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, and is the first GW observation which has been confirmed by non-gravitational means.GW170817 is a
The GW170817 signal as measured by the LIGO and Virgo gravitational wave detectors
|Event type||Gravitational wave event, neutron star merger|
|Date||17 August 2017|
|Right ascension||13h 09m 48.08s|
|Declination||−23° 22′ 53.3″|
|Distance||40 megaparsecs (130 Mly)|
|Related media on Wikimedia Commons|
Unlike all previous GW detections, which were of merging black holes not expected to produce a detectable electromagnetic signal,[a] the aftermath of this merger was also seen by 70 observatories on seven continents and in space, across the electromagnetic spectrum, marking a significant breakthrough for multi-messenger astronomy. The discovery and subsequent observations of GW170817 were given the Breakthrough of the Year award for 2017 by the journal Science.
Technically, there were three separate observations, and strong evidence that they came from the same astronomical source:
- GW170817, which 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.
- GRB 170817A, a short (~ 2 seconds duration) gamma-ray burst detected by the Fermi and INTEGRAL spacecraft beginning 1.7 seconds after the GW merger signal. 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.
- AT 2017gfo (originally, SSS17a), an optical astronomical transient found 11 hours later in the galaxy NGC 4993 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 showed the characteristics (a fast-moving, rapidly-cooling cloud of neutron-rich material) expected of debris ejected from a neutron-star merger.
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.
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!". 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. The collaboration had earlier declined to comment on the rumors, not adding to a previous announcement that there were several triggers under analysis.
Gravitational wave detectionEdit
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.: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 template (i.e. the prediction from general relativity) defined from the post-Newtonian expansion.: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 sec post-event), 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 followup teams at 40 minutes post-event.
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 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. The three detections localized the source to an area of 31 square degrees in the southern sky with a 90% probability. More detailed calculations refined the localization to within 28 square degrees later. In particular, the absence of a clear detection by the Virgo system meant that the source was in one of Virgo's blind spots; this fact contributed to considerably reduce the source containment area.
Gamma ray detectionEdit
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 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.
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. 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.
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.:5 The first to detect optical light associated with the collision was the Swope Supernova Survey, which found it in an image of NGC 4993 taken 10 hours and 52 minutes after the GW event by the 1 meter (3 ft 3 in) 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. By identifying the host galaxy of the merger, it is possible to provide an accurate distance consistent with that based on gravitational waves alone.:5
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. Over the following days, the color of the optical source changed from blue to red as the source expanded and cooled.
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.
15.3 hours after the trigger, the source was detected in the ultraviolet by the Swift Gamma-Ray Burst Mission.:6 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. More than 70 observatories covering the electromagnetic spectrum observed the source.
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.
No neutrinos consistent with the source were found in follow-up searches by the IceCube and ANTARES neutrino observatories and the Pierre Auger Observatory. 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.
Astrophysical origin and productsEdit
The gravitational wave signal indicated that it was produced by the collision of two neutron stars with a total mass of +0.47
−0.09 times the mass of the sun ( 2.82solar masses). If low spins are assumed, consistent with those observed in binary neutron stars that will merge within a Hubble time, the total mass is +0.04
−0.01 M☉. 2.74
The masses of the component stars have greater uncertainty. The larger (m1) has a 90% chance of being between , and the smaller ( 1.36 and 2.26 M☉m2) has a 90% chance of being between . 0.86 and 1.36 M☉ Under the low spin assumption, the ranges are for 1.36 to 1.60 M☉m1 and for 1.17 to 1.36 M☉m2.
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. 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.
A hypermassive neutron star is believed to have formed initially and then collapsed into a black hole within milliseconds, as evidenced by the large amount of ejecta (much of which would have been swallowed by an immediately forming black hole) and the lack of evidence for emissions being powered by neutron star spin-down, which would occur for longer-surviving neutron stars.
Scientific interest in the event was enormous, with dozens of preliminary papers (and almost 100 preprints) published the day of the announcement, including eight letters in Science, six in Nature, and 32 in a special issue of The Astrophysical Journal Letters devoted to the subject. The interest and effort was global: the paper describing the multi-messenger observations 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 seven continents and in space.
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.
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.[b] In addition, it allowed investigation of the equivalence principle (through Shapiro delay measurement) and Lorentz invariance. 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. GW170817 also excluded some alternatives to general relativity, including variants of scalar–tensor theory, Hořava–Lifshitz gravity, Dark Matter Emulators and bimetric gravity.
Gravitational wave signals such as GW170817 may be used as a standard siren to provide an independent measurement of the Hubble constant. An initial estimate of the constant derived from the observation is +12.0
−8.0 (km/s)/Mpc, broadly consistent with current 70.0best estimates.
Electromagnetic observations helped to support the theory that the mergers of neutron stars contribute to rapid neutron capture r-process nucleosynthesis and are significant sources of r-process elements heavier than iron, including gold and platinum.
- 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.
- Previous constraint on the difference between the light speed and the gravitational speed was about ±20%
- Abbott, B. P.; et al. (LIGO, Virgo and other collaborations) (October 2017). "Multi-messenger Observations of a Binary Neutron Star Merger" (PDF). The Astrophysical Journal. 848 (2): L12. arXiv: [astro-ph.HE]. Bibcode:2017ApJ...848L..12A. doi: .
The optical and near-infrared spectra over these few days provided convincing arguments that this transient was unlike any other discovered in extensive optical wide-ﬁeld surveys over the past decade.
- 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). arXiv: [gr-qc]. Bibcode:2017PhRvL.119p1101A. doi: .
- 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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Mergers of stellar-mass black holes (BHs) [...] are not expected to have electromagnetic counterparts. [...] I show that the [GW and gamma-ray] signals might be related if the BH binary detected by LIGO originated from two clumps in a dumbbell configuration that formed when the core of a rapidly rotating massive star collapsed.
- Schilling, Govert (16 October 2017). "Astronomers Catch Gravitational Waves from Colliding Neutron Stars". Sky & Telescope.
because colliding black holes don’t give off any light, you wouldn’t expect any optical counterpart.
- de Mink, S.E.; King, A. (April 2017). "Electromagnetic Signals Following Stellar-mass Black Hole Mergers" (PDF). The Astrophysical Journal Letters. 839 (1): L7. arXiv: [astro-ph.HE]. Bibcode:2017ApJ...839L...7D. doi:10.3847/2041-8213/aa67f3.
It is often assumed that gravitational-wave (GW) events resulting from the merger of stellar-mass black holes are unlikely to produce electromagnetic (EM) counterparts. We point out that the progenitor binary has probably shed a mass ≳10 M☉ during its prior evolution. If even a tiny fraction of this gas is retained in a circumbinary disk, the sudden mass loss and recoil of the merged black hole shocks and heats it within hours of the GW event. Whether the resulting EM signal is detectable is uncertain.
- Berger, Edo (16 October 2017). "Focus on the Electromagnetic Counterpart of the Neutron Star Binary Merger GW170817". The Astrophysical Journal Letters (Editorial). 848 (2).
It is rare for the birth of a new field of astrophysics to be pinpointed to a singular event. This focus issue follows such an event—the neutron star binary merger GW170817—marking the first joint detection and study of gravitational waves (GWs) and electromagnetic radiation (EM).
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