Neutron star merger

A neutron star merger is a type of stellar collision.

For the song by Muse, see Neutron Star Collision (Love Is Forever).
Artist's impression of neutron stars merging, producing gravitational waves and resulting in a kilonova
Artist's impression of neutron stars merging, producing gravitational waves and resulting in a kilonova

When two neutron stars orbit each other closely, they gradually spiral inward due to gravitational radiation. When the two neutron stars meet, their merger leads to the formation of either a more massive neutron star, or a black hole (depending on whether the mass of the remnant exceeds the Tolman–Oppenheimer–Volkoff limit). The merger can also create a magnetic field that is trillions of times stronger than that of Earth in a matter of one or two milliseconds. These events are believed to create short gamma-ray bursts.[1]

The merger of binary neutron stars is believed to be the origin of most elements with large atomic weights – the r-process elements.[2]

The mergers are also believed to produce kilonovae, which are transient sources of fairly isotropic longer wave electromagnetic radiation due to the radioactive decay of heavy r-process nuclei that are produced and ejected during the merger process.[3]

Observed mergers

On 17 August 2017, the LIGO/Virgo collaboration detected a pulse of gravitational waves,[7][8] named GW170817, associated with the merger of two neutron stars in NGC 4993, an elliptical galaxy in the constellation Hydra. GW170817 also seemed related to a short (≈2 second long) gamma-ray burst, GRB 170817A, first detected 1.7 seconds after the GW merger signal, and a visible light observational event first observed 11 hours afterwards, SSS17a.[9][4][5][6][10]

The association of GW170817 with GRB 170817A in both space and time is strong evidence that neutron star mergers do create short gamma-ray bursts. The subsequent detection of event Swope Supernova Survey 2017a (SSS17a)[11] in the area in which GW170817 and GRB 170817A were known to have occurred and its having the expected characteristics for a kilonova is strong evidence that neutron star mergers do produce kilonovae.[lower-alpha 1]

In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be directly related to the historic GW170817, a gravitational wave event detected in 2017, and associated with the merger of two neutron stars. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are "striking", suggesting the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova, which may be more common in the universe than previously understood, according to the researchers.[13][14][15][16]

Also in October 2018, scientists presented a new way to use information from gravitational wave events (especially those involving the merger of neutron stars, like GW170817) to determining the Hubble constant, which is essential in establishing the rate of expansion of the universe.[17][18] The two earlier methods for finding the Hubble constant, one based on redshifts and another based on the cosmic distance ladder, yield different values,[lower-alpha 2] which might be reconciled by another standard candle.[lower-alpha 1]

In April 2019 the LIGO and Virgo gravitational wave observatories announced the detection of candidate event that is, with a probability 99.94%, the merger of two neutron stars. Despite extensive follow-up observations, no electromagnetic counterpart could be identified.[20] [21] [22]

In February 2018 the Zwicky Transient Facility began to track neutron star events via gravitational wave observation,[23] as evidenced by "systematic samples of tidal disruption events".[24]

In 2023 an observation of the kilonova GRB 230307A was published, including likely observations of the spectra of tellurium and lanthanide elements.[25]

XT2 (magnetar)
"XT2" redirects here. For the camera, see Fujifilm X-T2.

In 2019, analysis of data from the Chandra X-ray Observatory revealed another binary neutron star merger at a distance of 6.6 billion light years, an x-ray signal called XT2. The merger produced a magnetar; its emissions could be detected for several hours.[26]

See also

Notes
  1. From 1989-1999, when a mechanism for the r-process was proposed, and then observed in 2017, in a multi-messenger event seen by multiple observatories, the event was accepted by the scientific community as a likely kilonova.[12]
  2. The 10% difference in values is called the Hubble tension.[19]

References
  1. Rosswog, Stephan (2013). "Astrophysics: Radioactive glow as a smoking gun". Nature. 500 (7464): 535–6. Bibcode:2013Natur.500..535R. doi:10.1038/500535a. PMID 23985867. S2CID 4401544.
  2. Stromberg, Joseph (16 July 2013). "All the Gold in the Universe Could Come from the Collisions of Neutron Stars". Smithsonian. Retrieved 27 April 2014.
  3. Tanvir, N. R.; Levan, A. J.; Fruchter, A. S.; Hjorth, J.; Hounsell, R. A.; Wiersema, K.; Tunnicliffe, R. L. (2013). "A "kilonova" associated with the short-duration γ-ray burst GRB 130603B". Nature. 500 (7464): 547–9. arXiv:1306.4971. Bibcode:2013Natur.500..547T. doi:10.1038/nature12505. PMID 23912055. S2CID 205235329.
  4. Cho, Adrian (16 October 2017). "Merging neutron stars generate gravitational waves and a celestial light show". Science. Retrieved 16 October 2017.
  5. Landau, Elizabeth; Chou, Felicia; Washington, Dewayne; Porter, Molly (16 October 2017). "NASA Missions Catch First Light from a Gravitational-Wave Event". NASA. Retrieved 16 October 2017.
  6. Overbye, Dennis (16 October 2017). "LIGO Detects Fierce Collision of Neutron Stars for the First Time". The New York Times. Retrieved 16 October 2017.
  7. Abbott, B. P.; et al. (LIGO Scientific Collaboration & Virgo Collaboration) (16 October 2017). "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral". Physical Review Letters. 119 (16): 161101. arXiv:1710.05832. Bibcode:2017PhRvL.119p1101A. doi:10.1103/PhysRevLett.119.161101. PMID 29099225. S2CID 217163611.
  8. Scharping, Nathaniel (18 October 2017). "Gravitational Waves Show How Fast The Universe is Expanding". Astronomy. Retrieved 18 October 2017.
  9. Abbott, B. P.; et al. (LIGO, Virgo and other collaborations) (October 2017). "Multi-messenger Observations of a Binary Neutron Star Merger" (PDF). The Astrophysical Journal. 848 (2): L12. arXiv:1710.05833. Bibcode:2017ApJ...848L..12A. doi:10.3847/2041-8213/aa91c9. The optical and near-infrared spectra over these few days provided convincing arguments that this transient was unlike any other discovered in extensive optical wide-field surveys over the past decade.
  10. Krieger, Lisa M. (16 October 2017). "A Bright Light Seen Across The Universe, Proving Einstein Right - Violent collisions source of our gold, silver". The Mercury News. Retrieved 16 October 2017.
  11. Pan, Y.-C.; et al. (2017). "The Old Host-galaxy Environment of SSS17a, the First Electromagnetic Counterpart to a Gravitational-wave Source". The Astrophysical Journal. 848 (2): L30. arXiv:1710.05439. Bibcode:2017ApJ...848L..30P. doi:10.3847/2041-8213/aa9116. S2CID 3516168.
  12. Nature Astronomy (16 Oct 2017) Kilonovae, short gamma-ray bursts & neutron star mergers
  13. "All in the family: Kin of gravitational wave source discovered". EurekAlert! (Press release). University of Maryland. 16 October 2018. Retrieved 17 October 2018.
  14. Troja, E.; et al. (16 October 2018). "A luminous blue kilonova and an off-axis jet from a compact binary merger at z=0.1341". Nature Communications. 9 (1): 4089. arXiv:1806.10624. Bibcode:2018NatCo...9.4089T. doi:10.1038/s41467-018-06558-7. PMC 6191439. PMID 30327476.
  15. Mohon, Lee (16 October 2018). "GRB 150101B: A Distant Cousin to GW170817". NASA. Retrieved 17 October 2018.
  16. Wall, Mike (17 October 2018). "Powerful Cosmic Flash Is Likely Another Neutron-Star Merger". Space.com. Retrieved 17 October 2018.
  17. Lerner, Louise (22 October 2018). "Gravitational waves could soon provide measure of universe's expansion". Phys.org. Retrieved 22 October 2018.
  18. Chen, Hsin-Yu; Fishbach, Maya; Holz, Daniel E. (17 October 2018). "A two per cent Hubble constant measurement from standard sirens within five years". Nature. 562 (7728): 545–547. arXiv:1712.06531. Bibcode:2018Natur.562..545C. doi:10.1038/s41586-018-0606-0. PMID 30333628. S2CID 52987203.
  19. Charlie Wood (13 Dec 2021) Cosmologists Parry Attacks on the Vaunted Cosmological Principle
  20. "Breaking: LIGO Detects Gravitational Waves From Another Neutron Star Merger". D-brief. 25 April 2019. Retrieved 13 August 2019.
  21. "GraceDB |". gracedb.ligo.org. Retrieved 13 August 2019.
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