Exactly half a century ago, astronomers caught a strange signal, which was initially mistaken for messages from aliens. How pulsars frightened scientists and what they became for astronomers 50 years later, said the leading researcher of the Moscow State University, Doctor of Physical and Mathematical Sciences, astrophysicist Sergei Popov.
- Sergey, exactly 50 years ago radio astronomers in Cambridge discovered a radio pulsar for the first time. How did this happen?- It was 1967, the whole UK was preparing for the 50th anniversary of Great October, Pink Floyd released their first album, The Beatles recorded Sgt. Pepper's Lonely Hearts Club Band, if I'm not mistaken. Jocelyn Bell, as a graduate student, received 30 meters of paper every day, where the data on radio signals were written with the gnarled hand of a recorder. And she worked with them. Slowly, she began to notice a strange signal that repeatedly comes from the same region of the sky. She saw that the signal comes every 23 hours 56 minutes, that is, for the period of the Earth's revolution relative to the stars. The first such signal on the recordings of the recorder, noted by her, refers to August 6. But they identified all this later. Then she reported this to the leader, Anthony Hewish, and they had many doubts about how real this signal was. It was decided to test this signal, and on November 28, their verification was crowned with success. Moreover, at that moment they realized that this signal comes with a period of 1.33 seconds. Then it was necessary to discard a bunch of all sorts of options, including aliens. We will never know how seriously which of them took this version - the time was like that, the consciousness of all was expanded. Shortly before Christmas, while leaving for the holidays, Jocelyn discovered a second source. Jocelyn discovered a second source. Jocelyn discovered a second source.
And they were in no hurry to inform the world about the discovery?
- There was a very serious possibility that this signal was artificial, and therefore Hewish came up with the idea that if the signal comes from a certain planet, and the planet revolves around its star, then a rather strong Doppler shift of the signal will be noticeable. They purposefully investigated this option and rejected it, that is, they realized that the source is not on an object that periodically moves around the star. Well, then they published an article in Nature, where, in accordance with the traditions and orders of that time, Huish was the first author, and Bell was the second.
Then there was a big discussion about the nature of the object, and less than seven years later, rather quickly, the Nobel Prize was awarded for this.
And it was not without a scandal - Bell was left without a prize
- Yes, Frel Hoyle wrote a letter to the newspaper and spoke about the fact that what she did was not accidental at all, and it was she who noticed that the signal comes from one part of the sky with a difference in sidereal days. There was some discussion about this, and Jocelyn herself later wrote that she was not offended and had no complaints. At least, we can say that no one pushed or pushed anyone there on purpose.
The strange object turned out to be a neutron star, but this was the case when their existence was predicted earlier?
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- Yes, neutron stars have been predicting since the 1930s. In the beginning, even before the discovery of neutrons, there was an abstract theoretical prediction made by Landau that there might be superdense stars with a density like an atomic nucleus. Then, in 1934, when the neutron was discovered, an article by Baade and Zwicky appeared, where it was correctly predicted that neutron stars can mainly consist of neutrons and that they are born in supernova explosions. They indicated important key parameters. Then, one way or another, the existence of neutron stars surfaced among theorists, somewhere in the mid-60s they began to model in detail the cooling of these sources. And generally speaking, in the 67th year, an article was written by Franco Pacini, where pulsar radiation was almost predicted.
So, with the discovery of 1967, a whole class of new objects of stellar masses the size of a large city became known to science. What are their types?
- Indeed, there are many different neutron stars. But this is mainly the achievement of recent years. At first it was believed that all young neutron stars are similar to the pulsar in the Crab Nebula. And we can see old neutron stars in binary systems if matter is flowing on them from a companion star. And then it turned out that young neutron stars can manifest themselves in a very diverse way. The most famous type of sources is probably magnetars.
Magnetars can be considered one of the brightest discoveries of Russian-Soviet astronomy - flashing objects, reaching a maximum of an absolutely fantastic radiation power, more than 10 billion solar luminosity.
On the other hand, there are still young neutron stars. But they are completely different from pulsars, i.e. do not manifest themselves as pulsars. These are, for example, cooling neutron stars in solar environs, the so-called. The Magnificent Seven. There are sources in supernova remnants. It is very beautiful when right in the center of the remnant we see a small point X-ray source that does not show any activity. It is a young neutron star, and we see radiation from its hot surface. There are also various interesting variants of pulsars, for example, such as rotating radio transients - objects that give an impulse not every revolution.
What role did pulsars begin to play in astronomy and in applied problems?
- In general, all scientists were stunned by the stability of the rotation of the pulsars, so the pulsar works like a very accurate clock.
And this provides an excellent opportunity to test General Relativity. The second Nobel Prize for neutron stars was given, in fact, for checking general relativity for these objects (in particular, the existence of gravitational waves was indirectly confirmed).
The substance in the depths of neutron stars is in a superdense state - in such a state that we cannot receive in laboratories on Earth. And this is interesting for physicists. There is a very strong magnetic field on their surface, which is also impossible to obtain in a laboratory. Pulsars sometimes show period faults that change abruptly. And the first idea was that this is due to a break in the crust. But, in fact, it seems that these are still not crustal faults, but there is an even more interesting effect associated with the fact that there are vortices of superfluid neutrons in the crust. And when the system of these vortices is rebuilt, then a period failure occurs - the star sharply accelerates its rotation.
And, as they say, pulsars are of national economic importance.
For a long time, it was believed that the most important thing is their rotation stability. Therefore, precise time standards based on radio pulsars were developed very seriously.
And the fact that they have not been implemented today is due only to the fact that there is also very serious progress in the field of creating atomic clocks. So neutron stars were not useful here, but they were needed to solve another problem.
In space research there is a problem of autonomous navigation of satellites. If we have a spacecraft flying somewhere between Jupiters and Saturn, then ideally it should decide for itself where and when to turn on the engine to correct the orbit. To do this, he needs to know his speed and location. Now this is being solved by constant contact with the Earth. But this is bad. Firstly, because the signal can go back and forth for several hours, and secondly, you need to power a powerful radio transmitter on board. It would be great if the satellite could decide this on its own. And pulsars are the perfect solution. Because they give stable impulses.
The satellite moves relative to the center of mass of the solar system. Respectively, If we calculate the arrival times of pulses for the barycenter, then from the delay in the measured arrival time we can determine the coordinates of the satellite in the Solar System.
If the satellite is moving, then the Doppler effect occurs. If it moves towards the pulsar, then the frequency of arrival of pulses increases. If in the opposite direction, then it decreases. If several such pulsars are observed, then the three-dimensional position and velocity of the apparatus can be accurately determined. Today, technological advances have made X-ray detectors quite cheap, lightweight, and energy efficient. And the first Chinese satellite with a prototype of such a navigation system is already flying. And the second prototype is now being tested at the International Space Station. There is an American device NICER, as part of its use, the SEXTANT experiment is being carried out, in which the X-ray navigation system is being tested. Most likely, next-generation interplanetary stations will already be guided by pulsars.
Pavel Kotlyar