Scientific breakthrough: a combination of gravitational and electromagnetic waves

Original author: Matt Strassler
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Gravitational waves have become the most important tool available to astronomers. They are already being used to confirm that large black holes (BHs) - with masses ten or more times that of the Sun - and the mergers of these large BHs, forming even larger BHs, are not so rare in the Universe. In October 2017, this tool made a leap forward .

It has long been known that neutron stars (NS), the collapsed remains of stars that exploded and become supernovae, are common in the Universe. And almost as much is known that NCs sometimes walk in pairs. (This is how gravitational waves were first indirectly discovered in the 1970s). Stars often form pairs, and sometimes both stars explode and become supernovae, and their remnants in the form of NSs revolve 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 fall on top of each other in a spiral. 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.

  1. A very bright flash of light arises - 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.
  2. Gravitational waves arise, the details of which are easier to calculate, because of which they can be distinguished, but could not be detected until LIGO and VIRGO started collecting data: LIGO for the last few years, and VIRGO for the last few months.

It is possible that we have already seen the light of the merger of the two NSs, 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 merger of NS? It will be like if you see a firework and hear an explosion - to see and hear at the same time is better than separately, each of the signals clarifies the other. (Warning: scientists often say that the detection of gravitational waves is like a rumor. This is only an analogy, and very distant. They are not at all similar to the acoustic waves that we hear with our ears, for many reasons - therefore, you do not need to take the analogy literally). If we can do this and that, we will be able to gain new knowledge about NS and their properties in a completely new way.

And we finally found out that this happened. LIGO, with the first two gravitational observatories, discovered waves from two merging NSs located 130 million light-years from Earth, August 17, 2017. (The merger of NS lasts much longer than the merger of BH, so they are easy to distinguish; specifically, this merger happened so (relatively) close that it could be observed for a long time). VIRGO, with a 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, because he told scientists that the merger occurred in a small region of the sky in which VIRGO has 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 still not entirely clear what happened at the end! Have the merged NS formed another NS 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 radiation coming from the far ends of the universe daily, and a two-second gamma-ray burst was not unusual. It was discovered by another experiment with gamma rays, INTEGRAL. The teams exchanged information in a few minutes. FERMI and INTEGRAL gamma ray detectors can quite roughly determine the area of ​​the sky where these gamma rays come from, and LIGO / VIRGO together give only an approximate area. But scientists saw the overlap of these sites, and the evidence was incontrovertible. So astronomy entered a new, long-awaited phase.

Only this in itself was already quite a major discovery. Brief flashes of gamma rays have occupied scientists for years. One of the best guesses about their origin was the assumption of the merger of NZ. Now the riddle has been solved - this assumption has obviously been justified. (What if not? The gamma rays detected 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 left the same source, traveled a path that took them more than 100 million years, confirms that the speed of light and the speed of gravitational the waves are the same - and both of them are equal to the cosmic velocity limit, in exact accordance with the predictions of Einstein's theory of gravity.

Then these teams quickly informed their fellow astronomers of the need to direct their telescopes to the area where the source should be. Dozens of telescopes, from all over the world and from space, searched for electromagnetic waves with a wide spread of frequencies, being directed in approximately 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 light was discovered! At all frequencies! The object turned out to be very bright, making it very easy to find the galaxy in which the merger took place. Bright light was visible in gamma rays, ultraviolet light, infrared light, x-ray and radio range. (This time neutrinos, particles, которые можно использовать как ещё один способ наблюдения за удалёнными взрывами, обнаружены не были).

And with so much information you can find out so much!

The most important thing, probably, is this: from the laws that are present in the light spectrum, the hypothesis confirms that fusions of neutron stars are important, possibly the predominant sources of the appearance of many heavy chemical elements - iodine, iridium, cesium, gold, platinum, and so on - arising at high temperatures in such collisions. The most probable source was considered to be the same supernovae that form NS. But now, apparently, it turned out that the second stage in the life of NZ — fusion, not birth — is just as important. This is amazing because NS fusion is much less common than supernova explosions. In our Galaxy, the Milky Way supernova erupts about once every hundred years, but tens of millennia pass between the appearance of such “kilon” ones in the NS mergers.

If something in this news is disappointing, it is this: almost everything that was observed in these experiments was predicted in advance. Sometimes it’s more important and more useful if your predictions are completely unjustified, because then you understand how much you still have to learn. Obviously, our understanding of gravity, NS, their mergers, all types of sources of electromagnetic radiation arising in these mergers, is much better than you might think. But, fortunately, there are several new puzzles. X-rays are late; gamma rays were faint - we will learn more about this soon, since NASA is due to hold a new conference.

Some topics of the conference:

  • New information was obtained on the insides of the NS, which affects how large they can be and how exactly they merge.
  • The first image of a source of gravitational waves in visible light, located at the back of a distant galaxy, was obtained using the Swope telescope. The center of the galaxy is a circle of light, and the arrows indicate the place of the explosion.
  • Theoretical calculations of the kilon explosion indicate that the fragments of the explosion should block visible light fairly quickly, so the explosion quickly dims in visible light - but infrared light remains 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 has become much dimmer and much redder.
  • Assessment: the total mass of gold and platinum that arose in this explosion is much greater than the mass of the Earth.
  • Rating: 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 only 130 million years ago, giving rise to a recently discovered explosion.
  • A big mystery: all the previous gamma-ray bursts that we recorded glowed in ultraviolet and X-rays in exactly the same way as in the gamma range. But this time, x-rays did not appear, at least not immediately. This was a big surprise. The Chandra telescope took 9 days to detect X-rays that were too dim for any other telescope. Does this mean that two NSs created a BH, which then created a jet ( relativistic stream ) of matter, directed not directly at us and illuminating matter in interstellar space? Such an opportunity was proposed 20 years ago, but some evidence in its favor was obtained for the first time.
  • Another surprise: it took 16 days to open the radio waves, and they opened them with the most powerful of the existing radio telescopes, Very Large Array . Since then, radio emission increases brightness! This, as in the case of x-rays, supports the idea of ​​a jet directed away from us.
  • Up to this point, we have not seen anything like this gamma-ray 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 strange and a little mysterious. It is more difficult to observe it than most flashes, because if the jet does not look directly at us, its afterglow quickly disappears. Moreover, if a 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 properties of the kilon. But this time, LIGO / VIRGO told scientists: “Yes, this is a fusion of NS”, which led to a detailed study at all electromagnetic frequencies, including a patient multi-day study of gamma rays and radio emissions. In other cases, these observations would cease shortly after the start, and the whole story might not be correctly interpreted.