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 neutron stars momentarily creates an environment of such extreme neutron flux that the r-process can occur. This reaction accounts for the nucleosynthesis of around half of the isotopes in elements heavier than iron. [2]
The mergers also produce kilonovae, [3] which are transient sources of isotropic longer wave electromagnetic radiation due to the radioactive decay of heavy r-process nuclei that are produced and ejected during the merger process. [4] 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.
On 17 August 2017, the LIGO/Virgo collaboration detected GW170817, [5] a gravitational wave associated with the merger of two neutron stars in NGC 4993, an elliptical galaxy in the constellation Hydra. [6] GW170817 co-occurred with a short (roughly 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. [7] [8] [9] [10] [11]
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) [12] 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. [13]
In February 2018, the Zwicky Transient Facility began to track neutron star events via gravitational wave observation, [14] as evidenced by "systematic samples of tidal disruption events". [15] Later that year, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be directly related to GW170817 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. [16] [17] [18] [19]
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. [20] [21] 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. [22]
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. [23] [24] [25]
In 2023, an observation of the kilonova GRB 230307A was published, including likely observations of the spectra of tellurium and lanthanide elements. [26]
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. [27]
The emission from binary neutron star mergers can result in an extinction-level event if one were to occur within 10s of parsecs of Earth [28] . If the short Gamma Ray Burst (sGRB) struck Earth, the range of lethal influence extends to hundreds of parsec [29] . When considering other emission components, like near-earth supernovae, the high energy photon emission in the form of gamma rays and x-rays depletes the ozone leaving Earth’s inhabitants susceptible to harmful UVB radiation from the Sun. However, neutron star mergers are unique in that they have multiple sources of harmful radiation, including emission from the radioactive decay of heavy elements [30] , scattered by the sGRB cocoon [31] , and the sGRB afterglow [32] as well as cosmic rays accelerated by the blast. In order of arrival, the photons are first after the merger, and the cosmic rays arrive 100 to 1000s of years later. The ejected material sweeps up the interstellar medium and creates supernovae-remnant-like bubble with a lethal dose of cosmic rays held within it. If the Earth were to be encapsulated by the remnant, the cosmic rays, like the gamma rays, would deplete the ozone and could interact with the atmosphere and yield weakly-interacting muons. These particles will be generated in a large enough quantity such that escape from them will not be possible, even in caves or underwater. The danger lies in their ability to cause birth defects and mutations when interacting with the DNA of organic beings [33] [34] .
Relative to supernovae, they influence a similar volume of space. They are also much rarer and have a stronger dependence on the orientation of the event with respect to Earth [28] . Thus, the overall the threat of binary neutron star mergers is quite small.
A neutron star is a collapsed core of a massive supergiant star. The stars that later collapse into neutron stars have a total mass of between 10 and 25 solar masses (M☉), possibly more if the star was especially rich in elements heavier than hydrogen and helium. Except for black holes, neutron stars are the smallest and densest known class of stellar objects. Neutron stars have a radius on the order of 10 kilometers (6 mi) and a mass of about 1.4 M☉. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.
In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant galaxies, described by NASA as "the most powerful class of explosions in the universe". They are the most energetic and luminous electromagnetic events since the Big Bang. Bursts can last from ten milliseconds to several hours. After an initial flash of gamma rays, a longer-lived "afterglow" is usually emitted at longer wavelengths.
Timeline of neutron stars, pulsars, supernovae, and white dwarfs
The Fermi Gamma-ray Space Telescope, formerly called the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is the Large Area Telescope (LAT), with which astronomers mostly intend to perform an all-sky survey studying astrophysical and cosmological phenomena such as active galactic nuclei, pulsars, other high-energy sources and dark matter. Another instrument aboard Fermi, the Gamma-ray Burst Monitor, is being used to study gamma-ray bursts and solar flares.
The Virgo interferometer is a large Michelson interferometer designed to detect the gravitational waves predicted by general relativity. It is located in Santo Stefano a Macerata, near the city of Pisa, Italy. The instrument's two arms are three kilometres long, housing its mirrors and instrumentation inside an ultra-high vacuum.
Gravitational-wave astronomy is a subfield of astronomy concerned with the detection and study of gravitational waves emitted by astrophysical sources.
Gamma-ray burst progenitors are the types of celestial objects that can emit gamma-ray bursts (GRBs). GRBs show an extraordinary degree of diversity. They can last anywhere from a fraction of a second to many minutes. Bursts could have a single profile or oscillate wildly up and down in intensity, and their spectra are highly variable unlike other objects in space. The near complete lack of observational constraint led to a profusion of theories, including evaporating black holes, magnetic flares on white dwarfs, accretion of matter onto neutron stars, antimatter accretion, supernovae, hypernovae, and rapid extraction of rotational energy from supermassive black holes, among others.
Nial Rahil Tanvir is a British astronomer at the University of Leicester. His research specialisms are the Extragalactic distance scale, Galaxy evolution and Gamma ray bursts. Tanvir has featured in various TV programs, including The Sky at Night hosted by Sir Patrick Moore, and Horizon
A kilonova is a transient astronomical event that occurs in a compact binary system when two neutron stars or a neutron star and a black hole merge. These mergers are thought to produce gamma-ray bursts and emit bright electromagnetic radiation, called "kilonovae", due to the radioactive decay of heavy r-process nuclei that are produced and ejected fairly isotropically during the merger process. The measured high sphericity of the kilonova AT2017gfo at early epochs was deduced from the blackbody nature of its spectrum.
Multi-messenger astronomy is astronomy based on the coordinated observation and interpretation of signals carried by disparate "messengers": electromagnetic radiation, gravitational waves, neutrinos, and cosmic rays. They are created by different astrophysical processes, and thus reveal different information about their sources.
The first direct observation of gravitational waves was made on 14 September 2015 and was announced by the LIGO and Virgo collaborations on 11 February 2016. Previously, gravitational waves had been inferred only indirectly, via their effect on the timing of pulsars in binary star systems. The waveform, detected by both LIGO observatories, matched the predictions of general relativity for a gravitational wave emanating from the inward spiral and merger of a pair of black holes of around 36 and 29 solar masses and the subsequent "ringdown" of the single resulting black hole. The signal was named GW150914. It was also the first observation of a binary black hole merger, demonstrating both the existence of binary stellar-mass black hole systems and the fact that such mergers could occur within the current age of the universe.
A hypernova is a very energetic supernova which is believed to result from an extreme core-collapse scenario. In this case, a massive star collapses to form a rotating black hole emitting twin astrophysical jets and surrounded by an accretion disk. It is a type of stellar explosion that ejects material with an unusually high kinetic energy, an order of magnitude higher than most supernovae, with a luminosity at least 10 times greater. Hypernovae release so much of gamma rays they usually appear similar to a type Ic supernova, but with unusually broad spectral lines indicating an extremely high expansion velocity. Hypernovae are one of the mechanisms for producing long gamma ray bursts (GRBs), which range from 2 seconds to over a minute in duration. They have also been referred to as superluminous supernovae, though that classification also includes other types of extremely luminous stellar explosions that have different origins.
GW 170817 was a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 17 August 2017, originating from the shell elliptical galaxy NGC 4993. The signal was produced by the last moments of the inspiral process of a binary pair of neutron stars, ending with their merger. It is the first GW observation that has been confirmed by non-gravitational means. Unlike the five previous GW detections—which were of merging black holes and thus not expected to produce a detectable electromagnetic signal—the aftermath of this merger was seen across the electromagnetic spectrum by 70 observatories on 7 continents and in space, marking a significant breakthrough for multi-messenger astronomy. The discovery and subsequent observations of GW 170817 were given the Breakthrough of the Year award for 2017 by the journal Science.
NGC 4993 is a lenticular galaxy located about 140 million light-years away in the constellation Hydra. It was discovered on 26 March 1789 by William Herschel and is a member of the NGC 4993 Group.
GW170608 was a gravitational wave event that was recorded on 8 June 2017 at 02:01:16.49 UTC by Advanced LIGO. It originated from the merger of two black holes with masses of and . The resulting black hole had a mass around 18 solar masses. About one solar mass was converted to energy in the form of gravitational waves.
PyCBC is an open source software package primarily written in the Python programming language which is designed for use in gravitational-wave astronomy and gravitational-wave data analysis. PyCBC contains modules for signal processing, FFT, matched filtering, gravitational waveform generation, among other tasks common in gravitational-wave data analysis.
Daryl Haggard is an American-Canadian astronomer and associate professor of physics in the Department of Physics at McGill University and the McGill Space Institute.
GRB 150101B is a gamma-ray burst (GRB) that was detected on 1 January 2015 at 15:23 UT by the Burst Alert Telescope (BAT) on board the Swift Observatory Satellite, and at 15:23:35 UT by the Gamma-ray Burst Monitor (GBM) on board the Fermi Gamma-ray Space Telescope. The GRB was determined to be 1.7 billion light-years (0.52 Gpc) from Earth near the host galaxy 2MASX J12320498-1056010 in the constellation Virgo. The characteristics of GRB 150101B are remarkably similar to the historic event GW170817, a merger of neutron stars.
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.
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