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]
Physical characteristics
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]
Dark matter content
NGC 4993 has a
dark matter halo with an estimated mass of 193.9×1010M☉.[13]
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.