GW170817

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 GW was produced by the last minutes of two neutron stars spiralling closer to each other and finally merging, and is the first GW observation which has been confirmed by non-gravitational means.[1][2] Unlike the five previous GW detections, which were of merging black holes not expected to produce a detectable electromagnetic signal,[3][4][5][lower-alpha 1] the aftermath of this merger was also seen by 70 observatories on 7 continents and in space, across the electromagnetic spectrum, marking a significant breakthrough for multi-messenger astronomy.[1][7][8][9][10] The discovery and subsequent observations of GW 170817 were given the Breakthrough of the Year award for 2017 by the journal Science.[11][12]

GW170817
The GW170817 signal as measured by the LIGO and Virgo gravitational wave detectors
Other designationsGW170817
Event typeGravitational wave event, neutron star merger 
Date17 August 2017 
InstrumentLIGO, Virgo
ConstellationHydra 
Right ascension 13h 09m 48.08s[1]
Declination−23° 22′ 53.3″[1]
EpochJ2000.0
Distance40 megaparsecs (130 Mly)
Redshift0.0099 
Preceded byGW170814 
Related media on Wikimedia Commons

The gravitational wave signal, designated GW 170817, had a duration of approximately 100 seconds, and shows the characteristics in 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 (~2 seconds' duration) gamma-ray burst, designated GRB 170817A, was detected by the Fermi and INTEGRAL spacecraft beginning 1.7 seconds after the GW merger signal.[1][13][14] These detectors have very limited directional sensitivity, but indicated a large area of the sky which overlapped the gravitational wave position. It has been a long-standing hypothesis that short gamma-ray bursts are caused by neutron star mergers.

An intense observing campaign then took place to search for the expected emission at optical wavelengths. An astronomical transient designated AT 2017gfo (originally, SSS 17a) was found, 11 hours after the gravitational wave signal, in the galaxy NGC 4993[15] during a search of the region indicated by the GW detection. It was observed by numerous telescopes, from radio to X-ray wavelengths, over the following days and weeks, and was shown 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 GRB 150101B, a gamma-ray burst event detected in 2015, may be analogous to GW 170817. 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 considered "striking", and this remarkable resemblance suggests the two separate and independent events may both be the result of the merger of neutron stars, and both may be a hitherto-unknown class of kilonova transients. Kilonova events, therefore, may be more diverse and common in the universe than previously understood, according to the researchers.[16][17][18][19] In retrospect, GRB 160821B, another gamma-ray burst event is now construed to be another kilonova,[20] by its resemblance of its data to GRB 170817A,[21] part of the multi-messenger now denoted GW170817.

Announcement
It's the first time that we've observed a cataclysmic astrophysical event in both gravitational waves and electromagnetic waves – our cosmic messengers.[22]

David Reitze, LIGO executive director

The observations were officially announced on 16 October 2017 at press conferences at the National Press Club in Washington, D.C. and at the ESO headquarters in Garching bei München in Germany.[13][14][15]

Some information was leaked before the official announcement, beginning on 18 August 2017 when astronomer J. Craig Wheeler of the University of Texas at Austin tweeted "New LIGO. Source with optical counterpart. Blow your sox off!".[5] He later deleted the tweet and apologized for scooping the official announcement protocol. Other people followed up on the rumor, and reported that the public logs of several major telescopes listed priority interruptions in order to observe NGC 4993, a galaxy 40 Mpc (130 Mly) away in the Hydra constellation.[23][24] The collaboration had earlier declined to comment on the rumors, not adding to a previous announcement that there were several triggers under analysis.[25][26]

Gravitational wave detection

The gravitational wave signal lasted for approximately 100 seconds starting from a frequency of 24 hertz. It covered approximately 3,000 cycles, increasing in amplitude and frequency to a few hundred hertz in the typical inspiral chirp pattern, ending with the collision received at 12:41:04.4 UTC.[2]:2 It arrived first at the Virgo detector in Italy, then 22 milliseconds later at the LIGO-Livingston detector in Louisiana, USA, and another 3 milliseconds later at the LIGO-Hanford detector in the state of Washington, USA. The signal was detected and analyzed by a comparison with a prediction from general relativity defined from the post-Newtonian expansion.[1]:3

An automatic computer search of the LIGO-Hanford datastream triggered an alert to the LIGO team about 6 minutes after the event. The gamma-ray alert had already been issued at this point (16 seconds post-event),[27] so the timing near-coincidence was automatically flagged. The LIGO/Virgo team issued a preliminary alert (with only the crude gamma-ray position) to astronomers in the follow-up teams at 40 minutes post-event.[28][29]

Sky localisation of the event requires combining data from the three interferometers; this was delayed by two problems. The Virgo data were delayed by a data transmission problem, and the LIGO Livingston data were contaminated by a brief burst of instrumental noise a few seconds prior to event peak, but persisting parallel to the rising transient signal in the lowest frequencies. These required manual analysis and interpolation before the sky location could be announced about 4.5 hours post-event.[30][29] The three detections localized the source to an area of 31 square degrees in the southern sky at 90% probability. More detailed calculations later refined the localization to within 28 square degrees.[28][2] In particular, the absence of a clear detection by the Virgo system implied that the source was in one of Virgo's blind spots; this absence of signal in Virgo data contributed to considerably reduce the source containment area.[31]

Gamma ray detection
Artistic concept: two neutron stars merge

The first electromagnetic signal detected was GRB 170817A, a short gamma-ray burst, detected 1.74±0.05 s after the merger time and lasting for about 2 seconds.[14][23][1]:5

GRB 170817A was discovered by the Fermi Gamma-ray Space Telescope, with an automatic alert issued just 14 seconds after the GRB detection. After the LIGO/Virgo circular 40 minutes later, manual processing of data from the INTEGRAL gamma-ray telescope also detected the same GRB. The difference in arrival time between Fermi and INTEGRAL helped to improve the sky localization.

This GRB was relatively faint given the proximity of the host galaxy NGC 4993, possibly due to its jets not being pointed directly toward Earth, but rather at an angle of about 30 degrees to the side.[15][32]

Electromagnetic follow-up
Hubble picture of NGC 4993 with inset showing GRB 170817A over 6 days. Credit: NASA and ESA
Optical lightcurves
The change in optical and near-infrared spectra

A series of alerts to other astronomers were issued, beginning with a report of the gamma-ray detection and single-detector LIGO trigger at 13:21 UTC, and a three-detector sky location at 17:54 UTC.[28] These prompted a massive search by many survey and robotic telescopes. In addition to the expected large size of the search area (about 150 times the area of a full moon), this search was challenging because the search area was near the Sun in the sky and thus visible for at most a few hours after dusk for any given telescope.[29]

In total six teams (One-Meter, Two Hemispheres (1M2H), DLT40, VISTA, Master, DECam, Las Cumbres Observatory (LCO) Chile) imaged the same new source independently in a 90-minute interval.[1]:5 The first to detect optical light associated with the collision was the 1M2H team running the Swope Supernova Survey, which found it in an image of NGC 4993 taken 10 hours and 52 minutes after the GW event[14][1][33] by the 1-meter diameter (3.3 ft) Swope Telescope operating in the near infrared at Las Campanas Observatory, Chile. They were also the first to announce it, naming their detection SSS 17a in a circular issued 12h26m post-event. The new source was later given an official International Astronomical Union (IAU) designation of AT 2017gfo.

The 1M2H team surveyed all galaxies in the region of space predicted by the gravitational wave observations, and identified a single new transient.[32][33] By identifying the host galaxy of the merger, it is possible to provide an accurate distance consistent with that based on gravitational waves alone.[1]:5

The detection of the optical and near-infrared source provided a huge improvement in localisation, reducing the uncertainty from several degrees to 0.0001 degree; this enabled many large ground and space telescopes to follow up the source over the following days and weeks. Within hours after localization, many additional observations were made across the infrared and visible spectrum.[33] Over the following days, the color of the optical source changed from blue to red as the source expanded and cooled.[32]

Numerous optical and infrared spectra were observed; early spectra were nearly featureless, but after a few days, broad features emerged indicative of material ejected at roughly 10 percent of light speed. There are multiple strong lines of evidence that AT 2017gfo is indeed the aftermath of GW 170817: The colour evolution and spectra are dramatically different from any known supernova. The distance of NGC 4993 is consistent with that independently estimated from the GW signal. No other transient has been found in the GW sky localisation region. Finally, various archive images pre-event show nothing at the location of AT 2017gfo, ruling out a foreground variable star in the Milky Way.[1]

The source was detected in the ultraviolet (but not in X-rays) 15.3 hours after the event by the Swift Gamma-Ray Burst Mission.[1]:6 After initial lack of X-ray and radio detections, the source was detected in X-rays 9 days later by the Chandra X-ray Observatory,[34][35] and 16 days later in the radio by the Karl G. Jansky Very Large Array (VLA) in New Mexico.[15] More than 70 observatories covering the electromagnetic spectrum observed the source.[15]

The radio and X-ray light continued to rise for several months after the merger,[36] and have been represented to be diminishing.[37] In September 2019, astronomers reported obtaining an optical image of GW170817 [putative] afterglow by the Hubble Space Telescope.[38][39] In March 2020, continued X-ray emission at 5-sigma was observed by the Chandra Observatory 940 days after the merger, demanding further augmentation or refutation of prior models that had previously been supplemented with additional post-hoc interventions.[40]

Other detectors

No neutrinos consistent with the source were found in follow-up searches by the IceCube and ANTARES neutrino observatories and the Pierre Auger Observatory.[2][1] A possible explanation for the non-detection of neutrinos is because the event was observed at a large off-axis angle and thus the outflow jet was not directed towards Earth.[41][42]

Astrophysical origin and products

The gravitational wave signal indicated that it was produced by the collision of two neutron stars[23][24][26][43] with a total mass of 2.82+0.47
−0.09 times the mass of the sun (solar masses)[2] If low spins are assumed, consistent with those observed in binary neutron stars that will merge within a Hubble time, the total mass is 2.74+0.04
−0.01 M☉.

The masses of the component stars have greater uncertainty. The larger (m1) has a 90% chance of being between 1.36 and 2.26 M☉, and the smaller (m2) has a 90% chance of being between 0.86 and 1.36 M☉.[44] Under the low spin assumption, the ranges are 1.36 to 1.60 M☉ for m1 and 1.17 to 1.36 M☉ for m2.

The chirp mass, a directly observable parameter which may be very roughly equated to the geometric mean of the masses, is measured at 1.188+0.004
−0.002 M☉.[44]

The neutron star merger event is thought to result in a kilonova, characterized by a short gamma-ray burst followed by a longer optical "afterglow" powered by the radioactive decay of heavy r-process nuclei. Kilonovae are candidates for the production of half the chemical elements heavier than iron in the Universe.[15] A total of 16,000 times the mass of the Earth in heavy elements is believed to have formed, including approximately 10 Earth masses just of the two elements gold and platinum.[45]

A hypermassive neutron star was believed to have formed initially, as evidenced by the large amount of ejecta (much of which would have been swallowed by an immediately forming black hole). The lack of evidence for emissions being powered by neutron star spin-down, which would occur for longer-surviving neutron stars, suggest it collapsed into a black hole within milliseconds.[46]

Later searches did find evidence of spin-down in the gravitational signal, suggesting a longer-lived neutron star.[47]

Scientific importance
Artist’s impression of strontium emerging from a neutron star merger.[48]

Scientific interest in the event was enormous, with dozens of preliminary papers (and almost 100 preprints[49]) published the day of the announcement, including 8 letters in Science,[15] 6 in Nature, and 32 in a special issue of The Astrophysical Journal Letters devoted to the subject.[7] The interest and effort was global: The paper describing the multi-messenger observations[1] is coauthored by almost 4,000 astronomers (about one-third of the worldwide astronomical community) from more than 900 institutions, using more than 70 observatories on all 7 continents and in space.[5][15]

This may not be the first observed event that is due to a neutron star merger; GRB 130603B was the first plausible kilonova suggested based on follow-up observations of short-hard gamma-ray bursts.[50] It is, however, by far the best observation, making this the strongest evidence to date to confirm the hypothesis that some mergers of binary stars are the cause of short gamma-ray bursts.[1][2]

The event also provides a limit on the difference between the speed of light and that of gravity. Assuming the first photons were emitted between zero and ten seconds after peak gravitational wave emission, the difference between the speeds of gravitational and electromagnetic waves, vGW − vEM, is constrained to between −3×10−15 and +7×10−16 times the speed of light, which improves on the previous estimate by about 14 orders of magnitude.[44][51][lower-alpha 2] In addition, it allowed investigation of the equivalence principle (through Shapiro delay measurement) and Lorentz invariance.[2] The limits of possible violations of Lorentz invariance (values of 'gravity sector coefficients') are reduced by the new observations, by up to ten orders of magnitude.[44] GW 170817 also excluded some alternatives to general relativity,[52] including variants of scalar–tensor theory,[53][54][55][56][57][58][59][60] Hořava–Lifshitz gravity,[61][62][63] Dark Matter Emulators[64] and bimetric gravity.[65]

Gravitational wave signals such as GW 170817 may be used as a standard siren to provide an independent measurement of the Hubble constant.[66][67] An initial estimate of the constant derived from the observation is 70.0+12.0
−8.0 (km/s)/Mpc, broadly consistent with current best estimates.[66] Further studies improved the measurement to 70.3+5.3
−5.0 (km/s)/Mpc.[68][69][70] Together with the observation of future events of this kind the uncertainty is expected to reach two percent within five years and one percent within ten years.[71][72]

Electromagnetic observations helped to support the theory that the mergers of neutron stars contribute to rapid neutron capture r-process nucleosynthesis[33] and are significant sources of r-process elements heavier than iron,[1] including gold and platinum, which was previously attributed exclusively to supernova explosions.[45]

In October 2017, Stephen Hawking, in his last broadcast interview, presented the overall scientific importance of GW 170817.[73]

In September 2018, astronomers reported related studies about possible mergers of neutron stars (NS) and white dwarfs (WD): including NS-NS, NS-WD, and WD-WD mergers.[74]

The first identification of r-process elements in a neutron star merger was obtained during a re-analysis of GW170817 spectra.[75] The spectra provided direct proof of strontium production during a neutron star merger. This also provided a direct proof that neutron stars are made of neutron-rich matter.

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See also

Notes
  1. Although acknowledged as unlikely, several mechanisms have been suggested by which a black hole merger could be surrounded by sufficient matter to produce an electromagnetic signal, which astronomers have been searching for.[4][6]
  2. Previous constraint on the difference between the light speed and the gravitational speed was about ±20%.[51]

References
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  2. Abbott, B. P.; et al. (LIGO Scientific Collaboration & Virgo Collaboration) (October 2017). "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral" (PDF). Physical Review Letters. 119 (16): 161101. arXiv:1710.05832. Bibcode:2017PhRvL.119p1101A. doi:10.1103/PhysRevLett.119.161101. PMID 29099225.
  3. Connaughton, Valerie (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.
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