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GW170817
The GW170817 signal as measured by the LIGO and gravitational wave detectors. Image includes Virgo data despite signal being in its blind spot (and thus not present)
Event typeGravitational wave
Date144 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
Distance144 million ly
Redshift0.0099
HostNGC 4993
Progenitor2 neutron stars
Other designationsGW170817
  Related media on Commons
[edit on Wikidata]

GW170817 was a gravitational wave (GW) observed by the LIGO and Virgo detectors on 17 August 2017, originating within the shell elliptical galaxy NGC 4993, about 140 million light years away.[2] The wave was produced by the last moments of the inspiral of a binary pair of neutron stars, ending with their merger. As of October 2025, it is the only GW detection to be definitively correlated with any electromagnetic observation.[1][3]

Unlike the five prior GW detections—which were of merging black holes and thus not expected to have detectable electromagnetic signals[4]—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] The discovery and subsequent observations of GW170817 were given the Breakthrough of the Year award for 2017 by the journal Science.[5][6]

GW170817 had an audible duration of approximately 100 seconds and exhibited 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 emitted by the merger.[1][7][8] These detectors have very limited directional sensitivity, but indicated a large region of the sky which overlapped the gravitational wave direction. 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 sGRB/GW 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.[9] It was captured by numerous telescopes in other electromagnetic bands, from radio to X-ray wavelengths, over the following days and weeks. It was found to be a fast-moving, rapidly-cooling cloud of neutron-rich material, as expected of debris ejected from a neutron-star merger.

Announcement

[edit]

It's the first time that we've observed a cataclysmic astrophysical event in both gravitational waves and electromagnetic waves—our cosmic messengers.[10]

Reitze D, 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.[7][8][9]

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!"[11] He later deleted the tweet and apologized for scooping the official announcement embargo. 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.[12][13] The collaboration had earlier declined to comment on the rumors, not adding to a previous announcement that there were several triggers under analysis.[14][15]

Gravitational wave detection

[edit]
Artist's impression of the collision of two neutron stars. This is a general illustration, not specific to GW170817. (00:23 video.)

The gravitational wave signal lasted for approximately 100 seconds (much longer than the few seconds measured for binary black hole mergers)[16] 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.[3]: 2  It arrived first at the Virgo detector in Italy, then 22 milliseconds later at the LIGO-Livingston detector in Louisiana, United States, and another 3 milliseconds later at the LIGO-Hanford detector in the state of Washington, in the United States. 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),[17] 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.[18][19]

Sky localisation of the event required combining data from the three interferometers, but 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 the event peak, which persisted 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 after the event.[20][19] 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.[18][3] In particular, the absence of a clear detection by the Virgo interferometer implied that the source was localized within one of its blind spots, a constraint which reduced the search area considerably.[21]

Gamma ray detection

[edit]
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.[8][12][1]: 5 

GRB 170817A was first recorded by the Fermi Gamma-ray Space Telescope, which issued an automatic alert just 14 seconds after the detection. After the LIGO/Virgo circular 40 minutes later, manual processing of data from the INTEGRAL gamma-ray telescope retrieved independent data for the event. 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 off axis.[9][22]

Electromagnetic follow-up

[edit]
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.[18] 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.[19]

In total six teams (One-Meter, Two Hemispheres (1M2H),[23] DLT40, VISTA, Master, DECam, and Las Cumbres Observatory (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[8][1][24] 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 SSS17a in a circular issued 12h26m post-event.[23] The new source was later given an official International Astronomical Union (IAU) designation 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.[22][24] 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.[24] Over the following days, the color of the optical source changed from blue to red as the source expanded and cooled.[22]

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 GW170817. The color 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 show nothing at the location of AT 2017gfo, ruling out a foreground variable star in the Milky Way.[23]

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.[25][5] After initial lack of X-ray and radio detections, the source was detected in X-rays 9 days later[26] using the Chandra X-ray Observatory,[27][28] and 16 days later in the radio[29] using the Karl G. Jansky Very Large Array (VLA) in New Mexico.[9] More than 70 observatories covering the electromagnetic spectrum observed the source.[9]

The radio and X-ray light increased to a peak 150 days after the merger,[30][31] diminishing afterwards.[32] Astronomers have monitored the optical afterglow of GW170817 using the Hubble Space Telescope.[33][34] In March 2020, continued X-ray emission at 5-sigma was observed by the Chandra Observatory 940 days after the merger.[35]

Other detectors

[edit]

No neutrinos consistent with the source were found in follow-up searches by the IceCube and ANTARES neutrino observatories and the Pierre Auger Observatory.[3][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.[36][37]

Astrophysical origin and products

[edit]

The origin and properties (masses and spins) of a double neutron star system like GW170817 are the result of a long sequence of complex binary star interactions.[38] The gravitational wave signal indicated that it was produced by the collision of two neutron stars[12][13][15][39] with a total mass of 2.82+0.47
−0.09 solar masses (M☉).[3] If low spins are assumed, consistent with those observed in binary neutron stars expected to merge within (twice[a]) the Hubble time, the total mass is 2.74+0.04
−0.01 M☉.

The masses of the progenitor stars have greater uncertainty. The chirp mass, a directly observable parameter which may be roughly equated to the geometric mean of the prior masses, was measured at 1.188+0.004
−0.002 M☉.[40] The larger progenitor (m1) has a 90% probability of being between 1.36 and 2.26 M☉, and the smaller (m2) has a 90% probability of being between 0.86 and 1.36 M☉.[40] 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, inside a 12 km radius.[41]

A hypermassive neutron star was believed to have formed initially, as evidenced by the large amount of ejecta (much of which would have been trapped by an immediately forming black hole). At first, the lack of evidence for emissions being powered by neutron star spindown, which would occur for longer-surviving neutron stars, suggested it collapsed into a black hole within milliseconds.[42] However, a more detailed analysis of the GW170817 signal tail later found evidence of further features consistent with the seconds-long spindown of an intermediate or remnant hypermassive magnetar,[43] and the energy of this spindown was estimated at ≃63 Foe, equivalent to 3.5% of the mass-energy of the Sun.[44] This was below the estimated sensitivity of the LIGO search algorithms at the time.[45] This was confirmed in 2023 by a statistically independent method of analysis revealing the central engine of GRB 170817A.[46]

The short gamma-ray burst was followed over the next several months by its slower-evolving kilonova counterpart, a spherically expanding optical afterglow powered by the radioactive decay of heavy r-process nuclei produced and ejected at the initial cataclysmic instant.[47][48] GW170817 therefore confirmed neutron star mergers to be viable sites for the r-process, where the nucleosynthesis of around half the isotopes in elements heavier than iron can occur.[9] 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.[49] The electromagnetic emission is estimated at 0.5% of the mass-energy of the Sun.[44]

As of 2025, the precise nature of the ultimately stable compact remnant remains uncertain.[43][35]

Scientific importance

[edit]
Artist's impression of strontium emerging from a neutron star merger.[50]

Scientific interest in the event was enormous, with dozens of preliminary papers (and almost 100 preprints[51]) published the day of the announcement, including 8 letters in Science,[9] 6 in Nature, and 32 in a special issue of The Astrophysical Journal Letters devoted to the subject.[52] 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.[11][9]

The event provided 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.[40][53][b]

In addition, GW170817 allowed investigation of the equivalence principle (through Shapiro delay measurement) and Lorentz invariance.[3] 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.[40]

The event also excluded some alternatives to general relativity,[54] including variants of scalar–tensor theory,[55][56][57][58][59][60][61][62] Hořava–Lifshitz gravity,[58][63][59] Dark Matter Emulators,[64] and bimetric gravity,[65] Furthermore, an analysis published in July 2018 used GW170817 to show that gravitational waves propagate fully through the 3+1 curved spacetime described by general relativity, ruling out hypotheses involving "leakage" into higher, non-compact spatial dimensions.[c][66]

Gravitational wave signals such as GW170817 may be used as a standard siren to provide an independent measurement of the Hubble constant.[67][68] 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.[67] Further studies improved the measurement to 70.3+5.3
−5.0 (km/s)/Mpc.[69][70][71] 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.[72][73]

Electromagnetic observations help support the theory that neutron star mergers contribute to rapid neutron capture (r-process) nucleosynthesis[24]—previously assumed to be associated with supernova explosions—and are therefore the primary source of r-process elements heavier than iron,[1] including gold and platinum.[49] The first identification of r-process elements in a neutron star merger was obtained during a re-analysis of GW170817 spectra.[74] The spectra provided direct proof of strontium production during a neutron star merger. This also provided the most direct proof that neutron stars are made of neutron-rich matter. Since then, several r-process elements have been identified in the ejecta including yttrium,[75] lanthanum and cerium.[76]

In October 2017, Stephen Hawking, in his last broadcast interview, discussed the overall scientific importance of GW170817.[77] 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.[78]

Retrospective comparisons

[edit]

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.[79][80][81][82]

Later research further construed GRB 160821B—another sGRB predating GW170817—also to belong to this class, again based on afterglow resemblance to the AT 2017gfo signature.[83]

See also

[edit]

Notes

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

GW170817

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GW170817 is a event detected on August 17, 2017, at 12:41:04 UTC by the Advanced detectors in , and , and the Advanced Virgo detector near , , originating from the inspiral and merger of a binary neutron star system approximately 40 megaparsecs (130 million light-years) away in the elliptical galaxy NGC 4993. The signal, with a signal-to-noise ratio of 32.4 and a false alarm rate of less than one per 80,000 years, was localized to a 28-square-degree region of the with 90% confidence, enabling rapid follow-up observations by over 70 ground- and space-based telescopes worldwide. The binary system's total mass was measured as 2.74 solar masses, with individual component masses estimated between 1.17 and 1.60 solar masses, consistent with stars rather than black holes. This detection confirmed the existence of binaries as sources of and provided precise in the strong-field regime during the merger. Just 1.7 seconds after the signal, the Fermi Gamma-ray Burst Monitor and the satellite detected a short (GRB 170817A), establishing a direct link between mergers and these cosmic explosions. Subsequent electromagnetic observations revealed a —a rapidly fading source of optical, , , and light—powered by the of heavy elements synthesized in the ejected neutron-rich material. Telescopes including Hubble, Swift, , and Spitzer captured this counterpart, confirming mergers as a major production site for elements heavier than iron, such as and , through rapid processes. The joint gravitational and electromagnetic detection of GW170817 heralded the era of multi-messenger astronomy, allowing astronomers to study the same cosmic event across multiple wavelengths and providing insights into extreme physics, including the equation of state of matter and the acceleration of relativistic jets. It also refined the Hubble constant measurement to 70 km/s/Mpc, aiding cosmological distance scale calibrations, and demonstrated the power of international collaborations in real-time alert systems for transient events.

Discovery

Gravitational Wave Detection

The gravitational wave event GW170817 was detected on August 17, 2017, at 12:41:04 UTC by the Advanced detectors at Hanford and Livingston observatories, along with the Advanced Virgo detector in . This marked the first observation of a binary neutron star inspiral by the LIGO-Virgo network, with the signal originating from the coalescence of two compact objects inferred to be neutron stars based on their low masses. The signal lasted approximately 100 seconds and exhibited a characteristic , with its frequency increasing from about 30 Hz to 400 Hz and a peak strain amplitude on the order of 10−2110^{-21}. The detection was achieved through matched filtering techniques applied to the detector data, using a bank of template waveforms modeled for the inspiral phase of binary neutron star systems. The combined (SNR) across the three detectors was 32.4, corresponding to a rate of less than one event per 8×1048 \times 10^4 years, confirming the astrophysical origin of the signal. Waveform reconstruction revealed the full sequence of the binary evolution, including the inspiral phase dominated by the gradual due to energy loss via gravitational , a brief merger phase, and a weak ringdown phase from the post-merger remnant. The inspiral was modeled using post-Newtonian approximations, while the merger and ringdown phases incorporated simulations to capture the highly nonlinear dynamics. A key parameter extracted was the , defined as Mchirp=(m1m2)3/5(m1+m2)1/5,\mathcal{M}_\text{chirp} = \frac{(m_1 m_2)^{3/5}}{(m_1 + m_2)^{1/5}}, estimated to be 1.188−0.002+0.0041.188^{+0.004}_{-0.002} solar masses, which provided initial constraints on the component masses. Low-latency analysis pipelines enabled rapid alerts to be issued through the GraceDB database and the Gamma-ray Coordinates Network (GCN), facilitating follow-up observations within minutes of detection. The gravitational wave signal was later associated with the short gamma-ray burst GRB 170817A.

Gamma-Ray Detection

The prompt gamma-ray counterpart to GW170817, designated GRB 170817A, was detected by the Fermi Gamma-ray Burst Monitor (GBM) approximately 1.7 seconds after the gravitational-wave trigger time. The burst was also independently detected by the Anti-Coincidence Shield (ACS) of the Spectrometer aboard (SPI-ACS) at about 2 seconds after the GW trigger. This short temporal offset provided initial evidence for a physical association between the electromagnetic and gravitational-wave signals. GRB 170817A exhibited properties consistent with a typical short gamma-ray burst, including a duration T90≈2T_{90} \approx 2 seconds in the 50--300 keV band. The time-integrated fluence was measured as (2.8±0.2)×10−7(2.8 \pm 0.2) \times 10^{-7} erg cm−2^{-2} over 10--1000 keV by Fermi GBM, while SPI-ACS reported (1.4±0.4±0.6)×10−7(1.4 \pm 0.4 \pm 0.6) \times 10^{-7} erg cm−2^{-2} in the 75--2000 keV range. Assuming a source distance of approximately 40 Mpc, the isotropic-equivalent energy release was estimated at around 5×10465 \times 10^{46} erg, though this value was later refined to account for structured jet geometry. Spectral analysis of the Fermi GBM data favored a Band function fit for the main pulse, with low-energy photon index α≈−0.6\alpha \approx -0.6, high-energy photon index β≈−2.3\beta \approx -2.3, and peak energy Epeak≈185E_{\rm peak} \approx 185 keV. A weaker tail emission was modeled separately as a blackbody with temperature kT≈10kT \approx 10 keV. No significant emission was detected by the Fermi Large Area Telescope (LAT) above 100 MeV, with upper limits on the flux of 4.5×10−104.5 \times 10^{-10} erg cm−2^{-2} s−1^{-1} (0.1--1 GeV) starting from about 20 minutes post-trigger, constraining the presence of high-energy components. The observed properties of GRB 170817A, including its relatively low isotropic luminosity compared to typical short gamma-ray bursts (underluminous by a factor of ∼1000\sim 1000), are consistent with an off-axis relative to a relativistic jet launched by the . This geometry explains the dimmer emission while still linking the burst to the event.

Announcement

The detection of the signal GW170817 on August 17, 2017, prompted an immediate coordinated alert to the astronomical community through the Gamma-ray Coordinates Network (GCN), issued by the LIGO-Virgo Collaboration shortly after the event. This was followed by reports of an associated short gamma-ray burst (GRB 170817A) detected by NASA's Fermi Gamma-ray Burst Monitor and ESA's satellite, with joint LIGO-Virgo-Fermi- notices disseminated via GCN circulars on the same day to facilitate rapid follow-up. The LIGO-Virgo Electromagnetic (LVE) working group coordinated the initial sky localization effort, providing an early 90% credible region of approximately 28–31 deg² based on the three-detector network data, which was subsequently refined to around 10–16 deg² through improved parameter estimation. These alerts triggered a global response, activating roughly 70 ground- and space-based telescopes across the within hours of the merger to search for counterparts within the localized region. The first optical detection, SSS17a, was reported by the Swope Survey 10.9 hours post-merger, marking the beginning of intensive multi-wavelength observations. The full public announcement occurred on October 16, 2017, via synchronized press releases from , Virgo, Fermi, and , highlighting the multimessenger nature of the event. Scientific publications followed immediately, with the analysis detailed in and multimessenger results in The Astrophysical Journal Letters, both released on October 16, 2017. To prevent false alarms and ensure rigorous verification, the initially withheld detailed public information, conducting internal reviews and cross-checks over nearly two months before the coordinated , a process that underscored the challenges of managing high-stakes multimessenger alerts.

Multi-Messenger Observations

Electromagnetic Follow-Up

Following the gravitational-wave alert from /Virgo, which localized GW170817 to a 28 deg² probability region in the southern sky, extensive electromagnetic follow-up campaigns targeted this area across multiple wavelengths. The optical counterpart, designated AT2017gfo, was discovered in the galaxy NGC 4993 approximately 11 hours post-merger by the One-Meter, Two Hemisphere (1M2H) collaboration using the , with rapid confirmation from the on the Blanco 4 m and the Las Cumbres global network of 1 m . The transient peaked at an of approximately -16 in the r-band around 1 day after the merger, exhibiting a rapid rise and subsequent decline characteristic of a . Infrared observations complemented the optical data, with the detecting AT2017gfo at 3.6 and 4.5 μm beginning 43 days post-merger. The (VLT) at Cerro Paranal provided near-infrared imaging and spectroscopy showing a color evolution from blue to red over the first few days, attributed to the presence of lanthanide-poor ejecta, with the peak luminosity reaching about 10^{41} erg s^{-1}. X-ray emission from the afterglow was first detected by the in an observation starting 9 days post-merger, with an unabsorbed flux of approximately 10^{-15} erg cm^{-2} s^{-1} in the 0.3–10 keV band. Radio emission emerged later, detected by the Karl G. Jansky Very Large Array (VLA) at 16 days post-merger with a flux density of about 100 μJy at 3 GHz. modeling of the indicated a two-component structure, comprising a "blue" component from fast-moving dynamical and a "red" component from slower tidal tail material, which together reproduced the observed multi-band evolution over the first ~10 days. The in radio wavelengths showed a rising , peaking around 150 days post-merger before declining. Ongoing monitoring, including radio observations and X-ray exposures, revealed a continued decline in flux at late times, with a 2025 reanalysis of up to 2043 days post-merger (as of March 2023) confirming the evolution consistent with models of a structured jet.

Neutrino and Other Non-Electromagnetic Searches

Searches for high-energy s associated with GW170817 were conducted using multiple observatories to probe potential emission from relativistic outflows or shocked in the binary neutron star merger. The , sensitive to neutrinos in the TeV to PeV energy band, analyzed in a ±500 s time window centered on the trigger time, as well as an extended 14-day period following the merger. No neutrino events were found directionally or temporally coincident with location. This non-detection yielded an upper limit on the spectral fluence of F(E)=0.19(E/GeV)−2 GeV−1cm−2F(E) = 0.19 (E / \mathrm{GeV})^{-2} \, \mathrm{GeV^{-1} cm^{-2}} at 90% confidence level for an E−2E^{-2} power-law spectrum in the 10 TeV to 100 PeV range during the prompt window. The neutrino telescope, focusing on lower-energy s in the GeV to TeV range, performed a complementary search for prompt emission within ±500 s of the merger and over the subsequent 14 days. No candidate events were identified within the sky localization uncertainty of GW170817. The resulting upper limits on the spectral fluence were F(E)=2.0(E/GeV)−2 GeV−1cm−2F(E) = 2.0 (E / \mathrm{GeV})^{-2} \, \mathrm{GeV^{-1} cm^{-2}} for the prompt window in the 100 GeV to 100 TeV band, providing stringent constraints on models involving production from choked jets or internal shocks in the merger . These limits imply that any emission efficiency in such scenarios is below 1% of the inferred jet energy. As the successor to , the KM3NeT detector has since enhanced sensitivity in the Mediterranean for similar low-energy searches, though its initial operations postdated GW170817; retrospective analyses align with results, reinforcing the null outcome. The Pierre Auger Observatory complemented these efforts by searching for ultra-high-energy s (above 100 PeV) that could arise from cosmic-ray interactions in the merger environment or associated outflows. No events were detected in association with GW170817 across the relevant time windows. The observatory set upper limits on the neutrino fluence at the level of 4×10−8 GeV cm−24 \times 10^{-8} \, \mathrm{GeV \, cm^{-2}} for an E−2E^{-2} above 100 PeV, at 90% level, further bounding potential high-energy neutrino contributions from cosmic-ray in the system. In addition to particle searches, the and Virgo detectors extended the gravitational wave analysis beyond the inspiral-merger-ringdown signal to probe for continuous emission from a potentially long-lived, deformed remnant. Searches targeted quasi-normal mode ringdown frequencies and possible persistent emission from non-axisymmetric deformations induced by the merger. No significant signals were found in the post-merger data up to several hundred seconds after the peak amplitude. Upper limits on the strain amplitude were placed at h∼10−22h \sim 10^{-22} (dimensionless) for frequencies around 1-2 kHz, ruling out continuous emission at levels that would indicate a , rapidly rotating remnant with significant ellipticity. The null results from these non-electromagnetic searches collectively indicate minimal production of high-energy and continuous in the GW170817 event. This supports models where the energy release in binary neutron star mergers is predominantly electromagnetic, via and mechanisms, rather than through efficient or persistent channels. The constraints particularly disfavor scenarios with substantial acceleration leading to neutrino emission, such as in on-axis relativistic jets or choked jet cocoons, aligning with the off-axis inferred from the observed electromagnetic signals.

Localization and Host Galaxy

Sky Localization

The sky localization of GW170817 was achieved through triangulation using arrival time and phase differences of the signal across the three detectors: LIGO Hanford, LIGO Livingston, and Virgo. These timing differences, on the order of a few milliseconds, constrained the source direction, with the small delay between Hanford and Livingston (approximately 1.5 ms) and the larger relative delay to Virgo (approximately 6 ms) indicating a location in the . This three-detector network provided a significant improvement over prior mergers, such as GW150914, which relied on only two detectors and resulted in a much larger 90% credible region of about 600 deg². Bayesian parameter estimation was performed using the LALInference pipeline within the Scientific Collaboration and Virgo Collaboration's inference framework, incorporating low-latency and full low-frequency approximations of the gravitational to sample the posterior distribution of source parameters, including sky position. The initial analysis yielded a 90% credible sky region of 28 deg² centered near 13h 09m and -23°, along with a distance estimate of 40^{+8}_{-14} Mpc derived from the signal and modeling. Subsequent refinements, incorporating electromagnetic counterpart data to update the joint posterior, reduced the effective 90% credible region to approximately 10 deg², facilitating targeted follow-up observations. The precision of this localization underscored the value of the Virgo detector's inclusion, as simulations had predicted that a third interferometer could shrink error regions by factors of 3–10 for similar events, enabling the multimessenger identification within hours.

Identification of NGC 4993

The electromagnetic counterpart to GW170817, designated AT2017gfo, was detected as an optical transient approximately 10 arcseconds from the nucleus of the galaxy NGC 4993, pinpointing it as the likely host within the initial gravitational-wave sky localization region of 28 square degrees. This localization encompassed roughly 150 bright galaxies suitable for targeted follow-up searches, enabling rapid identification through multi-wavelength observations that confirmed the transient's association with NGC 4993. To assess host galaxy probabilities, researchers cross-referenced the localization with comprehensive galaxy catalogs, including the Galaxy List for the Advanced Detector Era (GLADE), which integrates data from surveys like the Gravitational Wave Galaxy Catalog and . These catalogs facilitated ranking based on factors such as and luminosity-weighted probabilities; NGC 4993 emerged as a high-probability candidate due to its position and properties aligning with the expected merger environment. The galaxy, an early-type shell elliptical at z=0.0098z = 0.0098, exhibits morphological features indicative of a recent merger approximately 400 million years ago, including concentric stellar shells formed from the accretion of a smaller , and maintains a low rate of approximately 0.01 M⊙/yr0.01 \, M_\odot / \mathrm{yr}. The AT2017gfo transient is offset by about 2 kpc in projection from NGC 4993's center, a separation consistent with the natal kick velocities imparted to neutron stars during their formation. This identification via the electromagnetic counterpart position provided definitive confirmation, distinguishing NGC 4993 from other galaxies in the region.

Distance Measurement

The gravitational-wave signal of GW170817 enabled a direct inference of the source's luminosity from the of the detected , yielding DL=40.7−5.0+6.9D_L = 40.7^{+6.9}_{-5.0} Mpc (68.3% ). The association with host galaxy NGC 4993 provided an independent measurement of z=0.0099z = 0.0099 from the Hα emission line. Assuming a Hubble constant of H0=70H_0 = 70 km s−1^{-1} Mpc−1^{-1}, this corresponds to a comoving of approximately 40 Mpc at such low zz, where the approximation Dc≈cz/H0D_c \approx cz/H_0 holds. These distance estimates from gravitational waves and electromagnetic observations are mutually consistent, validating the use of GW170817 as a standard siren and previewing its application in cosmology; the low obviates the need for peculiar corrections. Subsequent analyses incorporating refined models have further tightened the gravitational-wave to 40±240 \pm 2 Mpc.

Merger Interpretation

Binary System Parameters

The gravitational-wave signal from GW170817 allowed for the inference of the pre-merger binary star system's intrinsic properties through Bayesian parameter estimation. The analysis employed numerical relativity-inspired models such as SEOBNRv4 and IMRPhenomPv2, which incorporate post-Newtonian approximations for the inspiral, effective-one-body methods for the merger, and tidal effects from the finite deformability of stars. (MCMC) sampling was used to explore the posterior distributions, providing 90% credible intervals for the parameters while marginalizing over extrinsic factors like and sky . The component masses were estimated to be in the range m1≈1.36−1.60 M⊙m_1 \approx 1.36 - 1.60 \, M_\odot for the primary and m2≈1.17−1.36 M⊙m_2 \approx 1.17 - 1.36 \, M_\odot for the secondary (low-spin prior), yielding a total mass M≈2.74 M⊙M \approx 2.74 \, M_\odot with 90% credible bounds of 2.69−2.78 M⊙2.69 - 2.78 \, M_\odot. These values are consistent with the masses inferred from observations, such as the double pulsar system PSR J0737−3039A/B, which has components of approximately 1.338 M⊙M_\odot and 1.249 M⊙M_\odot. The effective tidal deformability of the binary, Λ~\tilde{\Lambda}, which quantifies the quadrupolar deformation induced by the companion's tidal field, was constrained to Λ~=300−230+420\tilde{\Lambda} = 300^{+420}_{-230} at 90% credibility, indicating relatively compact neutron stars compared to stiffer equations of state. The spins of the neutron stars were found to be small, with dimensionless magnitudes χ<0.4\chi < 0.4 for both components under astrophysically motivated priors, and no evidence for misaligned spins; the effective aligned spin parameter χeff\chi_\mathrm{eff} is consistent with zero within uncertainties, suggesting the spins are largely aligned with the orbital angular momentum. The orbital eccentricity at merger was negligible, with an upper limit e≤0.02e \leq 0.02 at a gravitational-wave frequency of 10 Hz, as the binary had circularized during its long inspiral due to gravitational-wave emission. Post-merger, the remnant was a hypermassive neutron star with a gravitational mass of approximately 2.7 M⊙M_\odot, supported briefly against collapse by its rapid differential rotation before forming a black hole. Numerical relativity simulations indicate this collapse occurred within ≲10\lesssim 10 ms, consistent with the absence of a prolonged post-merger gravitational-wave signal in the data.

Dynamical Ejecta and Kilonova

The dynamical ejecta launched during the merger of the two neutron stars in consisted of neutron-rich material expelled on dynamical timescales, with an estimated mass of approximately 0.04 M_\sun and velocities ranging from 0.2c to 0.3c. In addition, post-merger winds from the accretion disk around the remnant contributed further ejecta, with a mass of about 0.03 M_\sun and lower velocities around 0.08c–0.1c. These components together powered the thermal emission observed as the kilonova AT 2017gfo, providing key insights into the merger's outflow structure. The kilonova's spectral evolution reflected changes in ejecta opacity driven by composition differences. An initial lanthanide-free "blue" component, with opacity κ ≈ 1 cm² g⁻¹, dominated early emission in the UV-optical bands, transitioning to a lanthanide-rich "red" component with higher opacity κ ≈ 10–100 cm² g⁻¹ that peaked in the infrared. Radiative transfer simulations using codes like SuperNu predicted this multi-wavelength peak, incorporating time-dependent Monte Carlo methods to model photon diffusion through expanding ejecta with power-law velocity distributions. The total observed radiated energy was approximately 10^{46} erg, consistent with the radioactive decay of r-process elements in the outflow. Spectral observations revealed evidence for asymmetric ejecta geometry, manifested as velocity gradients in line profiles. P-Cygni absorption-emission features in early spectra indicated outflow velocities varying from ≈0.15c to 0.3c across the line of sight, suggesting equatorial concentration of denser material and more isotropic polar winds. Recent late-time spectroscopy up to ≈10 days post-merger has confirmed persistent r-process signatures, with evolving emission lines from heavy elements like strontium and barium emerging hour-by-hour, supporting models of stratified ejecta composition.

Relativistic Jet and Structured Afterglow

The relativistic outflow from the binary neutron star merger is interpreted as a structured jet viewed off-axis, with the observer's line of sight at a viewing angle θv≈20∘\theta_v \approx 20^\circ relative to the jet axis and a core opening angle θc≈5∘\theta_c \approx 5^\circ--6∘6^\circ. This geometry explains the weak prompt gamma-ray emission of GRB 170817A as off-axis viewing of the relativistic core, where the beaming suppresses the observed flux. The jet structure features a highly relativistic, energetic core surrounded by broader wings of lower energy and velocity, enabling initial visibility of the slower wing material before the core's emission becomes dominant as it decelerates. The broadband afterglow arises from synchrotron radiation produced by the forward shock of the jet interacting with the circumburst interstellar medium, characterized by a low density n≈0.01 cm−3n \approx 0.01~\mathrm{cm}^{-3}. During the deceleration phase, the bulk Lorentz factor evolves as Γ∝t−3/8\Gamma \propto t^{-3/8}, consistent with the self-similar Blandford-McKee solution for an adiabatic blast wave in a constant-density medium. Multi-wavelength light curves in radio and X-ray bands peak around 150 days post-merger, marking the transition from coasting to deceleration and the increasing visibility of on-axis-like emission from the core. Bayesian analyses of the afterglow data constrain the structured jet properties, including a core Lorentz factor Γc≈100\Gamma_c \approx 100 and an energy profile often modeled with a Gaussian distribution in the core region transitioning to power-law wings. The energy in the wings follows E(θ)∝θ−2E(\theta) \propto \theta^{-2} beyond the core, allowing fits to the rising and peaking light curves without requiring multiple ejecta components. Recent late-time observations extending into 2024 and 2025 reveal a steepening in the afterglow light curve, signaling the increasing visibility of the jet edge as the Lorentz factor decreases and the beaming cone widens to encompass the structured outflow's boundaries. This evolution aligns with predictions for off-axis structured jets, where the post-peak decline transitions to a steeper slope once Γ−1≈θv+θc\Gamma^{-1} \approx \theta_v + \theta_c.

Scientific Significance

Tests of General Relativity

The multimessenger observation of GW170817 enabled precise tests of General Relativity (GR) in the strong-field regime, particularly during the inspiral, merger, and potential ringdown phases of the binary neutron star coalescence, as well as the propagation of the gravitational-wave (GW) signal across cosmic distances. These tests leveraged the high signal-to-noise ratio of the GW detection and the near-simultaneous arrival of the electromagnetic (EM) counterpart, GRB 170817A, to probe deviations from GR predictions in alternative gravity theories. Analyses consistently found agreement with GR, placing stringent bounds on modified gravity parameters without evidence for violations. A key test involved the propagation speed of gravitational waves, directly compared to the speed of light via the EM counterpart. The GW signal was detected at 12:41:04.4 UTC on August 17, 2017, followed by GRB 170817A approximately 1.7 seconds later, with the source distance estimated at about 40 Mpc. Assuming simultaneous emission or a short emission delay of up to 10 seconds, this small time offset over the propagation baseline constrains the relative speed difference to ∣cGW−cEM∣/c<5×10−16|c_\text{GW} - c_\text{EM}| / c < 5 \times 10^{-16}, consistent with GR's prediction of massless, light-speed propagation and ruling out many theories with differing GW speeds, such as some scalar-tensor models. The timing precision, with uncertainties below 10−510^{-5} seconds after accounting for detector calibration and emission physics, further tightens this bound and excludes modified dispersion relations that could cause frequency-dependent delays. In the parametrized post-Einstein (PPN) framework, GW170817 data were used to bound deviations in the phase evolution during inspiral, focusing on post-Newtonian coefficients that parameterize alternative theories. No significant deviations were found, with the dipole radiation amplitude constrained to B≤1.2×10−5B \leq 1.2 \times 10^{-5} at 90% credibility, aligning with GR's vanishing dipole for non-spinning, non-charged compact objects like neutron stars. Similarly, tests of polarization content confirmed the presence of only the two tensor modes predicted by GR, bounding contributions from scalar breathing modes (which would introduce longitudinal polarizations) to less than 10% of the tensor signal strength. These results, derived via Bayesian inference on hybrid inspiral-merger-ringdown waveforms like PhenomPNRT and SEOBNRT, support GR's quadrupole radiation dominance. Cumulative analyses including subsequent GW events have further tightened these bounds. The post-merger phase provided additional GR tests through the potential ringdown of the remnant. Although no clear ringdown signal was detected due to the event's low signal-to-noise ratio above ~1 kHz and the likely formation of a short-lived hypermassive neutron star, the observed signal cutoff is consistent with GR predictions for a prompt collapse to a Kerr black hole remnant of mass ~2.8 M_\sun and spin parameter a \approx 0.7. Numerical relativity simulations matching the inferred binary parameters predict the dominant (2,2) quasinormal mode frequency f_{220} \approx 2000 Hz for such a remnant, with no deviations observed in the available data. Upper limits on post-merger GW emission (network signal-to-noise ratio < 6.7 at 90% credibility) further align with GR expectations for damped oscillations. Searches for Lorentz violation and massive gravitons using GW170817 yielded null results, reinforcing GR. Modified dispersion relations, which could induce Lorentz-violating effects or graviton mass, were constrained by the non-dispersive propagation over 40 Mpc; the graviton mass is bounded to m_g < 1.2 \times 10^{-22} eV/c^2 (90% credibility), over an order of magnitude weaker than binary black hole bounds but still excluding many massive gravity models. The multimessenger timing consistency independently rules out frequency-dependent dispersion at levels below 10^{-22} eV^2 for the graviton mass parameter, with no evidence for birefringence or other violations. These tests collectively affirm GR's validity in the strong-field, dynamical regime probed by GW170817.

Neutron Star Equation of State Constraints

The detection of gravitational waves from allowed for the first measurement of the effective tidal deformability Λ~\tilde{\Lambda} of a binary neutron star system, which encodes information about tidal interactions during the inspiral phase and provides direct constraints on the neutron star equation of state (EOS). The dimensionless tidal deformability for a single neutron star is defined as Λ=23k2(c2RGM)5\Lambda = \frac{2}{3} k_2 \left( \frac{c^2 R}{G M} \right)^5, where k2k_2 is the dimensionless quadrupole Love number, RR is the radius, and MM is the mass. The effective binary parameter Λ~\tilde{\Lambda} is a mass-weighted combination of the individual Λ1\Lambda_1 and Λ2\Lambda_2. Analysis of the GW170817 signal yielded Λ~=300−230+420\tilde{\Lambda} = 300^{+420}_{-230} (90% credible interval), excluding stiff EOS models such as DD2 at beyond the 1.5σ\sigma level. This measurement implies tight bounds on neutron star radii, with the radius of a 1.4 M⊙M_\odot star constrained to R1.4<13R_{1.4} < 13 km at 90% confidence level, favoring softer EOS like SLy4 while showing tension with very stiff ones that predict larger radii. The binary component masses, approximately 1.4 M⊙M_\odot each assuming low spins, further support these inferences by linking Λ~\tilde{\Lambda} to the EOS via universal relations between tidal parameters and global properties. Additionally, the absence of a prolonged post-merger gravitational-wave signal indicates a remnant lifetime shorter than 10 ms, implying prompt collapse to a black hole and requiring the maximum stable neutron star mass MTOV>2.1M⊙M_\mathrm{TOV} > 2.1 M_\odot for proto-neutron star stability against collapse. Subsequent analyses incorporating improved numerical relativity waveforms have refined these constraints while maintaining compatibility with soft-to-intermediate EOS and excluding hadronic models with excessively stiff high-density behavior. These updates leverage enhanced waveform models that better capture tidal effects, providing more precise mapping to EOS parameters without altering the core implications for neutron star structure.

Standard Candle Measurements for Cosmology

GW170817 marked the first use of a gravitational-wave event as a standard siren to measure the Hubble constant H0H_0, providing a luminosity distance DLD_L of approximately 40 Mpc directly from the waveform analysis, independent of traditional distance ladders. The identification of the host galaxy NGC 4993, with a measured redshift z≈0.01z \approx 0.01, enabled the conversion of this distance to an expansion rate estimate. In the statistical dark siren method, which marginalizes over possible host galaxies using a catalog within the gravitational-wave localization volume, the analysis yielded H0=70−8+12H_0 = 70^{+12}_{-8} km s−1^{-1} Mpc−1^{-1}. Leveraging the electromagnetic counterpart for precise host association reduced inference degeneracies, leading to a direct bright siren measurement of H0=70−8+12H_0 = 70^{+12}_{-8} km s−1^{-1} Mpc−1^{-1}. This result sits between the cosmic microwave background constraint from Planck, H0=67.4±0.5H_0 = 67.4 \pm 0.5 km s−1^{-1} Mpc−1^{-1}, and the Cepheid-supernova distance ladder from SH0ES, H0=73.0±1.0H_0 = 73.0 \pm 1.0 km s−1^{-1} Mpc−1^{-1}, offering an intermediate value that does not resolve but informs the ongoing H0H_0 tension. The underlying methodology relates the observed luminosity distance to via the equation DL=(1+z)∫0zc dz′H(z′),D_L = (1 + z) \int_0^z \frac{c \, dz'}{H(z')}, where H(z)H(z) is the Hubble parameter at z′z', and H0H_0 is marginalized over in Bayesian analyses to derive the expansion rate.

Insights into Heavy Element

The merger of GW170817 provided neutron-rich conditions conducive to rapid neutron capture (r-process) , with fractions YeY_e ranging from approximately 0.04 in the dynamical component to 0.2–0.5 in the post-merger wind, enabling the production of heavy nuclei up to A∼254A \sim 254. These low YeY_e values in the tidally disrupted material favored neutron excess, driving the formation of third-peak r-process elements including actinides, while higher YeY_e regions contributed lighter r-process isotopes. Simulated under these conditions confirmed the of the r-process pathway, with fission cycling recycling heavy nuclei to match observed abundance patterns. Estimates of r-process yields from the kilonova AT 2017gfo associated with GW170817 indicate approximately 0.01 M⊙M_\odot of lanthanides (elements with 56≤Z≤7156 \leq Z \leq 71) and 0.05 M⊙M_\odot of lighter r-process elements (A<140A < 140), based on modeling the total mass of about 0.1 M⊙M_\odot and opacity constraints from the . These yields highlight the merger's role in synthesizing significant quantities of heavy elements, surpassing typical outputs from single stellar events. Spectroscopic observations of the revealed absorption features from r-process elements, including the Sr II line at 4077 Å, marking the first extragalactic confirmation of synthesis via this pathway at 2.4 days post-merger. Opacity calculations from nuclear networks incorporating these lines showed elevated values due to lanthanide-bound electrons, explaining the blue-to-red color evolution and validating models of neutron-rich composition. The inferred abundances from GW170817 closely align with the solar system's r-process isotopic distribution, particularly for elements beyond , supporting mergers as the primary production site over core-collapse supernovae, which produce insufficient for heavy nuclei. Galactic chemical evolution models incorporating GW170817-like events reproduce observed stellar enrichment patterns, indicating mergers dominate r-process contributions in dwarf galaxies and the .

Legacy

Comparisons to Subsequent Events

GW170817 marked the first binary neutron star (BNS) merger observed in both (GWs) and electromagnetic (EM) radiation, setting it apart from subsequent BNS candidates that lacked detectable EM counterparts. For instance, GW190425, detected during the O3a run in April 2019 by the Hanford and Livingston observatories, produced a signal consistent with a BNS merger at a luminosity distance of approximately 159 Mpc, but extensive optical and gamma-ray follow-up searches across a 90% credible sky area of 5340 deg² yielded no counterpart, despite expectations for a similar to GW170817's. Neutron star-black hole (NSBH) mergers, such as GW200105 and GW200115 detected in January 2020 during , further illustrate the rarity of EM counterparts. These events—with black hole masses of roughly 9 M⊙ (GW200105) and 6 M⊙ (GW200115), masses around 1.9 M⊙ and 1.5 M⊙, and masses of ~2.6 M⊙ and ~2.5 M⊙ respectively—produced no detectable kilonovae or relativistic jets, despite theoretical predictions that off-axis viewing or disrupted accretion might suppress emission in many cases. In the O4 run (2023–2025), the possible NSBH GW230529, observed in May 2023 with a lower-mass component of 1.2–2.0 M⊙ and a lower signal strength (SNR ≈11.6) than GW170817 (SNR=32.4), also lacked an EM counterpart, attributed in part to its large initial sky localization exceeding 24,000 deg² from single-detector trigger. These absences highlight that the structured jet and bright of GW170817, evidenced by its GRB 170817A and multi-wavelength , remain unique among detected compact binary mergers. By the end of O4 in November 2025, approximately 200 GW events were detected, predominantly mergers, with no BNS events identified. Localization capabilities have advanced since GW170817's 28 deg² 90% credible sky area, enabled by the full participation of in O4, which often reduces areas to under 10 deg² for well-localized events, facilitating more efficient EM follow-up compared to the broader uncertainties in O3 detections like GW190425. No other GW event has provided comparable evidence for a structured relativistic jet, as afterglow modeling for candidates like GW200115 showed no structured emission consistent with GW170817's Gaussian jet profile.

Advancements in Multimessenger Astronomy

The detection of GW170817 marked a pivotal advancement in multimessenger astronomy by establishing robust protocols for real-time coordination between gravitational-wave (GW) observatories and electromagnetic (EM) facilities. Following the event, the LIGO and Virgo collaborations implemented low-latency alert systems that disseminated preliminary GW candidate notices via the VOEvent protocol to approximately 100 partner facilities worldwide, enabling rapid EM follow-up within minutes. This infrastructure, refined during the O2 observing run, has since become standard, allowing telescopes to tile localization skies efficiently and capture transients like kilonovae before they fade. Significant upgrades to observational infrastructure were spurred by the need to detect faint, rapidly evolving counterparts to GW events. The , for instance, incorporates wide-field imaging capabilities optimized for rapid tiling of GW localizations, with its Legacy Survey of Space and Time designed to scan large sky areas repeatedly to identify kilonova-like transients. Integration of GW pipelines with EM alert brokers, such as the Astrophysical Multimessenger Observatory Network (AMON), has further streamlined this process by correlating GW triggers with high-energy data streams in real time, facilitating automated candidate vetting and follow-up prioritization. On the theoretical front, GW170817 prompted a shift toward models accommodating off-axis viewing angles for short gamma-ray bursts (sGRBs), as the associated GRB 170817A was underluminous compared to typical on-axis events, necessitating structured jet simulations to explain the observed emission. This has boosted searches for in untargeted surveys, with the (ZTF) enhancing its real-time analysis to detect rapidly evolving blue transients resembling AT2017gfo, the kilonova counterpart to GW170817. By 2025, advancements included AI-driven inference techniques for binary neutron star (BNS) mergers, achieving full parameter estimation—such as sky location and distance—in under one minute using on GW waveforms, dramatically improving multimessenger response times. Overall, GW170817 confirmed BNS mergers as progenitors of sGRBs, reshaping strategies for LIGO-Virgo-KAGRA's O4 and upcoming O5 runs by emphasizing joint GW-EM campaigns to maximize counterpart detections.

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