Superluminous supernovae are hydrogen-rich (SLSNe-II), or hydrogen-poor (SLSNe-I), explosions so bright that they require a power source beyond that of traditional supernovae. SLSNe-I rise to a peak over 20-90 days, then decline over a timescale roughly twice as long. At early times they have a blue continuum, peaking in the ultraviolet, have temperatures in excess of 14,000 K, and show ionized lines of carbon and oxygen out of thermodynamic equilibrium. As the supernovae cool, their spectra start to resemble SNe Ic, though with a time delay. They also favor environments with metallicities half solar or lower. Modeling indicates that they are explosions of stripped carbon oxygen stellar cores, similar to, but sometimes more massive than the progenitors of SNe Ic. SLSNe-I similar to SN 2007bi have broader lightcurves, and seemingly more massive progenitors. Some have proposed that these are pair instability supernovae, but in general the supernovae rise too quickly for this model. Most SLSNe-I show no signs of interaction, and instead seem to be powered by a central engine. The magnetar spin-down model has been the most successful at reproducing the lightcurves and peak luminosity of SLSNe, though it may not be unique. Most SLSNe-II seem to be powered by interaction of these SNe with circumstellar material, as in SNe IIn. However, there are a handful of hybrid cases, or SLSNe-II, with weak or little interaction, which may be related to SLSNe-I.
Superluminous supernovae (SLSNe), as the name attests, are supernovae that are brighter than usual. As a result of the overly broad name, the category is a catch-all describing several classes of supernovae – some with hydrogen, some without, some interacting, some probably not. A few authors have defined SLSNe as those brighter than \(M=-21\) at peak, though this arbitrary cut could leave out related physical phenomena. Instead, I define SLSNe as luminous SNe which cannot be explained by the power sources fueling traditional (Types I and II) supernovae: radioactive decay from a moderate amount of elements synthesized in the explosion, the energy deposited by a shock unbinding the star, or interaction with moderate but obvious amounts of circumstellar material (CSM) previously lost by the supernova progenitor or a companion.
This last point creates a gray area. Should Type IIn supernovae count as SLSNe? Type IIn supernovae are those with a strong blue continuum at early times, and narrow and intermediate width hydrogen emission lines at some points in their spectroscopic evolution. They are thought to be the collapse of massive stars whose ejecta shock CSM. On one hand, they have been recognized as a class since the 1980s, a large and diverse one, and the source of their luminosity is not a mystery. On the other, some SNe IIn are so bright that they have been considered SLSNe [e.g. SN 2006gy (Smith et al., 2007; Ofek et al., 2007), which reached a peak absolute magnitude of \(-22\)]. A complicating factor is that interaction should be considered as a possible power source for SLSNe, whether or not the spectra show narrow lines. Here I compromise – I will generally not include clear SNe IIn, as their power source is not a mystery. However, I will mention a few extraordinary cases where appropriate, and discuss interaction as a possible power source.