Today, at several simultaneous press conferences, scientists from the LIGO and Virgo gravitational observatories, as well as from other scientific institutions around the world, announced that in August of this year they were able to detect gravitational waves generated by the merger of two neutron stars for the first time. Previously, gravitational waves were noted by physicists four times, but in all cases they were generated by the merger of two black holes, not neutron stars.
© ESO/L. Calçada/M. Kornmesser
Moreover, for the first time in history, an event that caused gravitational waves was noted not only by gravitational interferometer detectors, but also observed by space and ground-based telescopes in various ranges (X-ray, ultraviolet, visible, infrared and radio). The discovery will not only enable the next step in the study of gravitational waves and gravity, but will also provide significant advances in the study of neutron stars. In particular, it confirms the hypothesis of the synthesis of heavy elements in the process of merging neutron stars and the nature of gamma-ray bursts. The discovery is described in a number of papers published in Nature, Nature Astronomy, Physical Review Letters and Astrophysical Journal Letters.
Gravitational waves are generated by any object that has mass and moves with uneven acceleration, but strong enough waves that can be detected using human-made devices are born during the interaction of objects of very large mass: black holes, components of binary stars, neutron stars. The current wave, designated GW170817, was detected by both detectors at the LIGO gravitational observatory in the US and the Virgo detector in Italy on August 17 of this year.
The presence of three detectors located at different points on the Earth allows scientists to approximately determine the position of the wave source. Two seconds after the gravitational observatories recorded the wave GW170817, a gamma-ray flash was noted in the area where its source should be located. This was done by the space gamma-ray telescopes Fermi (Fermi Gamma-ray Space Telescope) and INTEGRAL (INTERnational Gamma Ray Astrophysics Laboratory). After that, many ground and space observatories began to look for a possible source of these events. The area of the search area, determined from the data of gravitational observatories and gamma-ray telescopes, was quite large, amounting to about 35 square degrees, several hundred full lunar disks would fit in such a section of the sky, and the number of stars located on it is several million. But they still managed to find the source of the gravitational wave and the gamma-ray burst.
Eleven hours after the gamma-ray burst, the Swope reflecting telescope operating at the Las Campanas Observatory in Chile was the first to do this. After that, several large telescopes immediately interrupted their previously approved programs of their observations and switched to observing the small galaxy NGC 4993 in the constellation Hydra, at a distance of 40 parsecs from the solar system (about 130 million light years). This event caused the first rumors about the discovery, but scientists did not officially confirm anything until today's press conferences.
Indeed, the source of the waves and gamma rays was a star located near the galaxy NGC 4993. This star was monitored for several weeks by the Pan-STARRS and Subaru telescopes in Hawaii, the Very Large Telescope of the European Southern Observatory (VLT ESO), the New Technology Telescope (NTT), VLT Survey Telescope (VST), 2.2-meter MPG / ESO telescope, array of telescopes ALMA (Atacama Large Millimeter / submillimeter Array) - in total, about seventy observatories from around the world participated in the observations, as well as the Hubble Space Telescope. “It rarely happens that a scientist has the opportunity to witness the beginning of a new era in science,” astronomer Elena Pian of the Italian Astrophysical Institute INAF quoted an ESO press release as saying. “This is one of those cases!” Astronomers had little time, since the galaxy NGC 4993 was available for observation only in the evening in August, in September it turned out to be too close to the Sun in the sky and became unobservable.
The observed star was initially very bright, but during the first five days of observations, its brightness decreased by a factor of twenty. This star is located at the same distance from us as the galaxy NGC 4993 - 130 million light years. This means that the gravitational wave GW170817 originated at a record distance close to us. Calculations showed that the source of the gravitational wave was the merger of objects whose masses are from 1.1 to 1.6 solar masses, which means that they could not be black holes. So neutron stars became the only possible explanation.
Composite image of NGC 4993
and kilonova according to many ESO instruments
© ESO
The generation of gravitational waves by neutron stars occurs according to the same scenario as during the merger of black holes, only the waves generated by neutron stars are weaker. Rotating around a common center of gravity in a binary system, two neutron stars lose energy by emitting gravitational waves. Therefore, they gradually approach each other until they merge into one neutron star (there is a possibility that a black hole may also appear during the merger). The merger of two neutron stars is accompanied by a flash much brighter than a normal new star. Astronomers propose the name "kilon" for it. During the merger, part of the mass of two stars is converted into the energy of gravitational waves, which were noticed this time by earthly scientists.
Although kilon stars were predicted over 30 years ago, this is the first time such a star has been discovered. Its characteristics, determined as a result of observations, are in good agreement with previous predictions. As a result of the merger of two neutron stars and the explosion of a kilonova, radioactive heavy chemical elements are released, flying apart at a speed of one fifth of the speed of light. Within a few days - faster than any other stellar explosion - the color of the kilonova changes from bright blue to red. “When the spectrum of the object appeared on our monitors, I realized that this is the most unusual transient phenomenon that I have ever seen,” says Stephen Smartt, who made observations with the ESO NTT telescope. “I have never seen anything like it. Our data, as well as data from other research groups, clearly show that this was not a supernova or a background variable star, but something completely unusual.”
The emission spectra of the star show the presence of cesium and tellurium, ejected into space during the merger of neutron stars. This observation confirmed the theory of r-nucleosynthesis (r-process, fast neutron capture process) formulated earlier by astrophysicists in the interiors of superdense stellar objects. The chemical elements formed during the merger of neutron stars dispersed into space after the explosion of the kilonova.
Another theory of astronomers has also been confirmed, according to which short gamma-ray bursts occur during the merger of neutron stars. This idea has been expressed for a long time, but only the combination of data from the gravitational observatories LIGO and Virgo with the observations of astronomers made it possible to finally verify its correctness.
“So far, the data we have received is in excellent agreement with the theory. This is a triumph for theorists, confirmation of the absolute reality of the events recorded by the LIGO-VIRGO facilities, and a remarkable achievement by ESO, which managed to obtain such observations of the kilonova”, says astronomer Stefano Covino.
The LIGO-Virgo collaboration, together with astronomers from 70 observatories, announced today the observation of the merger of two neutron stars in the gravitational and electromagnetic ranges: they saw a gamma-ray burst, as well as X-ray, ultraviolet, visible, infrared and radio radiation.
An illustration of a neutron star collision. A narrow diagonal ejection is a stream of gamma rays. The glowing cloud around the stars is the source of visible light observed by telescopes after the merger. Credit: NSF/LIGO/Sonoma State University/Aurore Simonnet
The joint observation of a gamma-ray burst, gravitational waves and visible light made it possible to determine not only the region in the sky where the event occurred, but also the galaxy NGC 4993, to which the stars belonged.
Determining the location in the sky with different detectors
What can we say about neutron stars?
Astronomers have observed short bursts of gamma rays for many decades, but did not know exactly how they occur. The main assumption was that this burst is the result of a neutron star merger, and now the observation of gravitational waves from this event has confirmed the theory.When neutron stars collide, most of their matter merges into one supermassive object, emitting a "fireball" of gamma rays (the shortest gamma-ray burst recorded two seconds after gravitational waves). After that, the so-called kilonova occurs, when the matter left after the collision of neutron stars is carried away from the collision site, emitting light. Observation of the spectrum of this radiation made it possible to determine that heavy elements, such as gold, are born precisely as a result of kilones. Scientists observed the afterglow for weeks after the event, collecting data on the processes occurring in stars, and this was the first reliable observation of the kilonova.
Neutron stars are superdense objects that form after a supernova explosion. The pressure in the star is so high that individual atoms cannot exist, and inside the star is a liquid "soup" of neutrons, protons and other particles. To describe a neutron star, scientists use an equation of state that relates pressure and matter density. There are many possible equations of state, but scientists do not know which ones are correct, so gravitational observations can help resolve this issue. At the moment, the observed signal does not give an unambiguous answer, but it helps to give interesting estimates on the shape of the star (which depends on the gravitational attraction to the second star).
An interesting discovery was that the observed short gamma-ray burst is the closest to the Earth, but at the same time too dim for such a distance. Scientists have suggested several possible explanations: perhaps the beam of gamma rays was unevenly bright, or we saw only its very edge. In any case, the question arises: previously, astronomers did not assume that such faint bursts could be located so close, and could they then miss the same faint bursts, or misinterpret them as more distant? Joint observations in the gravitational and electromagnetic ranges can help provide an answer, but at a given level of detector sensitivity, such observations will be quite rare - on average 0.1-1.4 per year.
In addition to gravitational and electromagnetic radiation, neutron stars emit streams of neutrinos in the process of merging. Neutrino detectors also worked to find these streams from the event, but did not record anything. In general, this result was expected - as in the case of a gamma-ray burst, the event is too dim (or we observe it at a high angle) for the detectors to see it.
Gravitational wave speed
Since the gravitational waves and the light signal came from the same source with a very high probability (5.3 sigma), and the first light signal came 1.7 seconds after the gravitational one, we can limit the propagation speed of gravitational waves with very high accuracy. Assuming that light and gravitational waves were emitted at the same time, and the delay between the signals was due to the fact that gravity is faster, an upper bound can be obtained. A lower estimate can be obtained from models of neutron star mergers: assume that the light was emitted 10 seconds after the gravitational waves (at which point all processes should have been completed for sure) and caught up with the gravitational waves by the time it reached the Earth. As a result, the speed of gravity is equal to the speed of light with great precision.For a lower estimate, you can use a large delay between emission, and even assume that the light signal was emitted first, which will reduce the accuracy proportionally. But even in this case, the estimate is extremely accurate.
Using the same knowledge of the delay between signals, one can significantly improve the accuracy of estimates for Lorentz invariance (the difference between the behavior of gravity and light under the Lorentz transformation) and the equivalence principle.
Scientists measured the Hubble constant in another way - by observing the parameters of the cosmic microwave background on the Planck telescope, and obtained a different value of the Hubble constant, which is not consistent with the SHoES measurements. This difference is too large to be statistical, but the reasons for the discrepancies in the estimates are not yet known. Therefore, an independent measurement is needed.
Probability distribution for the Hubble constant using gravitational waves (blue). The dotted line indicates the intervals 1σ and 2σ (68.3% and 95.4%). For comparison, the 1σ and 2σ intervals for the previous estimates are shown: Planck (green) and SHoES (orange), which do not agree with each other.
Gravitational waves in this case play the role of standard candles (and are called standard sirens). By observing the amplitude of the signal on Earth and simulating its amplitude at the source, one can estimate how much it has decreased, and thereby know the distance to the source - regardless of any assumptions on the Hubble constant or previous measurements. Observation of the light signal made it possible to determine the galaxy where the neutron star pair was located, and the receding velocity of this galaxy was well known from previous measurements. The ratio between speed and distance is the Hubble constant. It is important that such an estimate is completely independent of previous estimates or the cosmic distance scale.
One measurement was not enough to solve the puzzle of the difference between the Planck and SHoES estimates, but in general the estimate is already in good agreement with the known values. Considering that previous estimates are based on statistics collected over many years, this is a very significant result.
A little about LIGO and glitches
The top panel shows a glitch in the LIGO-Livingston data, and also clearly shows the presence of a chirp. The bottom panel shows the dimensionless amplitude of the oscillation, ”strain" (the amount we use to describe signal magnitude in LIGO and Virgo) at the time of the glitch. This is a short
(only lasts about 1/4 second), but a very strong signal. Suppression reduces the glitch to the level of the orange curve, which represents the amount of background noise always present in LIGO detectors.
Only one of the LIGO detectors saw the signal in automatic mode, because the Livingston detector had a "glitch" at the time of the event. This term refers to a burst of noise, similar to the pop of static in a radio. Although the gravitational wave signal was clearly visible to the human eye, the automation cuts off such data. Therefore, it was necessary to clear the signal from the glitch before the data could be used by the detector. Glitches appear in detectors all the time - about once every few hours. Scientists classify them by shape and duration and use this knowledge to improve detectors. You can help them with this at the GravitySpy project, where users search for and classify glitches in LIGO data to help scientists.
Questions without answers
Known to us black holes, neutron stars and their mergers. There is an area of average masses, about the existence of compact objects with which we know nothing. Credit: LIGO-Virgo/Northwestern/Frank Elavsky
We registered gravitational waves from two compact objects, and the observation of electromagnetic radiation suggests that one of them was a neutron star. But the second could also be a black hole of low mass, and although no one has seen such black holes before, theoretically they can exist. From the observation of GW170817, it cannot be determined for sure whether this was a collision of two neutron stars, although this is more likely.
The second curious moment: what did this object become after the merger? It could become either a supermassive neutron star (the most massive known) or the lightest known black hole. Unfortunately, there is not enough observational data to answer this question.
Conclusion
The observation of neutron star mergers at all ranges is an amazingly physics-rich event. The amount of data received by scientists in just these two months has allowed for the preparation of several dozen publications, and there will be many more when the data become publicly available. The physics of neutron stars is much richer and more interesting than the physics of black holes - we can directly check the physics of the superdense state of matter, as well as quantum mechanics in conditions of strong gravitational fields. This unique opportunity may help us finally find the link between general relativity and quantum physics that has eluded us so far.This discovery once again shows how important the joint work of many collaborations of thousands of people is in modern physics.
Reddit AMA
Traditionally, scientists from LIGO answer questions from users on Reddit, I highly recommend it!This will happen from 18:00 Moscow time on October 17 and 18. The link to the event will be at the start time.
Today, at a press conference in Washington, scientists officially announced the registration of an astronomical event that no one has recorded before - the merger of two neutron stars. Based on the results of the observation, more than 30 scientific articles were published in five journals, so we cannot tell you everything at once. Here is a summary and the most important discoveries.
Astronomers have observed the merger of two neutron stars and the birth of a new black hole.
Neutron stars are objects that appear as a result of explosions of large and massive (several times heavier than the Sun) stars. Their dimensions are small (they are usually no more than 20 kilometers in diameter), but their density and mass are enormous.
As a result of the merger of two neutron stars 130 million light-years from Earth, a black hole was formed - an object even more massive and dense than a neutron star. The merger of stars and the formation of a black hole was accompanied by the release of enormous energy in the form of gravitational, gamma and optical radiation. All three types of radiation were recorded by terrestrial and orbital telescopes. The gravitational wave was registered by the LIGO and VIRGO observatories.
This gravitational wave was the highest energy wave ever observed so far.
All types of radiation reached the Earth on August 17. First, ground-based laser interferometers LIGO and Virgo registered periodic compression and expansion of space-time - a gravitational wave that circled the globe several times. The event that gave rise to the gravitational wave was named GRB170817A. A few seconds later, NASA's Fermi Gamma-ray Telescope detected high-energy gamma-ray photons.
On this day, large and small, ground-based and orbital telescopes, operating in all ranges, looked at one point in space.
Based on the results of observations at the University of California (Berkeley), they made a computer simulation of the merger of neutron stars. Both stars were, apparently, a mass slightly larger than the Sun (but with a much smaller radius). These two balls of incredible density swirled around each other, constantly accelerating. Here's how it was:
As a result of the merger of neutron stars, atoms of heavy elements - gold, uranium, platinum - fell into outer space; astronomers believe that such events are the main source of these elements in the universe. Optical telescopes first "saw" blue visible light, and then ultraviolet radiation, which was replaced by red light and infrared radiation.
This sequence coincides with theoretical predictions. According to the theory, colliding, neutron stars lose some of the matter - it is sprayed around the collision site with a huge cloud of neutrons and protons. When a black hole begins to form, an accretion disk forms around it, in which particles spin at a tremendous speed - so great that some overcome the black hole's gravity and fly away.
Such a fate awaits about 2% of the matter of the colliding stars. This substance forms a cloud around the black hole with a diameter of tens of thousands of kilometers and a density approximately equal to that of the Sun. The protons and neutrons that make up this cloud stick together to form atomic nuclei. Then the disintegration of these nuclei begins. The radiation of decaying nuclei was observed by terrestrial astronomers for several days. In the millions of years that have passed since the GRB170817A event, this radiation has filled the entire galaxy.
For the first time in human history, astronomers have detected gravitational waves from the merger of two neutron stars. The event in the galaxy NGC 4993 was "smelled" on August 17 by the gravitational observatories LIGO/Virgo. Following them, other astronomical instruments also joined the observations. As a result, 70 observatories observed the event, and according to the observational data, at least 20 (!) scientific articles were published today.
Rumors that the LIGO / Virgo detectors have finally registered a new event and this is not another black hole merger, have been crawling on social networks since August 18th. Statements about it were expected at the end of September, but then scientists limited themselves only to the next gravitational-wave event involving two black holes - it happened 1.8 billion light-years from Earth, not only American detectors participated in its observation on August 14, but also the European Virgo, which "joined" in the hunt for space-time fluctuations two weeks before.
New LIGO. Source with optical counterpart. Blow your sox off!
J Craig Wheeler (@ast309) August 18, 2017
After that, the collaboration earned its well-deserved Nobel Prize in Physics - for detecting gravitational waves and confirming the correctness of Einstein, who predicted their existence - and now she told the world about the discovery, which she saved "for dessert".
What exactly happened?
Neutron stars are very, very small and very dense objects that usually result from supernova explosions. The typical diameter of such a star is 10-20 km, and the mass is comparable to the mass of the Sun (whose diameter is 100,000,000 times greater), so that the density of matter in a neutron star is several times higher than the density of an atomic nucleus. At the moment, we know several thousand such objects, but there are only one and a half to two dozen binary systems.
The kilonova (by analogy with the "supernova"), the gravitational effect of which was registered by LIGO / Virgo on August 17, is located in the constellation Hydra at a distance of 130 million light years from Earth. It arose as a result of the merger of two neutron stars with masses in the range from 1.1 to 1.6 solar masses. About how close this event came to us is that while the signal from merging black hole binaries was usually within the sensitivity range of LIGO detectors for a fraction of a second, the signal recorded on August 17 lasted about 100 seconds.
“This is not the first registered kilonova,” said astrophysicist Sergei Popov, a leading researcher at the State Astronomical Institute named after A.I. PC. Sternberg, - but they could not even be listed on the fingers of one hand, but almost on the ears. There were literally one or two."
At almost the same time, about two seconds after the gravitational waves, NASA's Fermi Gamma-Ray Space Telescope and the INTERnational Gamma-Ray Astrophysics Laboratory/INTEGRAL Orbital Observatory detected bursts of gamma rays. In the following days, scientists recorded electromagnetic radiation in other ranges, including x-ray, ultraviolet, optical, infrared and radio waves.
Having received the coordinates, several observatories were able to start searching in the region of the sky where the event supposedly occurred within a few hours. A new bright dot, resembling a new star, was detected by optical telescopes, and as a result, about 70 observatories observed this event in various wavelength ranges.
“For the first time, in contrast to “lonely” black hole mergers, a “social” event was registered not only by gravitational detectors, but also by optical and neutrino telescopes. This is the first such circle of observations around a single event,” said Sergei Vyatchanin, professor at the Faculty of Physics of Moscow State University, who is part of a group of Russian scientists who participated in the observation of the phenomenon under the guidance of Professor of the Faculty of Physics of Moscow State University Valery Mitrofanov.
At the moment of collision, the main part of the two neutron stars merged into one ultra-dense object emitting gamma rays. The first measurements of gamma rays combined with the detection of gravitational waves confirm the prediction of Einstein's general theory of relativity, namely that gravitational waves propagate at the speed of light.
“In all previous cases, merging black holes have been the source of gravitational waves. Paradoxically, black holes are very simple objects, consisting entirely of curved space and therefore fully described by the well-known laws of general relativity. At the same time, the structure of neutron stars and, in particular, the equation of state of neutron matter is still not exactly known. Therefore, the study of signals from merging neutron stars will also provide a huge amount of new information about the properties of superdense matter in extreme conditions,” said Farit Khalili, professor at the Faculty of Physics at Moscow State University, who is also a member of Mitrofanov's group.
What is the significance of this discovery?
First, the observation of neutron star mergers is another clear demonstration of the effectiveness of astronomical observations pioneered by the LIGO and Virgo detectors.
“This is the birth of a new science! Today is such a day, - Vladimir Lipunov, head of the space monitoring laboratory of the SAI MSU and head of the MASTER project, told Attic. - It will be called gravitational astronomy. This is when all the thousand-year-old methods of astronomy, which thousands of astronomers have been using for many thousands of years, have been working out, will become useful for gravitational-wave topics. Until today, all this was pure physics, that is, even a fantasy from the point of view of the public, and now it is already a reality. New reality".
“A year and a half ago, when gravitational waves were discovered, a new way of studying the Universe, studying the nature of the Universe, was discovered. And this new method has already demonstrated its ability to give us important, deep information about various phenomena in the Universe in a year and a half. They only tried to detect gravitational waves for several decades, and then once - a year and a half ago they were detected, received the Nobel Prize, and now a year and a half has passed, and it is really shown that, apart from the flag that everyone raised - yeah, Einstein was right! - this is really working now, only at the beginning of the science of gravitational astronomy, it turns out to be so effective as to study various phenomena in the Universe, ”astrophysicist Yuri Kovalev, head of the Laboratory for Fundamental and Applied Research of Relativistic Objects of the Universe at MIPT, head of the laboratory, told the Attic correspondent FIAN, head of the scientific program of the Radioastron project.
In addition, a huge amount of new data was collected during the observations. In particular, it was recorded that heavy elements such as gold, platinum and uranium are formed during the merger of neutron stars. This confirms one of the existing theories of the origin of heavy elements in the Universe. Simulations had previously demonstrated that supernova explosions alone were not enough to synthesize heavy elements in the universe, and in 1999 a group of Swiss scientists suggested that neutron star mergers could be another source of heavy elements. And although kilonovae are much rarer than supernova explosions, they can generate most of the heavy elements.
“Imagine, you never found money on the street, and then you finally found it. And it's a thousand dollars at once, - says Sergey Popov. - Firstly, it is confirmation that gravitational waves propagate at the speed of light, confirmation with an accuracy of 10 -15 . This is a very important thing. Secondly, this is a certain number of purely technical confirmations of a number of provisions of the general theory of relativity, which is very important for fundamental physics in general. Thirdly - if we return to astrophysics - this is confirmation that short gamma-ray bursts are a merger of neutron stars. And as for the heavy elements, then, of course, it’s not that no one believed in such a thing before. But there was no such a chic data complex. ”
And this set of data already on the first day allowed scientists to publish, according to Attic estimates, at least 20 articles (eight in Science, five in Nature, two in Physical Review Letters and five in Astrophysical Journal Letters). According to journalists Science, the number of authors of the article describing the event, approximately corresponds to a third of all active astronomers. Are you looking forward to the sequel? We are yes.
The results of observations may in the future shed light on the mystery of the structure of neutron stars and the formation of heavy elements in the Universe.
Artistic depiction of gravitational waves generated by the merger of two neutron stars
Image: R. Hurt/Caltech-JPL
Moscow. 16 October. website - For the first time in history, scientists have recorded gravitational waves from the merger of two neutron stars - superdense objects with a mass as large as our Sun and the size of Moscow, the N + 1 website reports.
The gamma-ray burst and the kilonova flash that followed were observed by about 70 ground and space observatories - they were able to see the synthesis of heavy elements predicted by theorists, including gold and platinum, and confirm the correctness of the hypotheses about the nature of the mysterious short gamma-ray bursts, the press service of the collaboration reports. LIGO/Virgo, European Southern Observatory and Los Cumbres Observatory. The results of observations can shed light on the mystery of the structure of neutron stars and the formation of heavy elements in the Universe.
Gravitational waves are waves of fluctuations in the space-time geometry, the existence of which was predicted by the general theory of relativity. For the first time, the LIGO collaboration reported their reliable detection in February 2016 - 100 years after Einstein's predictions.
Reportedly, on the morning of August 17, 2017 (at 8:41 am East Coast time, when it was 3:41 pm in Moscow), automatic systems on one of the two detectors of the LIGO gravitational wave observatory registered the arrival of a gravitational wave from space. The signal received the designation GW170817, it was already the fifth case of detection of gravitational waves since 2015, from the moment they were first recorded. Just three days earlier, the LIGO observatory "heard" a gravitational wave for the first time along with the European project Virgo.
However, this time, just two seconds after the gravitational event, the Fermi space telescope recorded a flash of gamma radiation in the southern sky. Almost at the same moment, the flare was seen by the European-Russian space observatory INTEGRAL.
The automatic data analysis systems of the LIGO observatory came to the conclusion that the coincidence of these two events is extremely unlikely. During the search for additional information, it was found that the second LIGO detector, as well as the European gravitational observatory Virgo, saw the gravitational wave. Astronomers around the world have been put on "alert" as many observatories, including the European Southern Observatory and the Hubble Space Telescope, have begun hunting for the source of gravitational waves and the gamma-ray burst.
The task was not easy - the combined data from LIGO / Virgo, Fermi and INTEGRAL made it possible to delineate an area of \u200b\u200b35 square degrees - this is the approximate area of several hundred lunar disks. It wasn't until 11 hours later that the small Swope telescope with a meter mirror in Chile took the first picture of the alleged source - it looked like a very bright star next to the elliptical galaxy NGC 4993 in the constellation Hydra. Over the next five days, the brightness of the source dropped by a factor of 20, and the color gradually shifted from blue to red. All this time, the object was observed by many telescopes in the ranges from X-ray to infrared, until in September the galaxy was too close to the Sun, and became unobservable.
Scientists have come to the conclusion that the source of the outbreak was in the galaxy NGC 4993 at a distance of about 130 million light-years from Earth. This is incredibly close, so far gravitational waves have come to us from distances of billions of light years. Thanks to this proximity, we were able to hear them. The source of the wave was the merger of two objects with masses in the range from 1.1 to 1.6 solar masses - these could only be neutron stars.
Localization of the source of gravitational waves in the galaxy NGC 4993
The burst itself "sounded" for a very long time - about 100 seconds, gave bursts lasting a fraction of a second. A pair of neutron stars revolved around a common center of mass, gradually losing energy in the form of gravitational waves and approaching. When the distance between them was reduced to 300 km, gravitational waves became powerful enough to get into the zone of sensitivity of the LIGO/Virgo gravitational detectors. Neutron stars managed to make 1.5 thousand revolutions around each other. At the moment of the merger of two neutron stars into one compact object (a neutron star or a black hole), a powerful burst of gamma radiation occurs.
Astronomers call such gamma-ray bursts short gamma-ray bursts, gamma-ray telescopes record them about once a week. The brief gamma-ray burst from a neutron star merger reportedly lasted 1.7 seconds.
If the nature of long gamma-ray bursts is more understandable (their sources are supernova explosions), then there was no consensus on the sources of short bursts. There was a hypothesis that they are generated by mergers of neutron stars.
Now scientists have been able to confirm this hypothesis for the first time, because thanks to gravitational waves, we know the mass of the merged components, which proves that these are neutron stars.
"We've suspected for decades that short gamma-ray bursts are causing neutron star mergers. Now, thanks to the LIGO and Virgo data on this event, we have an answer. Gravitational waves tell us that the merged objects had masses corresponding to neutron stars, and the gamma-ray burst tells us that these objects could hardly be black holes, since the collision of black holes should not generate radiation, "says Julie McEnery, Fermi Project Officer at NASA's Goddard Space Flight Center.
Source of gold and platinum
In addition, for the first time, astronomers have received unambiguous confirmation of the existence of kilon (or "macron") flares, which are about 1 thousand times more powerful than ordinary nova flares. Theorists predicted that kilonovae could be produced by the merger of neutron stars or a neutron star and a black hole.
This starts the process of synthesis of heavy elements, based on the capture of neutrons by nuclei (r-process), as a result of which many of the heavy elements, such as gold, platinum or uranium, appeared in the Universe.
According to scientists, with one explosion of a kilonova, a huge amount of gold can appear - up to ten masses of the moon. So far, only once an event has been observed that could be a kilonova explosion.
Now, for the first time, astronomers have been able to observe not only the birth of the kilonova, but also the products of its "work". Spectra obtained with the Hubble and VLT (Very Large Telescope) telescopes showed the presence of cesium, tellurium, gold, platinum and other heavy elements formed during the merger of neutron stars.
Eleven hours after the collision, the temperature of the kilonova was 8,000 degrees, and its expansion rate reached about 100,000 kilometers per second, N + 1 notes, citing data from the Sternberg State Astronomical Institute (GAISh).
ESO reported that the observation coincided almost perfectly with the prediction of the behavior of two neutron stars during a merger.
“So far, the data we have obtained are in excellent agreement with the theory. This is a triumph for theorists, confirmation of the absolute reality of the events recorded by the LIGO and VIrgo observatories, and a remarkable achievement by ESO, which was able to obtain such observations of the kilonova,” says Stefano Covino, the first author of one of the articles in Nature Astronomy.
This is how astronomers saw the collision of neutron stars
Scientists do not yet have an answer to the question of what remains after the merger of neutron stars - it can be either a black hole or a new neutron star, in addition, it is not entirely clear why the gamma-ray burst turned out to be relatively weak.
