Hypernova

A hypernova (also known as a collapsar) is a very energetic

In February 1997, Dutch-Italian satellite BeppoSAX was able to trace GRB 970508 to 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 astronomer Bohdan Paczyński as supernovae from rapidly spinning stars.^[7]

The usage of the term hypernova from the late 20th century has since been refined to refer to those supernovae with unusually large kinetic energy.

Unusually bright radio supernovae have 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 normal

The collapsar model describes a type of 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.^[17][18]

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 WO

One proposed mechanism for producing gamma-ray bursts is induced gravitational collapse, where a neutron star is triggered to collapse into a black hole by 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-energy ejecta that have been difficult to model from a single star.^[20]

See also

• Gamma-ray burst progenitors – Types of celestial objects that can emit gamma-ray bursts
• Quark star – Compact exotic star which forms matter consisting mostly of quarks
• Quark-nova – Hypothetical violent explosion resulting from conversion of a neutron star to a quark star

References