Radiation astronomy/Gravitationals

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The images show LIGO and Livingston, Louisiana, measurement of gravitational waves. Credit: B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration).{{free media}}
This photo shows the Livingston LIGO detector. Credit: Caltech/MIT/LIGO Laboratory.{{free media}}
This gravitational wave spectrum includes sources and detectors. Credit: NASA Goddard Space Flight Center.{{free media}}

Gravitational radiation appears to be cylindrical waves of radiation produced by relativistic, undulatory gravitational fields in Euclidean space.[1]

Interaction Mediator Relative Magnitude Behavior Range
Strong interaction gluon 1038 1 10−15 m
Electromagnetic interaction photon 1036 1/r2 universal
Weak interaction W and Z bosons 1025 1/r5 to 1/r7 10−16 m
Gravitational interaction photon or graviton ? 10 1/r2 universal

As the gravitational interaction is 10-36 that of the electromagnetic interaction to produce gravitational radiation requires a massive oscillator.

At right are the results from the first gravitational radiation detection. The images show the radiation signals received by the Laser Interferometer Gravitational Observatory (LIGO) instruments at Hanford, Washington (left) and Livingston, Louisiana (right) and comparisons of these signals to the signals expected due to a black hole merger event.

The wavelength of the gravitational waves is given by for example: 3 x 108 m‧s-1/400 Hz = 750,000 m, which is way longer than radio waves but expected for such a weak oscillator. 35 Hz corresponds to 8,600,000 m.

LIGO operates two detectors located 3000 km (1800 miles) apart: One in eastern Washington near Hanford, and the other near Livingston, Louisiana. The photo on the left shows the Livingston detector.

"According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second, the two black holes collide at nearly half the speed of light and form a single, more massive black hole, converting a portion of the combined black holes' mass to energy, according to Einstein's formula E=mc2. This energy is emitted as a final strong burst of gravitational waves. These are the gravitational waves that LIGO observed."[2]

"LIGO’s twin interferometers bounce laser beams between mirrors at the opposite ends of 4-kilometre-long vacuum pipes that are set perpendicularly to each other. A gravitational wave passing through will alter the length of one of the arms, causing the laser beams to shift slightly out of sync."[3]

Later detection confirmed the fusion of two massive stellar-sized objects, a binary neutron star merger.[4]

"According to Einstein's field equations, photon matter subject to quadruple oscillations is a source of gravitational waves."[5]

"In this work, we present a solution to the first stage of a new two-stage global treatment of the vacuum binary black hole problem [1, 2]. The approach, based upon characteristic evolution, has been carried out in the regime of Schwarzschild perturbations where advanced and retarded solutions of the linearized problem can be rigorously identified [3]. Computational experiments are necessary to study the applicability of the approach to the nonlinear regime. From a time-reversed viewpoint, this first stage is equivalent to the determination of the outgoing radiation emitted from the fission of a white hole in the absence of ingoing radiation. This provides the physically correct “retarded” waveform for a white hole fission, were such events to occur in the universe. Although there is no standard astrophysical mechanism for producing white holes from a nonsingular matter distribution, white holes of primordial or quantum gravitational origin cannot be ruled out."[6]

"This fission problem has a simpler formulation as a characteristic initial value problem than the black hole merger problem. The boundary of the (conformally compactified) exterior spacetime contains two null hypersurfaces where boundary conditions must be satisfied: past null infinity I−, where the incoming radiation must vanish, and the white hole event horizon H−, which must describe a white hole, which is initially in equilibrium with no ingoing radiation and then distorts and ultimately fissions into two white holes with the emission of outgoing gravitational waves."[6]

An almost identical signal could originate from a comparable much more massive neutron star fission.

"This is an exciting time to study gravitation, astrophysics and cosmology. Through challenging cosmic microwave background (CMB) and supernovae observations cosmology has been turned on its head. Gravitational radiation astronomy should be the next contributor to this revolution in astrophysics and cosmology."[7]

Neutrinos[edit | edit source]

"New approaches to coherent interaction processes are presented, for the weak and the gravitational interactions. Very large cross sections appear possible. These developments provide new foundations for neutrino and gravitational radiation astronomy."[8]

Superluminals[edit | edit source]

Proper motion of the radio counterpart of GW170817 is displayed. Credit: K. P. Mooley, A. T. Deller, O. Gottlieb, E. Nakar, G. Hallinan, S. Bourke, D. A. Frail, A. Horesh, A. Corsi & K. Hotokezaka.{{fairuse}}

"The binary neutron-star merger GW1708171 was accompanied by radiation across the electromagnetic spectrum2 and localized2 to the galaxy NGC 4993 at a distance3 of about 41 megaparsecs from Earth. The radio and X-ray afterglows of GW170817 exhibited delayed onset4–7, a gradual increase8 in the emission with time (proportional to t0.8) to a peak about 150 days after the merger event8, followed by a relatively rapid decline9,10."[9]

The "compact radio source associated with GW170817 exhibits superluminal apparent motion between 75 days and 230 days after the merger event. This measurement breaks the degeneracy between the choked- and successful-jet cocoon models and indicates that, although the early-time radio emission was powered by a wide-angle outflow8 (a cocoon), the late-time emission was most probably dominated by an energetic and narrowly collimated jet (with an opening angle of less than five degrees) and observed from a viewing angle of about 20 degrees. The imaging of a collimated relativistic outflow emerging from GW170817 adds substantial weight to the evidence linking binary neutron-star mergers and short γ-ray bursts."[9]

Very "long-baseline interferometry (VLBI) observations with the High Sensitivity Array (HSA)—which consists of the Very Long Baseline Array (VLBA), the Karl G. Jansky Very Large Array (VLA) and the Robert C. Byrd Green Bank Telescope (GBT)—75 and 230 days after the GW170817 merger event [...] indicate that the centroid position of the radio counterpart of GW170817 changed from a right ascension of RA = 13 h 09 min 48.068638(8) s and declination of dec. = −23° 22′ 53.3909(4)′′ to RA = 13 h 09 m in 48.068831(11) s and dec. = −23° 22′ 53.3907(4)′′ between these epochs (1σ uncertainties in the last digits are given in parentheses). This implies an positional offset between the two observations of 2.67±0.19±0.21 mas in RA and 0.2±0.6±0.7 mas in dec. (1σ uncertainties; statistical and systematic, respectively; [...]). This corresponds to a mean apparent velocity of the source of the radio counterpart along the plane of the sky of βapp = 4.1 ± 0.5, where βapp is in units of the speed of light, c (1σ, including the uncertainty in the source distance). [...] Our VLBI data are consistent with the source being unresolved at both day 75 and day 230. Given the VLBI angular resolution and the signal-to-noise ratio of the detection, this puts an upper limit on the size of the source in both epochs of about 1 mas (0.2 pc at the distance of NGC 4993) in the direction parallel to its motion and 10 mas perpendicular to its motion [...]."[9]

"Although superluminal motion is seen frequently in active galactic nuclei and micro-quasars, it is extremely rare in extragalactic explosive transients. Superluminal motion has been measured in only one such transient: the long-duration γ-ray burst GRB 03032924. GRB 030329 had a measured superluminal expansion of βapp ≈ 3–5, but no proper motion, whereas GW170817 has measured proper motion, but no expansion. Although both were relativistic events of comparable energies, these differences suggest different geometries and/or viewing angles."[9]

Pulsars[edit | edit source]

As the pulsar picks up speed through accretion, it becomes distorted from a perfect sphere due to subtle changes in the crust, depicted here by an equatorial bulge. Credit: Dana Berry/NASA Goddard Space Flight Center.{{free media}}
A bizarre stellar pair consists of the most massive neutron star confirmed so far, orbited by a white dwarf star. Credit: ESO/L. Calçada.{{free media}}

"Gravitational radiation, ripples in the fabric of space predicted by Albert Einstein, may serve as a cosmic traffic enforcer, protecting reckless pulsars from spinning too fast and blowing apart, according to a report published in the July 3 issue of Nature. Pulsars, the fastest spinning stars in the Universe, are the core remains of exploded stars, containing the mass of our Sun compressed into a sphere about 10 miles across. Some pulsars gain speed by pulling in gas from a neighboring star, reaching spin rates of nearly one revolution per millisecond, or almost 20 percent light speed. These "millisecond" pulsars would fly apart if they gained much more speed."[10]

"Using NASA's Rossi X-ray Timing Explorer, scientists have found a limit to how fast a pulsar spins and speculate that the cause is gravitational radiation: The faster a pulsar spins, the more gravitational radiation it might release, as its exquisite spherical shape becomes slightly deformed. This may restrain the pulsar's rotation and save it from obliteration."[10]

"Nature has set a speed limit for pulsar spins. Just like cars speeding on a highway, the fastest-spinning pulsars could technically go twice as fast, but something stops them before they break apart. It may be gravitational radiation that prevents pulsars from destroying themselves."[11]

"Gravitational waves, analogous to waves upon an ocean, are ripples in four-dimensional spacetime. These exotic waves, predicted by Einstein's theory of relativity, are produced by massive objects in motion."[10]

"Created in a star explosion, a pulsar is born spinning, perhaps 30 times per second, and slows down over millions of years. Yet if the dense pulsar, with its strong gravitational potential, is in a binary system, it can pull in material from its companion star. This influx can spin up the pulsar to the millisecond range, rotating hundreds of times per second."[10]

"In some pulsars, the accumulating material on the surface occasionally is consumed in a massive thermonuclear explosion, emitting a burst of X-ray light lasting only a few seconds. In this fury lies a brief opportunity to measure the spin of otherwise faint pulsars. Scientists report in Nature that a type of flickering found in these X-ray bursts, called "burst oscillations," serves as a direct measure of the pulsar's spin rate. Studying the burst oscillations from 11 pulsars, they found none spinning faster than 619 times per second."[10]

"The Rossi Explorer is capable of detecting pulsars spinning as fast as 4,000 times per second. Pulsar break-up is predicted to occur at 1,000 to 3,000 revolutions per second. Yet scientists have found none that fast. From statistical analysis of 11 pulsars, they concluded that the maximum speed seen in nature must be below 760 revolutions per second."[10]

"This observation supports the theory of a feedback mechanism involving gravitational radiation limiting pulsar speeds. As the pulsar picks up speed through accretion, any slight distortion in the star's dense, half-mile-thick crust of crystalline metal will allow the pulsar to radiate gravitational waves. (Envision a spinning, oblong rugby ball in water, which would cause more ripples than a spinning, spherical basketball.) An equilibrium rotation rate is eventually reached where the angular momentum shed by emitting gravitational radiation matches the angular momentum being added to the pulsar by its companion star."[12]

"Accreting millisecond pulsars could eventually be studied in greater detail in an entirely new way, through the direct detection of their gravitational radiation. LIGO, the Laser Interferometer Gravitational-Wave Observatory now in operation in Hanford, Washington, and in Livingston, Louisiana, will eventually be tunable to the frequency at which millisecond pulsars are expected to emit gravitational waves."[12]

"The waves are subtle, altering spacetime and the distance between objects as far apart as the Earth and the Moon by much less than the width of an atom. As such, gravitational radiation has not been directly detected yet. We hope to change that soon."[13]

For the image second down on the right: "This artist’s impression shows the exotic double object that consists of a tiny, but very heavy neutron star that spins 25 times each second (right), orbited every two and a half hours by a white dwarf star (left). The neutron star is a pulsar named PSR J0348+0432 that is giving off radio waves that can be picked up on Earth by radio telescopes. Although this unusual pair is very interesting in its own right it is also a unique laboratory for testing the limits of physical theories."[14]

Black holes[edit | edit source]

Frame is from a simulation of the merger of two black holes and the resulting emission of gravitational radiation (colored fields). Credit: NASA/Ames Research Center/Christopher E. Henze.{{free media}}
Numerical simulations are of the gravitational waves emitted by the inspiral and merger of two black holes. Credit: NASA/Ames Research Center/C. Henze.{{free media}}

"According to Einstein, whenever massive objects interact, they produce gravitational waves — distortions in the very fabric of space and time — that ripple outward across the universe at the speed of light. While astronomers have found indirect evidence of these disturbances, the waves have so far eluded direct detection. Ground-based observatories designed to find them are on the verge of achieving greater sensitivities, and many scientists think that this discovery is just a few years away."[15]

"Catching gravitational waves from some of the strongest sources — colliding black holes with millions of times the sun's mass — will take a little longer. These waves undulate so slowly that they won't be detectable by ground-based facilities. Instead, scientists will need much larger space-based instruments, such as the proposed Laser Interferometer Space Antenna, which was endorsed as a high-priority future project by the astronomical community."[15]

"In the turbulent environment near the merging black holes, the magnetic field intensifies as it becomes twisted and compressed. [...] The most interesting outcome of the magnetic simulation is the development of a funnel-like structure — a cleared-out zone that extends up out of the accretion disk near the merged black hole. The most important aspect of the study is the brightness of the merger's flash. The team finds that the magnetic model produces beamed emission that is some 10,000 times brighter than those seen in previous studies, which took the simplifying step of ignoring plasma effects in the merging disks."[15]

In the image on the left: "Numerical simulations of the gravitational waves emitted by the inspiral and merger of two black holes. The colored contours around each black hole represent the amplitude of the gravitational radiation; the blue lines represent the orbits of the black holes and the green arrows represent their spins."[16]

Gravitational wave events[edit | edit source]

List of binary merger events[17][18]
GW event  Detection
time (UTC)
Chirp mass (M)[22] Effective spin The dimensionless effective inspiral spin parameter is:


where is the mass of a black hole, is its spin, and is the angle between the orbital angular momentum and a merging black hole's spin (ranging from when aligned to when antialigned). It is the mass-weighted linear combination of the components of the black holes' spins aligned with the orbital axis[23][18] and has values ranging from −1 to 1 (the extremes correspond to situations with both black hole spins exactly antialigned and aligned, respectively, with orbital angular momentum).[24] This is the spin parameter most relevant to the evolution of the inspiral gravitational waveform, and it can be measured more accurately than those of the premerger BHs.[25]}}

Primary Secondary Remnant Notes Ref
Type Mass (M) Type Mass (M) Type Mass (M) Spin Values of the dimensionless spin parameter cJ/GM2 for a black hole range from zero to a maximum of one. The macroscopic properties of an isolated astrophysical (uncharged) black hole are fully determined by its mass and spin. Values for other objects can potentially exceed one. The largest value known for a neutron star is ≤ 0.4, and commonly used equations of state would limit that value to < 0.7.[26]
GW150914 2015-09-14
2016-02-11 179; mostly to the south BH
Spin estimate is .[27]
Spin estimate is .[27]
BH First GW detection; first BH merger observed [28][29][27]
GW151012 [fr] 2015-10-12
2016-06-15 1555 BH BH BH Formerly candidate LVT151012; accepted as astrophysical since February 2019 [30][18][17]
GW151226 2015-12-26
2016-06-15 1033 BH BH BH [31][32]
GW170104 2017-01-04
2017-06-01 924 BH BH BH [23][33]
GW170608 2017-06-08
2017-11-16 396; to the north BH BH BH Smallest BH progenitor masses to date [34]
GW170729 2017-07-29
2018-11-30 1033 BH BH BH Largest progenitor masses and greatest spin to date [18]
GW170809 2017-08-09
2018-11-30 340; towards Cetus BH BH BH [18]
GW170814 2017-08-14
2017-09-27 87; towards Eridanus BH BH BH First announced detection by three observatories; first polarization measurement [35][36]
GW170817 2017-08-17
2017-10-16 16; NGC 4993 40±10 ≥ 0.04 NS NS NS
Based on a descending spin-down chirp observed in GW post-merger, a magnetar was produced that survived at least 5 seconds.[37]
≤ 2.8 Besides the loss of mass due to GW emission that occurred during the merger, the event is thought to have ejected 0.05±0.02 solar mass of material.[38] ≤ 0.89 First NS merger observed in GW; first detection of EM counterpart (GRB 170817A; Astronomical transient (AT) 2017gfo); nearest event to date [26][39][40]
GW170818 2017-08-18
2018-11-30 39; towards Pegasus BH BH BH [18]
GW170823 2017-08-23
2018-11-30 1651 1850±840 BH BH BH [18]
Gravitational Wave Transient Catalog 1. Credit:LIGO Scientific Collaboration and Virgo Collaboration/Georgia Tech/S. Ghonge & K. Jani
O1 & O2 events are from LIGO & Virgo. Credit: Spto.
O3 Superevents are from LIGO & Virgo. Credit: Spto.

Observation candidates from O3/2019[edit | edit source]

From the observation run O3/2019 on, observations are published as Open Public Alerts to facilitate multi-messenger observations of events.[41][42][43] Candidate event records can be directly accessed at the Gravitational Wave Candidate Event Database.[44] On 1 April 2019, the start of the third observation run was announced with a circular published in the public alerts tracker.[45] The first O3/2019 binary black hole detection alert was broadcast on 8 April 2019. A significant percentage of O3 candidate events detected by LIGO are accompanied by corresponding triggers at Virgo. False alarm rates are mixed, with more than half of events assigned false alarm rates greater than 1 per 20 years, contingent on presence of glitches around signal, foreground electromagnetic instability, seismic activity, and operational status of any one of the three LIGO-Virgo instruments. For instance, events S190421ar and S190425z weren’t detected by Virgo and LIGO’s Hanford site, respectively.

List of O3 event alerts[17][18]
GW event  Detection
time (UTC)
Detector[48] False Alarm
Rate (Hz)
Classification Notes Ref
BNS[49] NSBH[50] BBH[51] MassGap[52] Terrestrial[53]
S190408an 2019-04-08
387; towards Pegasus or Lacerta 1473±358 H,L,V 2.8 10−18 0.0 0.0 0.99999999999 0.0 9.8 10−12 [54][55]
S190412m 2019-04-12
156; towards Virgo or Boötes 812±194 H,L,V 1.7 10−27 0.0 0.0 1.0 0.0 1.7 10−20 [56]
S190421ar 2019-04-21
1444 1628±535 H,L 1.5 10−8 0.0 0.0 0.97 0.0 0.03 Initially marked with 96% chance of being terrestrial noise, but later upgraded to 97% chance of being a binary black hole merger. [57]
S190425z 2019-04-25
7461 156±41 L,V 4.5 10−13 0.9994 0.0 0.0 0.0 0.0006 [58][59]
S190426c 2019-04-26
1131 377±100 H,L,V 1.9 10−8 0.129 0.516 0.0 0.215 0.140 Initially marked with 49% chance of being binary neutron star merger, 13% neutron star-black hole merger, 24% mass gap merger. [60][61]


S190503bf 2019-05-03
448; towards Columba, Pictor, or Puppis 421±105 H,L,V 1.6 10−9 0.0 0.0047 0.9628 0.0323 0.0001 [63]
S190510g 2019-05-10
1166; towards Columba or Canis Major 227±92 H,L,V 8.8 10−9 0.42 0.0 0.0 0.0 0.58 Initially reported with a 2% chance of being noise, later downgraded to a 58% chance of being noise. [64]
S190512at 2019-05-12
399; towards Scorpius or Ophiuchus 1331±341 H,L,V 1.9 10−9 0.0 0.0 0.99 0.0 0.01 [65]
S190513bm 2019-05-13
691; towards Sagittarius, Capricornus, Perseus, or Camelopardalis 1987±501 H,L,V 3.7 10−13 0.0 0.005 0.943 0.052 6 10−8 [66]
S190517h 2019-05-17
939 2950±1038 H,L,V 2.4 10−9 0.0 0.00077 0.98255 0.01664 0.00004 [67]
S190519bj 2019-05-19
967 3154±791 H,L,V 5.8 10−9 0.0 0.0 0.96 0.0 0.04 [68]
S190521g 2019-05-21
765; towards Coma Berenices, Canes Venatici, or Phoenix 3931±953 H,L,V 3.8 10−9 0.0 0.0 0.97 0.0 0.03 [69]
S190521r 2019-05-21
488 1136±279 H,L 3.2 10−10 0.0 0.0 0.9993 0.0 0.0007 [70]
S190602aq 2019-06-02
1172 797±238 H,L,V 1.9 10−9 0.0 0.0 0.99 0.0 0.01 [71]
S190630ag 2019-06-30
8493 1059±307 L,V 1.3 10−13 0.0 0.005 0.943 0.052 1.8 10−7 [72]
S190701ah 2019-07-01 20:33:45 UTC 67; towards Eridanus or Cetus 1045±254 H,L,V 1.9 10−8 0.0 0.0 0.93 0.0 0.07 [73]
S190706ai 2019-07-06 22:26:57 UTC 1100 5725±1446 H,L,V 1.9 10−9 0.0 0.0 0.99 0.0 0.01 [74]
S190707q 2019-07-07 09:33:44 UTC 1375 810±234 H,L 5.3 10−12 0.0 0.0 0.99999 0.0 0.00001 [75]
S190720a 2019-07-20 00:08:36 UTC 1461 1071±323 H,L 3.8 10-9 0.0 0.0 0.989 0.0 0.011 Initially reported with a 71% chance of being noise, upgraded to 1% after a signal from the Virgo detector was found to be erroneous. [76]

GW170817[edit | edit source]

Time-frequency representations are of data containing the gravitational-wave event GW170817. Credit: LIGO Scientific Collaboration and Virgo Collaboration.{{free media}}
Hubble picture of NGC 4993 with inset showing GRB 170817A over 6 days. Credit: NASA and ESA.{{free media}}
This plot shows how the brightness of the kilonova GW170817 seen in the galaxy NGC 4993 changed when measured through different colour filters. Credit: Tanvir et al.{{free media}}
This montage of spectra taken using the X-shooter instrument on ESO's Very Large Telescope shows the changing behaviour of the kilonova AT 2017gfo. Credit: ESO/E. Pian et al./S. Smartt & ePESSTO.{{free media}}

At right, time-frequency representations of data containing the gravitational-wave event GW170817, were observed by the LIGO-Hanford (top), LIGO-Livingston (middle), and Virgo (bottom) detectors. Times are shown relative to August 17, 2017 12∶41:04 UTC. The amplitude scale in each detector is normalized to that detector’s noise amplitude spectral density.

GW170817, a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 17 August 2017, 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.[77][78] Unlike the five previous GW detections, which were of merging black holes not expected to produce a detectable electromagnetic signal, "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."[79]

"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."[80][81]

Because "colliding black holes don’t give off any light, you wouldn’t expect any optical counterpart."[80]

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.[80]

"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."[82]

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.[77][83][84][85][86]

The gravitational wave signal, designated GW170817, 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 Gamma-ray Space Telescope and INTEGRAL spacecraft beginning 1.7 seconds after the GW merger signal.[77][87][88]

An astronomical transient designated AT 2017gfo (originally, SSS17a) was found, 11 hours after the gravitational wave signal, in the galaxy NGC 4993[40] 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.

GRB 150101B, a gamma-ray burst event detected in 2015, may be analogous to GW170817 as 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, where kilonova events may be more diverse and common in the universe than previously understood, according to the researchers.[89][90][91][92]

"It’s the first time that we’ve observed a cataclysmic astrophysical event in both gravitational waves and electromagnetic waves — our cosmic messengers."[93]

The gravitational wave signal lasted for approximately 100 seconds starting from a frequency of 24 hertz covering 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.[78]: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, where the signal was detected and analyzed by a comparison with a prediction from general relativity defined from the post-Newtonian expansion.[77]:3

An automatic computer search of the LIGO-Hanford datastream triggered an alert to the LIGO team about 6 minutes after the event, whereas, the gamma-ray alert had already been issued (16 sec post-event),[94] 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.[95][96]

Sky localisation of the event requires combining data from the three interferometers: 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 persisted parallel to the rising transient signal in the lowest frequencies, requiring manual analysis and interpolation before the sky location could be announced about 4.5 hours post-event.[97][96] The three detections localized the source to an area of 31 square degrees in the southern sky at 90% probability, where more detailed calculations later refined the localization to within 28 square degrees.[95][78] 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.[98]

The first electromagnetic signal detected was GRB 170817A, a short gamma ray burst, detected 1.74±0.05 seconds after the merger time and lasting for about 2 seconds.[88][99][77]

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.[40][100]

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.[95] 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.[96]

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.[77] 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[88][77][101] 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.[100][101] By identifying the host galaxy of the merger, it is possible to provide an accurate distance consistent with that based on gravitational waves alone.[77]

The third image down on the right contains plots that show "how the brightness of the kilonova seen in the galaxy NGC 4993 changed when measured through different colour filters. In blue light the object faded rapidly, but at longer wavelengths, in the near infrared part of the spectrum, it brightened a little and then faded much more slowly. As a result this object changed colour from very blue to very red over the period of four weeks."[102]

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.[101] Over the following days, the color of the optical source changed from blue to red as the source expanded and cooled.[100]

In the fourth image down on the right is a "montage of spectra taken using the X-shooter instrument on ESO's Very Large Telescope shows the changing behaviour of the kilonova AT 2017gfo in the galaxy NGC 4993 over a period of 12 days after the explosion (GW170817) was detected on 17 August 2017. Each spectrum covers a range of wavelengths from the near-ultraviolet to the near-infrared and reveals how the object became dramatically redder as it faded."[103]

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.[77] 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.[40] More than 70 observatories covering the electromagnetic spectrum observed the source.[40]

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.[77]

On 9 December 2017, astronomers reported a brightening of X-ray emissions from GW170817/GRB 170817A/SSS17a.[104][105]

On 8 May 2018, researchers reported the first statistically significant decaying of X-ray emissions from GW170817.[106]

On 9 August 2018, astronomers reported a comparison of the X-ray light curve plateau of XMM-Newton and Chandra observations of GW170817, noting consistency at about 162 (XMM-Newton) and 159.7 (Chandra) days after the neutron star merger.[107]

On 13 August 2018, astronomers at the Chandra X-ray Observatory reported that the X-ray afterglow from the neutron star merger associated with GW170817 is fading at an increasingly rapid rate at 358.6 days after the event.[108]


  1. RA 13h 09m 48.08s (48.085±0.018)"[77]
  2. Dec −23° 22′ 53.3″ (53.343±0.218)"[77]
  3. epoch J2000.0
  4. distance 40 megaparsecs (130 Mly)

GW150914[edit | edit source]

The first direct observation of gravitational waves was made on 14 September 2015 and was announced by the LIGO and Virgo collaborations on 11 February 2016.[109][110][111]

The waveform, detected by both LIGO observatories,[112] matched the predictions of general relativity[113][114][115] for a gravitational wave emanating from the inward spiral and merger of a pair of black holes of around 36 and 29 solar masses and the subsequent "ringdown" of the single resulting black hole.

Technology[edit | edit source]

The LIGO Livingston control room is as it was during Advanced LIGO's first observing run (O1). This panoramic was sewn together from several images. Credit: Amber Stuver.{{free media}}

"The part which is of nongravitational radiation origin will provide information to improve the isolation of existing antennas. Only when the nature of the backgrounds is understood will it be possible to employ the antennas for gravitational radiation astronomy."[116]

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale observatory to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool.[117] Two large observatories: one at Livingston, Louisiana, and the other at Hanford, Washington, were built in the United States with the aim of detecting gravitational waves by laser interferometry which can detect a change in the 4 km mirror spacing of less than a ten-thousandth the charge diameter of a proton.[118]

"This is equivalent to measuring the distance from Earth to the nearest star (that is, to Proxima Centauri at 4.0208×1013 km to an accuracy smaller than the width of a human hair!"[118]

As of December 2018, LIGO has made eleven detections of gravitational waves, of which ten are from binary black hole mergers, and the other event GW170817 was the first detection of a collision of two neutron stars, on 17 August 2017 which simultaneously produced optical signals detectable by conventional telescopes, where all eleven events were observed in data from the first and second observing runs of Advanced LIGO.[119]

Virgo interferometers[edit | edit source]

Aerial view of the site of the Virgo experiment shows the central building, the Mode-Cleaner building, the full 3 km-long west arm and the beginning of the north arm (on the right). The other buildings include offices, workshops, the local computing center and the interferometer control room. When this picture was shot, the building hosting the project management and the canteen had not been built yet. Credit: The Virgo collaboration.
Basic scheme of a gravitational wave suspended interferometric detector like Virgo. Credit: Narnaud1974.
A sensitivity curve from the Virgo detector in the frequency band [10 Hz; 10 kHz], computed in August 2011[120]. Its shape is typical: the thermal noise of the mirror suspension pendulum mode dominates at low frequency while the increase at high frequency is due to the laser shot noise. In between these two frequency bands and superimposed to these fundamental noises, one can see resonances (for instance the suspension wire violin modes) such as contributions from various instrumental noises (among which the 50 Hz frequency from the power grid and its harmonics) which one is trying to reduce continuously. Credit: The Virgo collaboration.
Optical configuration of the first generation Virgo detector. On the schematics one can read the level of magnitude of the power stored in the various cavities. Credit: The Virgo collaboration.
Any Virgo mirror is supported, under vacuum, by a mechanical structure enormously damping seismic vibrations. A "Superattenuator" consists of a chain of pendula, hanging from an upper platform, supported by three long flexible legs clamped to ground, technically called inverted pendulum. In this way seismic vibrations above 10 Hz are reduced by more than 1012 times and the position of the mirror is very carefully controlled. Credit: The Virgo collaboration.

Since 2007, Virgo and LIGO have agreed to share and jointly analyze the data recorded by their detectors and to jointly publish their results.[121]

The first goal of Virgo is to directly observe gravitational waves, a straightforward prediction of Albert Einstein's general relativity.[122]

The study over three decades of the PSR B1913+16 (binary pulsar 1913+16) led to indirect evidence of the existence of gravitational waves. The observed evolution over time of this binary pulsar's orbital period is in excellent agreement with the hypothesis that the system is losing energy by emitting gravitational waves.[123]

A gravitational wave is a space-time perturbation which propagates at the speed of light. It then curves slightly the space-time, which changes locally the light path. Mathematically speaking, if is the amplitude (assumed to be small) of the incoming gravitational wave and the length of the optical cavity in which the light is in circulation, the change of the optical path due to the gravitational wave is given by the formula:[124]


with being a geometrical factor which depends on the relative orientation between the cavity and the direction of propagation of the incoming gravitational wave.

The signal induced by a potential gravitational wave is thus "embedded" in the light intensity variations detected at the interferometer output.[125] Several external causes—globally denoted as noises—changes the interference pattern perpetually and significantly so the design of detectors like Virgo and LIGO thus requires a detailed inventory of all noise sources which could impact the measurement, allowing a strong and continuing effort to reduce them as much as possible.[126][127]

The sensitivity varies with frequency as each noise has its own frequency range. For instance, it is foreseen that the sensitivity of the advanced Virgo detector be ultimately limited by:[127]

  • seismic noise (any ground motion whose sources are numerous: waves in the Mediterranean sea, wind, human activity for instance the traffic during daytime, etc.) in the low frequencies up to about 10 Hertz (Hz);
  • the thermal noise of the mirrors and their suspension wires, from a few tens of Hz up to a few hundreds;
  • the laser shot noise above a few hundreds of Hz.

Using an interferometer rather than a single optical cavity allows one to enhance significantly the sensitivity of the detector to gravitational waves.[128]

The mirror positions relative to a reference and their alignment are monitored accurately in real time[129] with a precision better than the tenth of a nanometre for the lengths;[127] at the level of a few nanoradians for the angles.

Reaching that working point from an initial configuration in which the various mirrors are moving freely is a control system challenge.[130]

The main components of the detector are the following:

  1. The laser is the light source of the experiment, powerful, while extremely stable in frequency as well as in amplitude.[131] To meet all these specifications which are somewhat opposing, the beam starts from a very low power, yet very stable, laser.[132]
  2. The large mirrors of the arm cavities are the most critical optics of the interferometer, making a resonant optical cavity in each arm and allowing an increase in power of the light stored in the 3-km arms; they are cylinders 35 cm in diameter and 20 cm thick,[133] made from the purest glass in the world.[134] The mirrors are polished to the atomic level in order to not diffuse (and hence lose) any light.[135] Finally, a reflective coating (a Bragg reflector made with ion beam sputtering, or IBS) is added, with the mirrors located at the end of the arms reflect all incoming light; less than 0.002% of the light is lost at each reflection.[136]
  3. All of the main mirrors are suspended by four thin fibers made of silica[137] (hence in glass) which are attached to a series of attenuators. This chain of suspension, called the 'superattenuator', is close to 10 meters high and is also under vacuum.[138]
  4. The largest ultra-high vacuum installation in Europe, with a total volume of 6,800 cubic meters.[139]

History[edit | edit source]

"Some of the experimental scientists may have felt they were testing a global theory some of the time, but sometimes they were also “getting a foot-hold” in the early stages of gravitational radiation astronomy, developing their instrumentation (detectors were built at Hughes Aircraft, IBM and Bell Labs), keeping up with the physics, trying to kill off an irritating nuisance who was making too much noise about some unbelievable results (see SO7S [Collins 1981a]), confirming the existence of gravity waves in the face of mindless criticism and much more besides."[140]

See also[edit | edit source]

References[edit | edit source]

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  2. Ivy F. Kupec (11 February 2016). Gravitational waves detected 100 years after Einstein's prediction. Alexandria, Virginia, USA: National Science Foundation. pp. 1. https://www.nsf.gov/news/news_summ.jsp?cntn_id=137628. Retrieved 3 January 2018. 
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  19. The area of the sky within which it was possible to localize the source.
  20. 1 Mpc is approximately 3.26 Mly.
  21. c2M is about 1.8×103 foe; 1.8×1047 J; 1.8×1054 erg; 4.3×1046 cal; 1.7×1044 BTU; 5.0×1040 kWh, or 4.3×1037 tonnes of TNT.
  22. The chirp mass is the binary parameter most relevant to the evolution of the inspiral gravitational waveform, and thus is the mass that can be measured most accurately. It is related to, but less than, the geometric mean of the binary masses, according to , thus ranging from ~87% of when the masses are the same to ~78% when they differ by an order of magnitude.
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  47. 1 Mpc is approximately 3.26 Mly.
  48. Which instruments observed the event. (H = LIGO Hanford, L=LIGO Livingston, V=Virgo)
  49. Both components have mass < 3 M.
  50. One components has mass > 5 M, the other has mass < 3 M.
  51. Both components have mass > 5 M.
  52. At least one component has mass in the range 3-5 M.
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