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Template:Use dmy dates Template:Infobox astronomical event Template:Sky GW170817 is a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 17 August 2017. The GW was produced by the last minutes of two neutron stars spiralling closer to each other and finally merging.

Unlike all previous GW detections, which were of merging black holes not expected to produce a detectable electromagnetic signal,<ref>Template:Cite journal</ref><ref name=Loeb>Template:Cite journal</ref>Template:Efn the aftermath of the merger was also seen by many conventional telescopes, marking a significant breakthrough for multi-messenger astronomy.Template:R<ref name="NASA-20171016">Template:Cite news</ref><ref>Template:Cite news</ref><ref>Template:Cite arxiv</ref>

Technically, there are three separate observations, and strong evidence that they come from the same astronomical source:

  • The GW signal, which had a duration of approximately 100 seconds, 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, calculated by triangulation.<ref>Template:Cite web</ref>
  • The short gamma-ray burst GRB 170817ATemplate:R detected by the Fermi and INTEGRAL spacecraft 1.7 seconds after the GW signal ended. These detectors have very limited directional sensitivity, but indicated a large area of the sky which overlapped the gravitational wave position. It has long been theorized that short gamma-ray bursts are caused by neutron star mergers.
  • The optical transient AT 2017gfo, found 11 hours later in the galaxy NGC 4993<ref name="SM-20171016" /> during a search of the region indicated by the GW detection. This was observed by numerous telescopes, from radio to X-ray wavelengths, over the following days and weeks, and shows the characteristics (a fast-moving, rapidly-cooling cloud of neutron-rich material) expected of debris ejected from a neutron-star merger.


Template:Quote box The event was officially announced on 16 October 2017<ref name="NYT-20171016">Template:Cite news</ref><ref name="MN-20171016">Template:Cite news</ref> at press conferences at the National Press Club in Washington, D.C. and at the ESO headquarters in Garching bei München in Germany.<ref name="SM-20171016">Template:Cite news</ref>

The first public information about the event was tweeted by astronomer J. Craig Wheeler of the University of Texas at Austin on 18 August 2017. 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 Template:Convert away in the Hydra constellation.<ref name="NAT-20170825">Template:Cite news</ref><ref name="NS-20170823">Template:Cite news</ref> The collaboration had earlier declined to comment on the rumors, not adding to a previous announcement that there were several triggers under analysis.<ref name="LIGO-20170825">Template:Cite news</ref><ref name="NG-20170825" />

Gravitational wave detection

The gravitational wave signal lasted for approximately 100 seconds starting from a frequency of 24 hertz (cycles per second). It covered approximately 3000 cycles, increasing in amplitude and frequency to a few hundred Hz in the typical inspiral chirp pattern, ending with the collision received at 12:41:04.4 UTC. 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.<ref>Template:Cite web</ref> The signal was detected and analyzed by a comparison with a template (i.e. the prediction from general relativity) defined from the post-Newtonian approximation.<ref>Luc Blanchet, Gravitational radiation from post-Newtonian sources and inspiralling compact binaries</ref>

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 (14 sec post-event), so the timing near-coincidence was automatically flagged and spurred frantic excitement and further analysis. The LIGO/Virgo team issued a preliminary alert (with only the crude gamma-ray position) to astronomers in the followup teams at 40 minutes post-event.Template:R

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 corrupted by a glitch (brief burst of instrument noise) a few seconds before the climax. These required manual analysis before the sky location could be announced about 4.5 hours post-event.<ref>Template:Cite web</ref> The three detections localized the source to an area of 28 square degrees in the southern sky with a 90% probability.<ref name="PhysRev2017" />

Gamma ray detection

The first electromagnetic signal detected was GRB 170817A, a short gamma ray burst, detected Template:Val after the merger time and lasting for a few seconds.Template:R

GRB 170817A was discovered by the Fermi gamma-ray 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 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.Template:R<ref name=spacecom>Template:Cite news</ref>

Electromagnetic follow-up

File:NGC 4993 and GRB170817A after glow.gif
Hubble picture of NGC 4993 with inset showing GRB170817A over 6 days. Credit: NASA and ESA
Optical lightcurves
File:Eso1733j X-shooter spectra montage of kilonova in NGC4993.png
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, and a three-detector sky location at 17:54 UTC.<ref name=GCN>Template:Cite web</ref> These prompted a massive search by many survey and robotic telescopes. In addition to its expected large size (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 only an hour after twilight for any given telescope.

In total six teams (SSS, DLT40, VISTA, Master, DECam, Las Cumbres Observatory (LCO) Chile) imaged the same new source independently in a 90-minute interval.Template:R The first to detect optical light associated with the collision was the Swope Supernova Survey, which found it in an image of NGC4993 taken 10 hours and 52 minutes after the eventTemplate:R by the Template:Convert diameter Swope Telescope operating in the near infrared at Las Campanas Observatory, Chile. They were also the first to announce it, naming their detection SSS17a in a circular issued 12h 26min post-event. The new source was later given an official International Astronomical Union (IAU) designation of AT 2017gfo.

The SSS team surveyed all galaxies in the region of space predicted by the gravitational wave observations, and identified a single new transient.Template:R By identifying the host galaxy of the merger, it is possible to provide a more accurate distance than based on gravitational waves alone. The detection of the optical/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.Template:R Over the following days, the color of the optical source changed from blue to red as the source expanded and cooled.Template:R

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.

Nine days later, the source was detected in X-rays by the Chandra X-ray Observatory (after non-detections at earlier times). Sixteen days after the merger event, the source was detected in radio with the Karl G. Jansky Very Large Array (VLA) in New Mexico.Template:R More than 70 observatories covering the electromagnetic spectrum observed the event.Template:R

Other detectors

No neutrinos consistent with the source were found in follow-up searches.<ref name="PhysRev2017">Template:Cite journal</ref><ref name="APJ">Template:Cite journal</ref> A possible explanation for the non-detection of neutrinos is because the event was observed at a large off-axis angle, i.e. the outflow jet was not directed towards Earth.<ref name="IceCube">Template:Cite web</ref>

Astrophysical origin and products

The gravitational wave signal indicated that the gravitational wave event was associated with the collision of two neutron starsTemplate:R<ref name="NG-20170825">Template:Cite news</ref><ref name="WIRED-20170825">Template:Cite news</ref> with a total mass of Template:Val times the mass of the sun (solar masses).Template:R If low spins are assumed, consistent with those observed in binary neutron stars that will merge within a Hubble time, the total mass is Template:Val.

The masses of the component stars have greater uncertainty. The larger (Template:Math) has a 90% chance of being between Template:Val, and the smaller (Template:Math) has a 90% chance of being between Template:Val.Template:R Under the low spin assumption, the ranges are Template:Val for Template:Math and Template:Val for Template:Math.

The chirp mass, a directly observable parameter which may be very roughly equated to the geometric mean of the masses, is measured at Template:Val.Template:R

The neutron star merger event is thought to be followed by a kilonova. Kilonovae are candidates for the production of half the chemical elements heavier than iron in the Universe.Template:R A total of 16,000 times the mass of the Earth in heavy elements is believed to have formed, including approximately ten Earth masses just of the two elements gold and platinum.<ref>Edo Berger, 1 hr 48 min into LIGO/Virgo press conference, 16 October 2017.</ref>

It is not known what object was produced by the merger. It could be either a neutron star heavier than any known neutron star, or a black hole lighter than any known black hole.Template:R

Scientific importance

Scientific interest in the event was enormous, with dozens of preliminary papers (and almost 100 preprints<ref>Template:Cite web</ref>) published the day of the announcement, including eight letters in Science,<ref>Template:Cite web</ref> six in Nature, and 23 in a special issue of The Astrophysical Journal Letters devoted to the subject.<ref name=ApJL>Template:Cite journal</ref>

This is not the first observation that is known to be of a neutron star merger; GRB 130603B was the first observed kilonova. It is however, by far the best observation, making this the strongest evidence to date to confirm the hypothesis that mergers of binary stars are the cause of short gamma-ray bursts.Template:R

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.<ref name = "Abbott^3_AJT">Template:Cite journal</ref> In addition, it allowed investigation of the equivalence principle (through Shapiro delay measurement) and Lorentz invariance.Template:R 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.Template:R GW170817 also excluded some alternatives to general relativity, including variants of scalar–tensor theory, Hořava–Lifshitz gravity<ref>Template:Cite arxiv</ref><ref>Template:Cite arxiv</ref><ref>Template:Cite arxiv</ref> and bimetric gravity.<ref>Template:Cite arxiv</ref>

Gravitational wave signals such as GW170817 may be used as a standard siren to provide an independent measurement of the Hubble constant.<ref name="Nat24471">Template:Cite journal</ref><ref name="AM-20170818">Template:Cite web</ref>

Electromagnetic observations helped to support the theory that the mergers of neutron stars contribute to r-process nucleosynthesis.<ref name='Drout'>Template:Cite journal</ref>


See also





External links

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