Scientists for the first time in history recorded gravitational waves from the merger of two neutron stars - superdense objects with a mass of our Sun and the size of Moscow. The resulting gamma-ray burst and the kilonova burst were observed by about 70 ground-based and space observatories - they were able to see the process of synthesis of heavy elements, including gold and platinum, predicted by theorists, and to confirm the correctness of hypotheses about the nature of mysterious short gamma-ray bursts, the press service of the collaboration reported. LIGO / Virgo, European Southern Observatory and Los Cumbres Observatory. Observational results can shed light on the mystery of the structure of neutron stars and the formation of heavy elements in the Universe.
On the morning of August 17, 2017 (at 8:41 a.m. US East Coast time, when it was 15:41 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, this was the fifth case of fixing gravitational waves since 2015, since they were first recorded. Just three days earlier, the LIGO observatory first "heard" a gravitational wave together with the European project Virgo.
However, this time, just two seconds after the gravitational event, the Fermi space telescope detected a gamma-ray burst in the southern sky. Almost at the same moment, the European-Russian space observatory INTEGRAL saw the outbreak.
The automatic data analysis systems of the LIGO observatory concluded that the coincidence of these two events is extremely unlikely. During the search for additional information, it was discovered that the gravitational wave was seen by the second LIGO detector, as well as the European gravitational observatory Virgo. Astronomers around the world were alerted to the hunt for the source of gravitational waves and gamma-ray bursts, many observatories, including the European Southern Observatory and the Hubble Space Telescope, began.
Changing the brightness and color of the kilonova after the explosion.
The task was not easy - the combined data from LIGO / Virgo, Fermi and INTEGRAL allowed us to outline an area of 35 square degrees - this is an approximate area of several hundred lunar disks. Only 11 hours later, the small Swope telescope with a meter mirror located 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 20 times, 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 inaccessible for observation.
Scientists concluded that the source of the outbreak was located in the galaxy NGC 4993 at a distance of about 130 million light-years from Earth. It's incredibly close, until now gravitational waves have come to us from distances of billions of light years. Thanks to this closeness, 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.
Photo of the source of gravitational waves - NGC 4993, with a flash in the center.
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The burst itself "sounded" for a very long time - about 100 seconds, merging of black holes 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 converging. When the distance between them was reduced to 300 kilometers, the gravitational waves became powerful enough to hit the zone of sensitivity of the LIGO / Virgo gravitational detectors. When two neutron stars merge into one compact object (neutron star or 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. If the nature of long GRBs is more understandable (their sources are supernova explosions), there was no consensus about the sources of short bursts. There was a hypothesis that they are generated by mergers of neutron stars.
Now scientists were 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 precisely neutron stars.
“For decades, we have suspected that short GRBs are generating mergers of neutron stars. Now, thanks to data from LIGO and Virgo about 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, project officer at Fermi Center. space flight NASA named Goddard.
In addition, astronomers for the first time have received unequivocal confirmation of the existence of kilon (or "macron") flares, which are about 1000 times more powerful than conventional nova flares. Theorists predicted that kilonovs could arise from the merger of neutron stars or a neutron star and a black hole.
This triggers the 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 arise - up to ten times the mass of the moon. Until now, only one event has been observed that could be a kilonova explosion.
Now astronomers were able to observe for the first time 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 from merging neutron stars.
“So far, the data we have received are in excellent agreement with theory. This is a triumph for theorists, a confirmation of the absolute reality of events recorded by LIGO and VIrgo, and a remarkable achievement for ESO to obtain such observations of the kilonova,”says Stefano Covino, the first author of an article in Nature Astronomy.
Scientists do not yet have an answer to the question of what is left after the merger of neutron stars - it can be either a black hole or a new neutron star, moreover, it is not entirely clear why the gamma-ray burst was relatively weak.
Gravitational waves are waves of oscillation of the geometry of space-time, the existence of which was predicted by the general theory of relativity. For the first time, the LIGO collaboration announced their reliable detection in February 2016 - 100 years after Einstein's predictions.
Alexander Voytyuk