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A superluminous supernova (SLSN, plural superluminous supernovae or SLSNe) is a type of stellar explosion with an luminosity substantially higher than that of standard supernovae. It is often popularly referred to as a hypernova (plural hypernovae)[verification needed], which is a highly energetic (kinetic energy ~ 10^52 erg) core-collapse supernova.[verification needed]
Like the more common supernovae of typical energy, SLSNe seem to be produced by several different 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.
History of the term
Before the late-1990s, the term "hypernova" was used sporadically to describe the theoretical extremely energetic explosions of extremely massive population III stars.[verification needed] It has also been used to describe other extreme energy events, such as mergers of supermassive black holes. They were also referred to as luminous supernovae and occasionally super-luminous or ultra-luminous supernovae.
In 1998, a paper suggesting a link between gamma-ray bursts and young massive stars formally proposed to use the term "hypernova" for the visible after-glow from those gamma-ray bursts.[verification needed] The energy of such events was speculated to be up to several hundred times that of known supernovae.[verification needed]
Almost simultaneously, various over-luminous supernovae were being discovered and investigated  These events were described as hypernovae and varied from less than five to around 50 times as energetic as other supernovae and up to 20 times as luminous as a standard type Ia supernova at its peak. This definition has become standard for the term "hypernova", although not all of them are associated with gamma-ray bursts. The word hypernova itself was coined by S.E. Woosley.
Investigation of these types of luminous supernovae suggests that some of them are due to destruction of extremely massive low metallicity stars by the pair instability mechanism, although not with the energies that were speculated for them decades earlier.
The term superluminous supernova was not common before 2012, but was brought into widespread use by the publication of a paper categorising anf formalising the definition, ironically title Luminous Supernovae. The abbreviation SLSN was also used as a more manageable term.
Discoveries of many SLSNe in the 21st century showed that not only were they more luminous by an order of magnitude than most supernovae, but they were not powered by the radioactive decay that was responsible for the observed electromagnetic radiation of conventional supernovae.[verification needed] Such events clearly needed a separate classification scheme to distinguish them from the well-known type Ia, type Ib/Ic, and type II supernovae.
The initial classification of SLSNe loosely followed the classification for conventional supernovae, with a distinction between hydrogen-rich and hydrogen-poor events.[verification needed] The hydrogen-rich SLSNe were classified as type SLSN-II, with observed radiation mediated through the changing opacity of a thick expanding hydrogen envelope. Most of the hydrogen-poor events were classified as type SLSN-I, with visible radiation produced from a large expanding envelope of material, but ultimately powered by an unknown mechanism. A third less common group of SLSNe is also hydrogen-poor and abnormally luminous, but clearly powered by radioactivity from a large mass of 56Ni.
With an increasing number of discoveries, some SLSNe do not fit cleanly into these three classes and further sub-classes or unique events have been described. Many or all SLSN-I show spectra that have no hydrogen or helium and also have lightcurves comparable to conventional type Ic supernovae. These have been 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.
Gamma-ray bursts are some of the most energetic events observed in the universe, but their origin was entirely speculative until around the year 2000. Now, some supernova explosions are known to cause at least some gamma-ray bursts, although many likely occur from unrelated events.
A nearby gamma-ray burst could destroy life on Earth; however, no likely candidate progenitors are close enough to cause a cataclysmic extinction. Some astrophysicists have suggested that a gamma-ray burst may have caused the Ordovician–Silurian mass extinction on Earth 440 million years ago, but no categorical evidence for this hypothesis exists.
A wide variety of astrophysical models have been proposed to explain events an order of magnitude or more greater than standard supernovae. The collapsar and CSM (circumstellar material) models are widely accepted and a number of events are well-observed. Other models are still only tentatively accepted or remain entirely theoretical.
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 a 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 Sn1998bw, 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.
Circumstellar material Model
Almost all observed SLSNe have had spectra similar to either a type Ic or type IIn supernova. The type Ic SLSNe are thought to be produced by jets from fallback to a black hole, but type IIn SLSNe have significantly different light curves and are not associated with gamma-ray bursts. Type IIn supernovae are all embedded in a dense nebula probably expelled from the progenitor star itself, and this circumstellar material (CSM) is thought to be the cause of the extra luminosity. When material expelled in an initial normal supernova explosion meets dense nebular or material or dust close to the star, the shockwave converts kinetic energy efficiently into visible radiation. This effect greatly enhances these extended duration and extremely luminous supernovae, even though the initial explosive energy was the same as that of normal supernovae.
Although any supernova type could potentially produce Type IIn SLSNe, theoretical constraints on the surrounding CSM sizes and densities do suggest that it will almost always be produced from the central progenitor star itself immediately prior to the observed supernova event. Such stars are likely candidates of hypergiants or LBVs that appear undergoing substantial mass loss due to Eddington instability, for example SN2005gl.
Another type of suspected SLSN is a pair-instability supernova, of which SN 2006gy may possibly be the first observed example. This supernova event was observed in a galaxy about 238 million light years (73 megaparsecs) from Earth.
The theoretical basis for pair-instability collapse has been known for many decades and was suggested as a dominant source of higher mass elements in the early universe as super-massive population III stars exploded. In a pair-instability supernova, the pair production effect causes a sudden pressure drop in the star's core, leading to a rapid partial collapse. Gravitational potential energy from the collapse causes runaway fusion of the core which entirely destroys the star, leaving no remnant.
Models show that this phenomenon only happens in stars with extreme low metallicity and masses between about 140 and 260 times the Sun, making them extremely unlikely in the local universe. Although originally expected to produce SLSN explosions hundreds of times greater than a supernova, current models predict that they actually produce luminosities ranging from about the same as a normal core collapse supernova to perhaps 50 times brighter, although remaining bright for much longer.
Magnetar energy release
Models of the creation and subsequent spin down of a magnetar yield much higher luminosities than regular supernova events and match the observed properties of at least some SLSNe. In cases where pair-instability supernova may not be a good fit for explaining a SLSN, a magnetar explanation is more plausible.
There are still models for SLSN explosions produced from binary systems, white dwarf or neutron stars in unusual arrangements or undergoing mergers, and some of these are proposed to account for some observed gamma-ray bursts.
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