Hypernova

Ahypernovais a very energeticsupernovawhich is believed to result from an extreme core collapse scenario. In this case, a massive star (>30solar masses) collapses to form arotating black holeemitting twin astrophysical jets and surrounded by anaccretion disk.It is a type ofstellar explosionthat ejects material with an unusually highkinetic energy,an order of magnitude higher than most supernovae, with a luminosity at least 10 times greater. Hypernovae release such intensegamma raysthat they often appear similar to atype Ic supernova,but with unusually broad spectral lines indicating an extremely high expansion velocity. Hypernovae are one of the mechanisms for producing longgamma ray bursts (GRBs),which range from 2 seconds to over a minute in duration. They have also been referred to assuperluminous supernovae,though that classification also includes other types of extremely luminous stellar explosions that have different origins.

History

In the 1980s, the termhypernovawas used to describe a theoretical type of supernova now known as apair-instability supernova.It referred to the extremely high energy of the explosion compared to typicalcore collapse supernovae.^[1]^[2]^[3]The term had previously been used to describe hypothetical explosions from diverse events such ashyperstars,extremely massivepopulation IIIstars in the early universe,^[4]or from events such asblack holemergers.^[5]

In February 1997, Dutch-Italian satelliteBeppoSAXwas able to traceGRB 970508to a faint galaxy roughly 6 billion light years away.^[6]From analyzing the spectroscopic data for both the GRB 970508 and its host galaxy, Bloom et al. concluded in 1998 that a hypernova was the likely cause.^[6]That same year, hypernovae were hypothesized in greater detail by Polish astronomerBohdan Paczyńskias supernovae from rapidly spinning stars.^[7]

The usage of the termhypernovafrom the late 20th century has since been refined to refer to those supernovae with unusually large kinetic energy.^[8]The first hypernova observed wasSN 1998bw,with a luminosity 100 times higher than a standard Type Ib.^[9]This supernova was the first to be associated with a gamma-ray burst (GRB) and it produced a shockwave containing an order of magnitude more energy than a normal supernova. Other scientists prefer to call these objects simply broad-linedtype Ic supernovae.^[10]Since then the term has been applied to a variety of objects, not all of which meet the standard definition; for exampleASASSN-15lh.^[11]

In 2023, the observation of the highly energetic,non-quasartransient eventAT2021lwxwas published with an extremely strong emission frommid-infraredtoX-raywavelengths and an overall energy of 1.5 10⁴⁶Joule.^[12]This object is not thought to be a hypernova; instead, it is likely to be a huge gas cloud being absorbed by a massive black hole. The event was also assigned the random name "ZTF20abrbeie" by theZwicky Transient Facility.This name and the seeming ferocity of the event led to nickname "Scary Barbie", drawing the attention of the mainstream press.[1]

Properties

Hypernovae are thought to be supernovae with ejecta having a kinetic energy larger than about10⁴⁵joule,an order of magnitude higher than a typical core collapse supernova. The ejected nickel masses are large and the ejection velocity up to 99% of thespeed of light.These are typically of type Ic, and some are associated with long-durationgamma-ray bursts.Theelectromagneticenergy released by these events varies from comparable to other type Ic supernova, to some of the most luminous supernovae known such asSN 1999as.^[13]^[14]

The archetypal hypernova, SN 1998bw, was associated withGRB 980425.Its spectrum showed no hydrogen and no clearheliumfeatures, but strong silicon lines identified it as a type Ic supernova. The mainabsorption lineswere extremely broadened and the light curve showed a very rapid brightening phase, reaching the brightness of atype Ia supernovaat day 16. The total ejected mass was about 10M_☉and the mass of nickel ejected about 0.4M_☉.^[13]All supernovae associated with GRBs have shown the high-energy ejecta that characterises them as hypernovae.^[15]

Unusually brightradio supernovaehave been observed as counterparts to hypernovae, and have been termed "radio hypernovae".^[16]

Astrophysical models

Models for hypernova focus on the efficient transfer of energy into the ejecta. In normalcore collapse supernovae,99% ofneutrinosgenerated in the collapsing core escape without driving the ejection of material. It is thought that rotation of the supernova progenitor drives a jet that accelerates material away from the explosion at close to the speed of light. Binary systems are increasingly being studied as the best method for both stripping stellar envelopes to leave a bare carbon-oxygen core, and for inducing the necessary spin conditions to drive a hypernova.

Collapsar model

The collapsar model describes a type of supernova that produces a gravitationally collapsed object, orblack hole.The word "collapsar", short for "collapsedstar",was formerly used to refer to the end product of stellargravitational collapse,astellar-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 theSun'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 5–15M_☉— 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 producerelativistic jets.Those powerful jets plough through stellar material produce strong shock waves, with the vigorous winds of newly-formed⁵⁶Ni blowing off the accretion disk, detonating the hypernova explosion. The ejected radioactive decay of⁵⁶Ni renders the visible outburst substantially more luminous than a standard supernova.^[17]The jets also beam high energy particles and gamma rays directly outward and thereby producex-rayorgamma-raybursts; 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.^[18]^[19]

Binary models

The mechanism for producing the stripped progenitor, a carbon-oxygen star lacking any significant hydrogen or helium, of type Ic supernovae was once thought to be an extremely evolved massive star, for example a type WOWolf-Rayet starwhose densestellar windexpelled all its outer layers. Observations have failed to detect any such progenitors. It is still not conclusively shown that the progenitors are actually a different type of object, but several cases suggest that lower-mass "helium giants" are the progenitors. These stars are not sufficiently massive to expel their envelopes simply by stellar winds, and they would be stripped by mass transfer to a binary companion. Helium giants are increasingly favoured as the progenitors of type Ib supernovae, but the progenitors of type Ic supernovae is still uncertain.^[20]

One proposed mechanism for producing gamma-ray bursts is inducedgravitational collapse,where aneutron staris triggered to collapse into ablack holeby the core collapse of a close companion consisting of a stripped carbon-oxygen core. The induced neutron star collapse allows for the formation of jets and high-energyejectathat have been difficult to model from a single star.^[21]

References

^Woosley, S. E.; Weaver, T. A. (1981). "Theoretical models for supernovae".NASA Sti/Recon Technical Report N.83:16268.Bibcode:1981STIN...8316268W.
^Janka, Hans-Thomas (2012)."Explosion Mechanisms of Core-Collapse Supernovae".Annual Review of Nuclear and Particle Science.62(1): 407–451.arXiv:1206.2503.Bibcode:2012ARNPS..62..407J.doi:10.1146/annurev-nucl-102711-094901.S2CID 118417333.
^Gass, H.; Liebert, James; Wehrse, R. (1988). "Spectrum analysis of the extremely metal-poor carbon dwarf star G 77-61".Astronomy and Astrophysics.189:194.Bibcode:1988A&A...189..194G.
^Barrington, R. E.; Belrose, J. S. (1963). "Preliminary Results from the Very-Low Frequency Receiver Aboard Canada's Alouette Satellite".Nature.198(4881): 651–656.Bibcode:1963Natur.198..651B.doi:10.1038/198651a0.S2CID 41012117.
^Park, Seok J.; Vishniac, Ethan T. (1991). "Are Hypernovae Detectable?".The Astrophysical Journal.375:565.Bibcode:1991ApJ...375..565P.doi:10.1086/170217.
^^a ^bBloom (1998). "The Host Galaxy of GRB 970508".The Astrophysical Journal.507(507): L25–28.arXiv:astro-ph/9807315.Bibcode:1998ApJ...507L..25B.doi:10.1086/311682.S2CID 18107687.
^Paczynski (1997).GRBs as Hypernovae.Huntsville Gamma-Ray Burst Symposium.arXiv:astro-ph/9712123.Bibcode:1997astro.ph.12123P.
^David S. Stevenson (5 September 2013).Extreme Explosions: Supernovae, Hypernovae, Magnetars, and Other Unusual Cosmic Blasts.Springer Science & Business Media.ISBN 978-1-4614-8136-2.Archivedfrom the original on 25 January 2022.Retrieved18 August2019.
^Woosley (1999). "Gamma-Ray Bursts and Type Ic Supernovae: SN 1998bw".The Astrophysical Journal.516(2): 788–796.arXiv:astro-ph/9806299.Bibcode:1999ApJ...516..788W.doi:10.1086/307131.S2CID 17690696.
^Moriya, Takashi J.; Sorokina, Elena I.; Chevalier, Roger A. (2018). "Superluminous Supernovae".Space Science Reviews.214(2): 59.arXiv:1803.01875.Bibcode:2018SSRv..214...59M.doi:10.1007/s11214-018-0493-6.S2CID 119199790.
^Jessica Orwig (January 14, 2016)."Astronomers are baffled by a newly discovered cosmic explosion that shines 570 billion times brighter than the sun".Business Insider.Archivedfrom the original on April 2, 2016.RetrievedMarch 22,2016.
^Wiseman, P.; et al. (2023).""Multiwavelength observations of the extraordinary accretion event AT2021lwx"".Monthly Notices of the Royal Astronomical Society.522(3): 3992–4002.arXiv:2303.04412.doi:10.1093/mnras/stad1000.
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