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

A neutron star merger is the stellar collision of neutron stars.

When two neutron stars fall into mutual orbit, they gradually spiral inward due to gravitational radiation. When they finally meet, their merger leads to the formation of either a more massive neutron star, or—if the mass of the remnant exceeds the Tolman–Oppenheimer–Volkoff limit—a black hole. The merger can 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 momentarily creates an environment of such extreme neutron flux that the r-process can occur. This reaction is believed to account for the nucleosynthesis of around half of the elements heavier than iron.[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] Kilonovae had been discussed as a possible r-process site since the reaction was first proposed in 1999, but the mechanism became widely accepted after multi-messenger event GW170817 was observed in 2017.

Observed mergers

On 17 August 2017, the LIGO/Virgo collaboration detected GW170817,[4] a gravitational wave associated with the merger of two neutron stars in NGC 4993, an elliptical galaxy in the constellation Hydra.[5] GW170817 co-occurred with 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.[6][7][8][9][10]

The co-occurrence of GW170817 with GRB 170817A in both space and time strongly implies that neutron star mergers create short gamma-ray bursts. The subsequent detection of Swope Supernova Survey event 2017a (SSS17a)[11] in the area where GW170817 and GRB 170817A were known to have occurred—and its having the expected characteristics of a kilonova—strongly imply that neutron star mergers are responsible for kilonovae as well.[12]

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 determine the Hubble constant, which establishes 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—disagree by about 10%, This difference, the Hubble tension, might be reconciled by using kilonovae as another type of standard candle.[19]

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)

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

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. ↑ 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.
  5. ↑ Scharping, Nathaniel (18 October 2017). "Gravitational Waves Show How Fast The Universe is Expanding". Astronomy. Retrieved 18 October 2017.
  6. ↑ Cho, Adrian (16 October 2017). "Merging neutron stars generate gravitational waves and a celestial light show". Science. Retrieved 16 October 2017.
  7. ↑ 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.
  8. ↑ Overbye, Dennis (16 October 2017). "LIGO Detects Fierce Collision of Neutron Stars for the First Time". The New York Times. Retrieved 16 October 2017.
  9. ↑ 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.
  10. ↑ 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.
  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.
  22. ↑ Hosseinzadeh, G.; Cowperthwaite, P. S.; Gomez, S.; Villar, V. A. (18 July 2019). "Follow-up of the Neutron Star Bearing Gravitational Wave Candidate Events S190425z and S190426c with MMT and SOAR". Astrophys. J. 880 (1): L4. arXiv:1905.02186. Bibcode:2019ApJ...880L...4H. doi:10.3847/2041-8213/ab271c. hdl:10150/633863. S2CID 146121014.
  23. ↑ Pease, Roland (2 May 2019). "Gravitational waves hunt now in overdrive". BBC News.
  24. ↑ Eric C. Bellm, Shrinivas R. Kulkarni, Matthew J. Graham, Richard Dekany, Roger M. Smith, Reed Riddle, Frank J. Masci, George Helou, Thomas A. Prince, Scott M. Adams (2018 December 7) The Zwicky Transient Facility: System Overview, Performance, and First Results
  25. ↑ Levan, Andrew; Gompertz, Benjamin P.; Salafia, Om Sharan; Bulla, Mattia; Burns, Eric; Hotokezaka, Kenta; Izzo, Luca; Lamb, Gavin P.; Malesani, Daniele B.; Oates, Samantha R.; Ravasio, Maria Edvige; Rouco Escorial, Alicia; Schneider, Benjamin; Sarin, Nikhil; Schulze, Steve (25 October 2023). "Heavy element production in a compact object merger observed by JWST". Nature. doi:10.1038/s41586-023-06759-1. ISSN 0028-0836.
  26. ↑ Klesman, Alison (18 April 2019). "A new neutron star merger is caught on X-ray camera". Astronomy. Retrieved 18 April 2019.
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