Superluminous supernova

A superluminous supernova (SLSN, plural superluminous supernovae or SLSNe) is a type of stellar explosion with a luminosity 10 or more times higher than that of standard supernovae. Like supernovae, SLSNe seem to be produced by several mechanisms, which is readily revealed by their light-curves and spectra. There are multiple models for what conditions may produce an SLSN, including core collapse in particularly massive stars, millisecond magnetars, interaction with circumstellar material (CSM model), or pair-instability supernovae. The first confirmed superluminous supernova connected to a gamma ray burst was not found until 2003, when GRB 030329 illuminated the Leo constellation. SN 2003dh represented the death of a star 25 times more massive than the Sun, with material being blasted out at over a tenth the speed of light. Stars with M ≥ 40 M☉ are likely to produce superluminous supernovae.

Classification Discoveries of many SLSNe in the 21st century showed that not only were they more luminous by an order of magnitude than most supernovae, their remnants were also unlikely to be powered by the typical radioactive decay that is responsible for the observed energies of conventional supernovae. SLSNe events use a separate classification scheme to distinguish them from the conventional Type Ia, Type Ib/Ic, and Type II supernovae, roughly distinguishing between the spectral signature of hydrogen-rich and hydrogen-poor events. Hydrogen-rich SLSNe are classified as Type SLSN-II, with observed radiation passing through the changing opacity of a thick expanding hydrogen envelope. Most hydrogen-poor events are classified as Type SLSN-I, with its visible radiation produced from a large expanding envelope of material powered by an unknown mechanism. A third less common group of SLSNe is also hydrogen-poor and abnormally luminous, referred to as SLSN-R, clearly powered by radioactivity from 56Ni. Increasing number of discoveries find that some SLSNe do not fit cleanly into these three classes, so further sub-classes or unique events have been described. Many or all SLSN-I show spectra without hydrogen or helium but have lightcurves comparable to conventional Type Ic supernovae, and are now classed as SLSN-Ic. PS1-10afx is an unusually red hydrogen-free SLSN with an extremely rapid rise to a near-record peak luminosity and an unusually rapid decline. PS1-11ap is similar to a Type Ic SLSN but has an unusually slow rise and decline.

Astrophysical models A wide variety of causes have been proposed to explain events that are an order of magnitude or more greater than standard supernovae. The collapsar and CSM (circumstellar material) models are generally accepted and a number of events are well-observed. Other models are still only tentatively accepted or remain entirely theoretical.

Collapsar model

The collapsar model is a type of superluminous supernova that produces a gravitationally collapsed object, or black hole. The word "collapsar", short for "collapsed star", was formerly used to refer to the end product of stellar gravitational collapse, a stellar-mass black hole. The word is now sometimes used to refer to a specific model for the collapse of a fast-rotating star. When core collapse occurs in a star with a core at least around fifteen times the Sun's mass (M☉)—though chemical composition and rotational rate are also significant—the explosion energy is insufficient to expel the outer layers of the star, and it will collapse into a black hole without producing a visible supernova outburst. A star with a core mass slightly below this level—in the range of 5–15 M☉—will undergo a supernova explosion, but so much of the ejected mass falls back onto the core remnant that it still collapses into a black hole. If such a star is rotating slowly, then it will produce a faint supernova, but if the star is rotating quickly enough, then the fallback to the black hole will produce relativistic jets. The energy that these jets transfer into the ejected shell renders the visible outburst substantially more luminous than a standard supernova. The jets also beam high energy particles and gamma rays directly outward and thereby produce x-ray or gamma-ray bursts; the jets can last for several seconds or longer and correspond to long-duration gamma-ray bursts, but they do not appear to explain short-duration gamma-ray bursts. Stars with 5–15 M☉ cores have an approximate total mass of 25–90 M☉, assuming the star has not undergone significant mass loss. Such a star will still have a hydrogen envelope and will explode as a Type II supernova. Faint Type II supernovae have been observed, but no definite candidates for a Type II SLSN (except Type IIn, which are not thought to be jet supernovae). Only the very lowest metallicity population III stars will reach this stage of their life with little mass loss. Other stars, including most of those visible to us, will have had most of their outer layers blown away by their high luminosity and become Wolf-Rayet stars. Some theories propose these will produce either Type Ib or Type Ic supernovae, but none of these events so far has been observed in nature. Many observed SLSNe are likely Type Ic. Those associated with gamma-ray bursts are almost always Type Ic, being very good candidates for having relativistic jets produced by fallback to a black hole. However, not all Type Ic SLSNe correspond to observed gamma-ray bursts but the events would only be visible if one of the jets were aimed towards us. In recent years, much observational data on long-duration gamma-ray bursts have significantly increased our understanding of these events and made clear that the collapsar model produces explosions that differ only in detail from more or less ordinary supernovae and have energy ranges from approximately normal to around 100 times larger. A good example of a collapsar SLSN is SN 1998bw, which was associated with the gamma-ray burst GRB 980425. It is classified as a Type Ic supernova due to its distinctive spectral properties in the radio spectrum, indicating the presence of relativistic matter.

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