Scientific breakthrough: a combination of gravitational and electromagnetic waves
Gravitational waves have become the most important tool available to astronomers. They are already used to confirm that large black holes (BHs) - with masses ten or more times than the Sun - and mergers of these large BHs, which form even larger BHs, do not so rarely occur in the Universe. In October 2017, this tool made a breakthrough .
It has long been known that neutron stars (NZ), the collapsed remains of stars that exploded and became supernovae, are common in the Universe. And almost as much it is known that NZ sometimes go in pairs. (This was the first way that gravitational waves were indirectly discovered in the 1970s). Stars often form pairs, and sometimes both stars explode and become supernova, and their remnants in the form of NCs orbit around each other. According to Einstein's theory of relativity, a pair of stars should gradually lose energy, emitting gravitational waves into space, and slowly but surely these two objects should spiral onto each other. As a result, after millions or even billions of years, they collide and merge into a larger NZ or BH. As a result of this collision two events occur.
It is possible that earlier we had already seen the light of the merger of two NCs, but no one can be sure of this. Wouldn't it be great if we could see gravitational waves And electromagnetic waves emanating from the confluence of the NZ? It will be similar to how if you see fireworks and hear an explosion - to see and hear at the same time better than separately, each of the signals specifies the other. (Warning: scientists often say that the detection of gravitational waves is similar to hearing. This is just an analogy, and very distant. They are not at all like the acoustic waves that we hear with our ears, for many reasons - so there is no need to take the analogy literally). If we can do both this and that, we will be able to gain new knowledge about the NT and their properties in a completely new way.
And we finally found out that it happened. LIGO, with the first two gravity observatories, discovered waves from two merging NSs located 130 million light years from Earth, on August 17, 2017. (The merger of NZ lasts much longer than the merger of BH, so they are easy to distinguish; specifically, this merger happened so (relatively) closely that it could be observed for a long time). VIRGO, with the third detector, allowed scientists to triangulate and roughly determine the location of the merger. They received a very weak signal, but it turned out to be extremely important, as he informed the scientists that the merger had occurred in a small region of the sky, in which VIRGO had a blind spot. And this made scientists understand where to look.
The merger was observed for more than a minute - it can be compared with BH, the merger of which occurs in less than a second. But it is not quite clear what happened at the end! Have merged NZs formed another NZ or BH? It's not clear yet.
Almost exactly at the moment when gravitational waves reached a maximum, another team of scientists, from the FERMI project, recorded a flash of gamma rays - high-frequency electromagnetic waves. FERMI observes gamma rays coming from the far ends of the Universe daily, and the two-second gamma flash was not unusual. She was discovered by another gamma-ray experiment, INTEGRAL. The teams exchanged information in a few minutes. FERMI and INTEGRAL gamma-ray detectors can quite roughly determine the part of the sky where these gamma-rays come from, and LIGO / VIRGO together also give only an approximate part. But scientists saw the overlap of these sites, and the evidence was irrefutable. This is how astronomy entered a new, long-awaited phase.
Only this in itself was already quite a major discovery. Brief flashes of gamma rays for years occupied by scientists. One of the best guesses about their origin was the assumption of the merger of NZ. Now the riddle is solved - this assumption, obviously, was justified. (What if no? The detected gamma rays were unexpectedly weak, so questions still remain).
Also, the fact that these signals came with a gap of a couple of seconds from each other, after they, having come from the same source, made a journey that had occupied them for more than 100 million years, confirms that the speed of light and the speed of gravitational waves are the same - and both are equal to the cosmic speed limit, in exact accordance with the predictions of Einstein's theory of gravity.
Then, these teams quickly informed their astronomers about the need to send their telescopes to the area where the source should be. Dozens of telescopes, from all continents and from space, searched for electromagnetic waves with a large frequency spread, being directed approximately in the right direction, and scanned the sky in search of something unusual. (One of the problems was that the desired object was in the sky close to the Sun, so it could be seen only in the dark and only an hour every night). And the light was discovered! At all frequencies! The object was very bright, so it was very easy to find the galaxy in which the merger took place. Bright light was visible in gamma rays, ultraviolet radiation, infrared light, X-rays, and radio frequency. (At this time, neutrinos, particles that can be used as another way of observing remote explosions, were not detected).
And with so much information you can learn so much everything!
The most important thing, perhaps, is this: from the patterns that are present in the spectrum of light, the hypothesis confirms that neutron star fusions are important, possibly the predominant sources of the appearance of many heavy chemical elements — iodine, iridium, cesium, gold, platinum, and so on. occurring at high temperatures in such collisions. The most likely sources were the same supernovae that form the NZ. But now, apparently, it turned out that the second stage of the life of the NT — merging, not birth — is just as important. This is amazing, because the NZ merger occurs much less frequently than supernova explosions. In our Milky Way Galaxy, a supernova flashes about once every hundred years, but tens of thousands of years elapse between the appearance of such kilon-like ones in mergers of the NT.
If something in this news is disappointing, this is the following: almost everything that was observed in these experiments was predicted in advance. Sometimes it is more important and more useful if your predictions are completely unjustified, because then you understand how much you still need to know. Obviously, our understanding of gravity, NZ, their mergers, all types of sources of electromagnetic radiation arising in these mergers, turns out to be much better than one could think. But, fortunately, there are several new puzzles. X-rays are late; the gamma rays were weak — we will soon find out more about this, since NASA must hold a new conference.
Some topics of the conference:
It has long been known that neutron stars (NZ), the collapsed remains of stars that exploded and became supernovae, are common in the Universe. And almost as much it is known that NZ sometimes go in pairs. (This was the first way that gravitational waves were indirectly discovered in the 1970s). Stars often form pairs, and sometimes both stars explode and become supernova, and their remnants in the form of NCs orbit around each other. According to Einstein's theory of relativity, a pair of stars should gradually lose energy, emitting gravitational waves into space, and slowly but surely these two objects should spiral onto each other. As a result, after millions or even billions of years, they collide and merge into a larger NZ or BH. As a result of this collision two events occur.
- There is some very bright flash of light - electromagnetic waves - the details of which we can only guess. Some of these waves will be visible light, and most of them will be invisible, for example, gamma radiation.
- Gravitational waves arise, the details of which are easier to calculate, which is why they can be distinguished, but could not be detected until LIGO and VIRGO started collecting data: LIGO over the past few years, and VIRGO over the past few months.
It is possible that earlier we had already seen the light of the merger of two NCs, but no one can be sure of this. Wouldn't it be great if we could see gravitational waves And electromagnetic waves emanating from the confluence of the NZ? It will be similar to how if you see fireworks and hear an explosion - to see and hear at the same time better than separately, each of the signals specifies the other. (Warning: scientists often say that the detection of gravitational waves is similar to hearing. This is just an analogy, and very distant. They are not at all like the acoustic waves that we hear with our ears, for many reasons - so there is no need to take the analogy literally). If we can do both this and that, we will be able to gain new knowledge about the NT and their properties in a completely new way.
And we finally found out that it happened. LIGO, with the first two gravity observatories, discovered waves from two merging NSs located 130 million light years from Earth, on August 17, 2017. (The merger of NZ lasts much longer than the merger of BH, so they are easy to distinguish; specifically, this merger happened so (relatively) closely that it could be observed for a long time). VIRGO, with the third detector, allowed scientists to triangulate and roughly determine the location of the merger. They received a very weak signal, but it turned out to be extremely important, as he informed the scientists that the merger had occurred in a small region of the sky, in which VIRGO had a blind spot. And this made scientists understand where to look.
The merger was observed for more than a minute - it can be compared with BH, the merger of which occurs in less than a second. But it is not quite clear what happened at the end! Have merged NZs formed another NZ or BH? It's not clear yet.
Almost exactly at the moment when gravitational waves reached a maximum, another team of scientists, from the FERMI project, recorded a flash of gamma rays - high-frequency electromagnetic waves. FERMI observes gamma rays coming from the far ends of the Universe daily, and the two-second gamma flash was not unusual. She was discovered by another gamma-ray experiment, INTEGRAL. The teams exchanged information in a few minutes. FERMI and INTEGRAL gamma-ray detectors can quite roughly determine the part of the sky where these gamma-rays come from, and LIGO / VIRGO together also give only an approximate part. But scientists saw the overlap of these sites, and the evidence was irrefutable. This is how astronomy entered a new, long-awaited phase.
Only this in itself was already quite a major discovery. Brief flashes of gamma rays for years occupied by scientists. One of the best guesses about their origin was the assumption of the merger of NZ. Now the riddle is solved - this assumption, obviously, was justified. (What if no? The detected gamma rays were unexpectedly weak, so questions still remain).
Also, the fact that these signals came with a gap of a couple of seconds from each other, after they, having come from the same source, made a journey that had occupied them for more than 100 million years, confirms that the speed of light and the speed of gravitational waves are the same - and both are equal to the cosmic speed limit, in exact accordance with the predictions of Einstein's theory of gravity.
Then, these teams quickly informed their astronomers about the need to send their telescopes to the area where the source should be. Dozens of telescopes, from all continents and from space, searched for electromagnetic waves with a large frequency spread, being directed approximately in the right direction, and scanned the sky in search of something unusual. (One of the problems was that the desired object was in the sky close to the Sun, so it could be seen only in the dark and only an hour every night). And the light was discovered! At all frequencies! The object was very bright, so it was very easy to find the galaxy in which the merger took place. Bright light was visible in gamma rays, ultraviolet radiation, infrared light, X-rays, and radio frequency. (At this time, neutrinos, particles that can be used as another way of observing remote explosions, were not detected).
And with so much information you can learn so much everything!
The most important thing, perhaps, is this: from the patterns that are present in the spectrum of light, the hypothesis confirms that neutron star fusions are important, possibly the predominant sources of the appearance of many heavy chemical elements — iodine, iridium, cesium, gold, platinum, and so on. occurring at high temperatures in such collisions. The most likely sources were the same supernovae that form the NZ. But now, apparently, it turned out that the second stage of the life of the NT — merging, not birth — is just as important. This is amazing, because the NZ merger occurs much less frequently than supernova explosions. In our Milky Way Galaxy, a supernova flashes about once every hundred years, but tens of thousands of years elapse between the appearance of such kilon-like ones in mergers of the NT.
If something in this news is disappointing, this is the following: almost everything that was observed in these experiments was predicted in advance. Sometimes it is more important and more useful if your predictions are completely unjustified, because then you understand how much you still need to know. Obviously, our understanding of gravity, NZ, their mergers, all types of sources of electromagnetic radiation arising in these mergers, turns out to be much better than one could think. But, fortunately, there are several new puzzles. X-rays are late; the gamma rays were weak — we will soon find out more about this, since NASA must hold a new conference.
Some topics of the conference:
- New information on the insides of the NC has been obtained, which affects how large they can be and how exactly they merge.
- The first image of the source of gravitational waves in visible light, located on the outskirts of a distant galaxy, using a Swope telescope was obtained. The center of the galaxy is a circle of light, and the arrows indicate the location of the explosion.
- Theoretical calculations of the kilon explosion indicate that the debris of an explosion should quickly block visible light, so the explosion quickly dims in visible light - but infrared light stays much longer. Observations of telescopes in the visible and infrared ranges confirm this aspect of the theory; this evidence can be seen in the picture above, where after four days the bright spot became much dimmer and much redder.
- Assessment: the total mass of gold and platinum arising from this explosion is much greater than the mass of the Earth.
- Assessment: these neutron stars formed about 10 billion years ago. They revolved around each other most of the history of the universe, and ended their existence just 130 million years ago, giving rise to the explosion we recently discovered.
- Big mystery: all the previous gamma-ray flashes recorded by us glowed in the ultraviolet and X-ray in the same way as in the gamma range. But this time the X-rays did not appear, at least not immediately. This was a big surprise. It took 9 days for the Chandra telescope to detect X-rays, which are too dim for any other telescope. Does this mean that two NZs created a BH, which then created a jet ( relativistic jet ) of matter, directed not directly at us and highlighting matter in interstellar space? This opportunity was offered 20 years ago, but some evidence in her favor was obtained for the first time.
- Another surprise: the discovery of radio waves took 16 days, and opened them with the most powerful of the existing radio telescopes, Very Large Array . Since then, the radio emission increases the brightness! This, as in the case of x-rays, supports the idea of the presence of a jet directed away from us.
- Up to this point, we have not seen anything like this gamma flash - or, more precisely, did not recognize it. When gamma rays do not have an X-ray component that appears almost immediately, it just looks weird and a little mysterious. It is harder to watch than most flashes, because if the jet does not look directly at us, its afterglow quickly fades away. Moreover, if the jet looks at us, then it turns out so bright that it blinds us and does not allow us to recognize the details of the kilon properties. But this time, LIGO / VIRGO told the scientists: “Yes, this is the NZ fusion,” which led to a detailed study at all electromagnetic frequencies, including a patient lasting many days of research on gamma rays and radio emission. In other cases, these observations would cease soon after the beginning, and the whole story could not be interpreted correctly.
Source: https://habr.com/ru/post/410565/