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GW170817

GW170817
The GW170817 signal as measured by the LIGO and Virgo gravitational wave detectors. Signal is invisible in the Virgo data
Event typeGravitational wave
Datec. 130 million years ago
(detected 17 August 2017, 12:41:04.4 UTC)
Durationc. 1 minute and 40 seconds
InstrumentLIGO, Virgo
Right ascension13h 09m 48.08s[1]
Declination−23° 22′ 53.3″[1]
EpochJ2000.0
Distancec. 130 million ly
Redshift0.0099
HostNGC 4993
Progenitor2 neutron stars
Other designationsGW170817
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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. As of February 2025, it is the only GW detection to be definitively correlated with any electromagnetic observation.[1][2] Unlike the five prior GW detections—which were of merging black holes and thus not expected to have detectable electromagnetic signals[3]—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.[1][2][4][5][6][7][8][9] The discovery and subsequent observations of GW170817 were given the Breakthrough of the Year award for 2017 by the journal Science.[6][10]

The gravitational wave signal, designated GW170817, had an audible duration of approximately 100 seconds, and showed the characteristic intensity and frequency expected of the inspiral of two neutron stars. Analysis of the slight variation in arrival time of the GW at the three detector locations (two LIGO and one Virgo) yielded an approximate angular direction to the source. Independently, a short gamma-ray burst (sGRB) of around 2 seconds, designated GRB 170817A, was detected by the Fermi and INTEGRAL spacecraft beginning 1.7 seconds after the GW merger signal.[1][5][11] These detectors have very limited directional sensitivity, but indicated a large area of the sky which overlapped the gravitational wave position. The co-occurrence confirmed a long-standing hypothesis that neutron star mergers describe an important class of sGRB progenitor event.

An intense observing campaign was prioritized, to scan the region indicated by the gravitational wave detection for the expected emission at optical wavelengths. During this search, 11 hours after the signal, an astronomical transient SSS17a, later designated kilonova AT 2017gfo,[1] was observed in the galaxy NGC 4993.[8] It was captured by numerous telescopes, from radio to X-ray wavelengths, over the following days and weeks, and was found to be a fast-moving, rapidly-cooling cloud of neutron-rich material, as expected of debris ejected from a neutron-star merger.

In October 2018, astronomers reported that, in retrospect, an sGRB event detected in 2015 (GRB 150101B) may represent an earlier case of the same astrophysics reported for GW170817. 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, were considered "striking", suggesting that the earlier event may also be the result of a neutron star merger, and that together these may signify a hitherto-unknown class of kilonova transients, making kilonovae more diverse and common in the universe than previously understood.[12][13][14][15] Later research further construed GRB 160821B—another sGRB predating GW170817—also to be a kilonova, again based its resemblance to the AT 2017gfo signature.[16]

  1. ^ a b c d e f Cite error: The named reference ApJ was invoked but never defined (see the help page).
  2. ^ a b Cite error: The named reference PhysRev2017 was invoked but never defined (see the help page).
  3. ^ Connaughton V (2016). "Focus on electromagnetic counterparts to binary black hole mergers". The Astrophysical Journal Letters (Editorial). The follow-up observers sprang into action, not expecting to detect a signal if the gravitational radiation was indeed from a binary black-hole merger. [...] most observers and theorists agreed: the presence of at least one neutron star in the binary system was a prerequisite for the production of a circumbinary disk or neutron star ejecta, without which no electromagnetic counterpart was expected.
  4. ^ Landau E, Chou F, Washington D, Porter M (16 October 2017). "NASA missions catch first light from a gravitational-wave event". NASA. Retrieved 16 October 2017.
  5. ^ a b Cite error: The named reference NYT-20171016 was invoked but never defined (see the help page).
  6. ^ a b Cho A (December 2017). "Cosmic convergence". Science. 358 (6370): 1520–1521. Bibcode:2017Sci...358.1520C. doi:10.1126/science.358.6370.1520. PMID 29269456.
  7. ^ Cite error: The named reference SkyandTelescope was invoked but never defined (see the help page).
  8. ^ a b Cite error: The named reference SM-20171016 was invoked but never defined (see the help page).
  9. ^ Cite error: The named reference NAT-20170825 was invoked but never defined (see the help page).
  10. ^ "Breakthrough of the year 2017". Science | AAAS. 22 December 2017.
  11. ^ Cite error: The named reference MN-20171016 was invoked but never defined (see the help page).
  12. ^ "All in the family: Kin of gravitational wave source discovered – New observations suggest that kilonovae – immense cosmic explosions that produce silver, gold and platinum – may be more common than thought". University of Maryland. 16 October 2018. Retrieved 17 October 2018 – via EurekAlert!.
  13. ^ Troja E, Ryan G, Piro L, van Eerten H, Cenko SB, Yoon Y, et al. (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.
  14. ^ Mohon L (16 October 2018). "GRB 150101B: A distant cousin to GW170817". NASA. Retrieved 17 October 2018.
  15. ^ Wall M (17 October 2018). "Powerful cosmic flash is likely another neutron-star merger". Space.com. Retrieved 17 October 2018.
  16. ^ Troja E, Castro-Tirado AJ, Becerra González J, Hu Y, Ryan GS, Cenko SB, et al. (2019). "The afterglow and kilonova of the short GRB 160821B". Monthly Notices of the Royal Astronomical Society. 489 (2): 2104. arXiv:1905.01290. Bibcode:2019MNRAS.489.2104T. doi:10.1093/mnras/stz2255. S2CID 145047934.
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