Contents

NGC 4993

NGC 4993
NGC 4993 and GRB 170817A afterglow as taken by Hubble Space Telescope[1]
Observation data (J2000 epoch)
Constellation	Hydra
Right ascension	13h09m 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]
Grouporcluster	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]byWilliam Herschel^[6][7] and is a member of the NGC 4993 Group.^[3]

NGC 4993 is the site of GW170817, the first astronomical event detected in both electromagnetic and gravitational radiation, the collision of two neutron stars, 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 large dust lane with 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. suggested 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]}

Anoptical 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.

• Gravitational-wave astronomy
• List of gamma-ray bursts
• List of gravitational wave observations
• Neil Gehrels Swift Observatory
• Ultra-Fast Flash Observatory Pathfinder

Wikimedia Commons has media related to NGC 4993.

• "NGC 4993". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 28 August 2017.
• "NGC 4993". DSO Browser. Archived from the original on 15 February 2015. Retrieved 28 August 2017.
• Courtney Seligman. "New General Catalog Objects: NGC 4950 – 4999". Retrieved 28 August 2017.
• Hartmut Frommert. "Revised NGC Data for NGC 4993". SEDS. Retrieved 28 August 2017.
• GRB 170817A – NASA/IPAC Extragalactic Database (NED)
• GRB 170817A – Max Planck Institute for Extraterrestrial Physics (MPE)
• GRB 170817A - INTEGRAL Science Data Center (ISDC)
• The galaxy NGC 4993 in the constellation of Hydra Starmap

• NGC 4993 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images