NGC 4993 | |
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![]() NGC 4993 and GRB 170817A afterglow as taken by Hubble Space Telescope [1] | |
Observation data (J2000 epoch) | |
Constellation | Hydra |
Right ascension | 13h 09m 47.7s [2] |
Declination | −23° 23′ 02″ [2] |
Redshift | 0.009727 [2] |
Heliocentric radial velocity | 2916 km/s [2] |
Distance | 44.1 Mpc (144 Mly) [2] |
Group or cluster | NGC 4993 Group [3] |
Apparent magnitude (V) | 13.32 [2] |
Characteristics | |
Type | (R')SAB0^-(rs) [2] |
Size | ~55,000 ly (17 kpc) (estimated) [2] |
Apparent size (V) | 1.3 x 1.1 [2] |
Notable features | Host of neutron star merger detected as gravitational wave GW170817 and gamma-ray burst GRB 170817A |
Other designations | |
NGC 4994, ESO 508-18, AM 1307-230, MCG -4-31-39, PGC 45657, WH III 766 [4] |
NGC 4993 (also catalogued as NGC 4994 in the New General Catalogue) is a lenticular galaxy [5] located about 140 million light-years away [2] in the constellation Hydra. [6] It was discovered on 26 March 1789 [7] by William Herschel [6] [7] and is a member of the NGC 4993 Group. [3]
NGC 4993 was the site of GW170817, a collision of two neutron stars, the first astronomical event detected in both electromagnetic and gravitational radiation, a discovery given the Breakthrough of the Year award for 2017 by the journal Science. [8] [9] Detecting a gravitational wave event associated with the gamma-ray burst provided direct confirmation that binary neutron star collisions produce short gamma-ray bursts. [10]
NGC 4993 has several concentric shells of stars and a large dust lane—with a diameter of approximately a few kiloparsecs—which surrounds the nucleus and is stretched out into an "s" shape. The dust lane appears to be connected to a small dust ring with a diameter of ~330 ly (0.1 kpc ). [11] These features in NGC 4993 may be the result [12] of a recent merger with a gaseous late-type galaxy that occurred about 400 million years ago. [13] However, Palmese et al. suggest that the galaxy involved in the merger was a gas-poor galaxy. [14]
NGC 4993 has a dark matter halo with an estimated mass of 193.9×1010 M☉ . [13]
NGC 4993 has an estimated population of 250 globular clusters. [5]
The luminosity of NGC 4993 indicates that the globular cluster system surrounding the galaxy may be dominated by metal-poor globular clusters. [15]
NGC 4993 has a supermassive black hole with an estimated mass of roughly 80 to 100 million solar masses (8×107 M☉ ). [16]
The presence of weak O III, NII and SII emission lines in the nucleus of NGC 4993 and the relatively high ratio of [NII]λ6583/Hα suggest that NGC 4993 is a low-luminosity AGN (LLAGN). [16] The activity may have been triggered by gas from the late-type galaxy as it merged with NGC 4993. [13]
In August 2017, rumors circulated [17] regarding a short gamma-ray burst designated GRB 170817A, of the type conjectured to be emitted in the collision of two neutron stars. [18] On 16 October 2017, the LIGO and Virgo collaborations announced that they had detected a gravitational wave event, designated GW170817. The gravitational wave signal matched prediction for the merger of two neutron stars, two seconds before the gamma-ray burst. The gravitational wave signal, which had a duration of about 100 seconds, was the first gravitational wave detection of the merger of two neutron stars. [1] [19] [20] [21] [22]
An optical transient, AT 2017gfo (also known as SSS 17a), was detected in NGC 4993 11 hours after the gravitational wave and gamma-ray signals, allowing the location of the merger to be determined. The optical emission is thought to be due to a kilonova. The discovery of AT 2017gfo was the first observation (and first localisation) of an electromagnetic counterpart to a gravitational wave source. [19] [21] [22] [23] [24]
GRB 170817A was a gamma-ray burst (GRB) detected by NASA's Fermi and ESA's INTEGRAL on 17 August 2017. [17] [25] [26] [27] Although only localized to a large area of the sky, it is believed to correspond to the other two observations, [23] in part due to its arrival time 1.7 seconds after the GW event.
A neutron star is the collapsed core of a massive supergiant star. It results 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. Except for black holes, neutron stars are the smallest and densest known class of stellar objects. They have a radius on the order of 10 kilometers (6 mi) and a mass of about 1.4 M☉. Stars that collapse into neutron stars have a total mass of between 10 and 25 solar masses (M☉), or possibly more for those that are especially rich in elements heavier than hydrogen and helium.
The following is a timeline of gravitational physics and general relativity.
Timeline of neutron stars, pulsars, supernovae, and white dwarfs
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Einstein@Home is a volunteer computing project that searches for signals from spinning neutron stars in data from gravitational-wave detectors, from large radio telescopes, and from a gamma-ray telescope. Neutron stars are detected by their pulsed radio and gamma-ray emission as radio and/or gamma-ray pulsars. They also might be observable as continuous gravitational wave sources if they are rapidly spinning and non-axisymmetrically deformed. The project was officially launched on 19 February 2005 as part of the American Physical Society's contribution to the World Year of Physics 2005 event.
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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 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.
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.
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James Michael Lattimer is a nuclear astrophysicist who works on the dense nuclear matter equation of state and neutron stars. He is currently a distinguished professor at Stony Brook University.
GW170817 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, about 140 million light years away. The signal was produced by the last moments of the inspiral process of a binary pair of neutron stars, ending with their merger. It was the first GW observation to be 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 GW170817 were given the Breakthrough of the Year award for 2017 by the journal Science.
GW170608 was a gravitational wave event that was recorded on 8 June 2017 at 2:01:16.49 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.
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