundamental differences in the explosion or progenitor. Motivated by this, we develop a new diagnostic, the Brightness-Timescale-Temperature-Radius diagram, and a simple toy model for the evolution of the photospheric velocity, to show that diversity in the light curve rise time (likely due to differences in ejected mass) naturally explains why SLSNe with broader light curves generally have weaker O II lines, lower photospheric velocities after maximum, and slower changes in photospheric velocity over time. We show that the velocity distribution of the known SLSN population favours a relatively flat ejecta density profile, consistent with a hot bubble inflated by a central engine.
One such supernova can outshine an entire galaxy, shining brighter than a hundred billion suns.