User:DeadWood/Kilonova

A kilonova (also called a ‘macronova’ or an ‘r-process supernova’) occurs when two neutron stars merge or sometimes when a neutron star and a black hole merge. Strong electromagnetic radiation is emitted due to the decay of heavy r-process ions that are produced and ejected fairly isotropically during the merger process—similar to a faint, short-lived supernova. A kilonova is the result of merging of two compact objects, namely a black hole and neutron star or two neutron stars in a binary system. The inspiral and merging of these compact objects are a strong source of gravitational waves (GWs).

It was also hypothesized to be the progenitor of short gamma-ray bursts, which was confirmed in 2017 when the gravitational wave observatories LIGO and Virgo both detected the signature (named GW170817) of a neutron star collision in the galaxy NGC 4993. The corresponding gamma-ray burst (named GRB 170817A) was detected by the Fermi and INTEGRAL space telescopes and multiple space-based and ground-based observatories studied the visible kilonova (named SSS17a/DLT17ck/AT 2017gfo) across other parts of the electromagnetic spectrum.

Kilonovae are believed to be the predominant source of stable r-process elements in the Universe.

Properties

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Evolution

System formation

Main article: Binary star § Compact binary

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Inspiral

The energy loss of a binary system by gravitational radiation has been measured ever since the Nobel-Prize winning discovery of the Hulse-Taylor binary pulsar as well as subsequent binary pulsars have made it possible to accurately measure the orbital period of close compact binary systems. The observed increase in the frequency of these systems as the binaries spiral closer together closely matches the predictions of general relativity, making it a major corroboration of that theory.

Current gravitational wave detectors are limited by environmental noise on Earth and the size of the facilities, renedering them unable to observe a kilonova before the last few seconds.

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Merger

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Optical kilonova

In the immediate aftermath of the neutron star merger, large amounts of neutron-rich material are ejected by winds coming from the accretion disk around the newly formed black hole. About 0.03 to 0.05 solar masses were ejected by kilonova SSS17a/DLT17ck, or about half the mass of the smallest main sequence stars. Theoretically the ejected mass can be as low as about 0.01% of a solar mass. This material glows brightly in visible and near-infrared light due to r-process nucleosynthesis taking place within it.

The high neutron density within the kilonova allows the r-process to take place, as it also does to an extent in for example a core-collapse supernova. The r-process, or rapid neutron capture nucleosynthesis, is a process allowing elements lighter than an atomic mass of about 56 (roughly that of iron or nickel) to grow into much heavier elements. Neutron capture allows nuclei to grow in mass but in the lower neutron density of stellar nucleosynthesis, where slow neutron capture processes dominate, the radioactive species that are formed tend to decay faster than more neutrons can be added, when forming heavy elements. With the large number of neutrons present in a kilonova, so many neutrons are being absorbed so quickly that heavy elements can form readily. S-processes and r-processes combined account for the abundance of all elements with high atomic mass.

As a large number of the newly formed nuclei are unstable, they tend to undergo radioactive decay. This decay heats the gas, which then glows visibly.

Aftermath

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See also

• Nova
• Nova remnant
• Hypernova
• Supernova impostor