Neutron star merger
A neutron star merger is a type of stellar collision. It occurs in a fashion similar to the rare brand of type Ia supernovae resulting from merging white dwarfs. When two neutron stars orbit each other closely, they spiral inward as time passes 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. 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.
On 17 August 2017, LIGO/Virgo collaboration detected a pulse of gravitational waves, 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.
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) 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.
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.
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 essential in establishing the rate of expansion of the universe. The two earlier methods, one based on redshifts and another based on the cosmic distance ladder, gave results that do not agree.
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.  
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.
- 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.
- 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.
- Cho, Adrian (16 October 2017). "Merging neutron stars generate gravitational waves and a celestial light show". Science. Retrieved 16 October 2017.
- 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.
- Overbye, Dennis (16 October 2017). "LIGO Detects Fierce Collision of Neutron Stars for the First Time". The New York Times. Retrieved 16 October 2017.
- 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.
- Scharping, Nathaniel (18 October 2017). "Gravitational Waves Show How Fast The Universe is Expanding". Astronomy. Retrieved 18 October 2017.
- 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.
- 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.
- 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.
- "All in the family: Kin of gravitational wave source discovered". EurekAlert! (Press release). University of Maryland. 16 October 2018. Retrieved 17 October 2018.
- 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.
- Mohon, Lee (16 October 2018). "GRB 150101B: A Distant Cousin to GW170817". NASA. Retrieved 17 October 2018.
- Wall, Mike (17 October 2018). "Powerful Cosmic Flash Is Likely Another Neutron-Star Merger". Space.com. Retrieved 17 October 2018.
- Lerner, Louise (22 October 2018). "Gravitational waves could soon provide measure of universe's expansion". Phys.org. Retrieved 22 October 2018.
- 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.
- "Breaking: LIGO Detects Gravitational Waves From Another Neutron Star Merger". D-brief. 25 April 2019. Retrieved 13 August 2019.
- "GraceDB |". gracedb.ligo.org. Retrieved 13 August 2019.
- 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.
- Pease, Roland (2 May 2019). "Gravitational waves hunt now in overdrive". BBC News.
- Klesman, Alison (18 April 2019). "A new neutron star merger is caught on X-ray camera". Astronomy. Retrieved 18 April 2019.