MOSCOW, August 28 - RIA Novosti. Scientists have discovered a record-breaking neutron star with twice the mass of the Sun, which will force them to reconsider a number of theories, in particular, the theory that "free" quarks may be present inside the superdense matter of neutron stars, according to an article published on Thursday in journal Nature.
A neutron star is the "corpse" of a star left after a supernova explosion. Its size does not exceed the size of a small city, but the density of matter is 10-15 times higher than the density of the atomic nucleus - a "pinch" of neutron star matter weighs more than 500 million tons.
Gravity "presses" electrons into protons, turning them into neutrons, which is why neutron stars got their name. Until recently, scientists believed that the mass of a neutron star cannot exceed two solar masses, since otherwise gravity would "collapse" the star into a black hole. The state of the interior of neutron stars is largely a mystery. For example, the presence of "free" quarks and such elementary particles as K-mesons and hyperons in the central regions of a neutron star is discussed.
The authors of the study, a group of American scientists led by Paul Demorest from the National Radio Observatory, studied the binary star J1614-2230, three thousand light-years from Earth, one of whose components is a neutron star, and the other a white dwarf.
At the same time, a neutron star is a pulsar, that is, a star that emits narrowly directed radio emission streams; as a result of the rotation of the star, the radiation flux can be caught from the Earth's surface using radio telescopes at different time intervals.
A white dwarf and a neutron star rotate relative to each other. However, the speed of the radio signal from the center of the neutron star is affected by the gravity of the white dwarf, it "slows down" it. Scientists, measuring the time of arrival of radio signals on Earth, can determine with high accuracy the mass of the object "responsible" for the signal delay.
"We're very lucky with this system. A fast-spinning pulsar gives us a signal coming from an orbit that's perfectly placed. What's more, our white dwarf is quite large for a star of this type. This unique combination allows us to take full advantage of the Shapiro effect (gravitational signal delay) and simplifies measurements," says co-author Scott Ransom.
The binary system J1614-2230 is located in such a way that it can be observed almost edge-on, that is, in the plane of the orbit. This makes it easier to accurately measure the masses of its constituent stars.
As a result, the mass of the pulsar was equal to 1.97 solar masses, which was a record for neutron stars.
"These mass measurements tell us that if there are quarks at all in the core of a neutron star, they cannot be 'free', but, most likely, they must interact with each other much more strongly than in 'ordinary' atomic nuclei," explains the head group of astrophysicists dealing with this issue, Feryal Ozel (Feryal Ozel) from the University of Arizona.
"It surprises me that something as simple as the mass of a neutron star can say so much in so many different areas of physics and astronomy," says Ransom.
Astrophysicist Sergei Popov of the Sternberg State Astronomical Institute notes that the study of neutron stars can provide crucial information about the structure of matter.
"In terrestrial laboratories, it is impossible to study matter at a density much higher than nuclear. And this is very important for understanding how the world works. Fortunately, such dense matter exists in the depths of neutron stars. To determine the properties of this matter, it is very important to know what the maximum mass can be have a neutron star and not turn into a black hole," Popov told RIA Novosti.
Supernova remnant Korma-A, at the center of which is a neutron star
Neutron stars are the remnants of massive stars that have reached the end of their evolutionary path in time and space.
These interesting objects are born from once massive giants that are four to eight times the size of our Sun. It happens in a supernova explosion.
After such an explosion, the outer layers are ejected into space, the core remains, but it is no longer able to support nuclear fusion. Without external pressure from the overlying layers, it collapses and shrinks catastrophically.
Despite their small diameter - about 20 km, neutron stars boast 1.5 times the mass of our Sun. Thus, they are incredibly dense.
A small spoonful of star matter on Earth would weigh about a hundred million tons. In it, protons and electrons are combined into neutrons - this process is called neutronization.
Compound
Their composition is unknown; it is assumed that they may consist of a superfluid neutron liquid. They have an extremely strong gravitational pull, much stronger than that of the Earth and even the Sun. This gravitational force is especially impressive because it has a small size.
All of them rotate around an axis. During compression, the angular momentum of rotation is preserved, and due to a decrease in size, the rotation speed increases.
Due to the huge speed of rotation, the outer surface, which is a solid “crust”, periodically cracks and “starquakes” occur, which slow down the rotation speed and dump “excess” energy into space.
The overwhelming pressure that exists in the core may be similar to that which existed at the time of the big bang, but unfortunately it cannot be simulated on Earth. Therefore, these objects are ideal natural laboratories where we can observe energies inaccessible on Earth.
radio pulsars
Radio pulsars were discovered in late 1967 by graduate student Jocelyn Bell Burnell as radio sources that pulsate at a constant frequency.
The radiation emitted by the star is visible as a pulsating radiation source or pulsar.
Schematic representation of the rotation of a neutron star
Radio pulsars (or simply a pulsar) are spinning neutron stars whose jets of particles move at nearly the speed of light, like a spinning beacon beam.
After continuous rotation, for several million years, pulsars lose their energy and become normal neutron stars. Only about 1,000 pulsars are known today, although there may be hundreds of them in the galaxy.
Radio pulsar in the Crab Nebula
Some neutron stars emit X-rays. The famous Crab Nebula is a good example of such an object, formed during a supernova explosion. This supernova explosion was observed in 1054 AD.
Pulsar wind, Chandra video
A radio pulsar in the Crab Nebula photographed by the Hubble Space Telescope through a 547nm filter (green light) from August 7, 2000 to April 17, 2001.
magnetars
Neutron stars have a magnetic field millions of times stronger than the strongest magnetic field produced on Earth. They are also known as magnetars.
Planets near neutron stars
So far, four are known to have planets. When it is in a binary system, it is possible to measure its mass. Of these binary systems in the radio or X-ray range, the measured masses of neutron stars were about 1.4 times the mass of the Sun.
Double systems
A completely different type of pulsar is seen in some X-ray binaries. In these cases, a neutron star and an ordinary one form a binary system. A strong gravitational field pulls material from an ordinary star. Material falling on it during the accretion process heats up so much that it produces X-rays. Pulsed X-rays are visible when hot spots on a spinning pulsar pass through the line of sight from Earth.
For binary systems containing an unknown object, this information helps to distinguish whether it is a neutron star, or, for example, a black hole, because black holes are much more massive.
The objects that will be discussed in the article were discovered by chance, although the scientists L. D. Landau and R. Oppenheimer predicted their existence back in 1930. We are talking about neutron stars. The characteristics and features of these cosmic bodies will be discussed in the article.
The neutron and the star of the same name
After the prediction in the 30s of the XX century about the existence of neutron stars and after the discovery of the neutron (1932), Baade V., together with Zwicky F. in 1933 at the Congress of Physicists in America, announced the possibility of the formation of an object called neutron star. This is a cosmic body that occurs in the process of a supernova explosion.
However, all the calculations were only theoretical, since it was not possible to prove such a theory in practice due to the lack of appropriate astronomical equipment and the too small size of the neutron star. But in 1960 X-ray astronomy began to develop. Then, quite unexpectedly, neutron stars were discovered thanks to radio observations.
Opening
1967 was a significant year in this area. Bell D., being a graduate student of Hewish E., was able to discover a space object - a neutron star. This is a body emitting constant radiation of radio wave impulses. The phenomenon has been compared to a cosmic radio beacon due to the narrow focus of the radio beam, which came from a very fast rotating object. The fact is that any other standard star could not maintain its integrity at such a high rotational speed. Only neutron stars are capable of this, among which the pulsar PSR B1919+21 was the first to be discovered.
The fate of massive stars is very different from small ones. In such luminaries there comes a moment when the pressure of the gas no longer balances the gravitational forces. Such processes lead to the fact that the star begins to shrink (collapse) indefinitely. With a star mass exceeding the solar one by 1.5-2 times, the collapse will be inevitable. During the compression process, the gas inside the stellar core heats up. Everything happens very slowly at first.
Collapse
Reaching a certain temperature, the proton is able to turn into neutrinos, which immediately leave the star, taking energy with them. The collapse will intensify until all the protons turn into neutrinos. Thus, a pulsar, or neutron star, is formed. This is a collapsing core.
During the formation of a pulsar, the outer shell receives compression energy, which will then be at a speed of more than one thousand km / s. thrown into space. In this case, a shock wave is formed that can lead to new star formation. This one will be billions of times higher than the original one. After such a process, for a period of one week to a month, the star emits light in excess of an entire galaxy. Such a celestial body is called a supernova. Its explosion leads to the formation of a nebula. At the center of the nebula is a pulsar, or neutron star. This is the so-called descendant of the star that exploded.
Visualization
In the depths of the entire space of space, amazing events take place, among which is the collision of stars. Thanks to the most complex mathematical model, NASA scientists were able to visualize the rampage of a huge amount of energy and the degeneration of the matter involved in it. An incredibly powerful picture of a cosmic cataclysm is playing out before the eyes of observers. The probability that a collision of neutron stars will occur is very high. The meeting of two such luminaries in space begins with their entanglement in gravitational fields. Possessing a huge mass, they, so to speak, exchange hugs. Upon collision, a powerful explosion occurs, accompanied by an incredibly powerful release of gamma radiation.
If we consider a neutron star separately, then these are the remnants after a supernova explosion, in which the life cycle ends. The mass of a star surviving its age exceeds the solar one by 8-30 times. The universe is often illuminated by explosions of supernovae. The probability that neutron stars will meet in the universe is quite high.
Meeting
Interestingly, when two stars meet, the development of events cannot be unambiguously predicted. One of the options is described by a mathematical model proposed by NASA scientists from the Space Flight Center. The process begins when two neutron stars are located from each other in outer space at a distance of approximately 18 km. By cosmic standards, neutron stars with a mass of 1.5-1.7 times that of the sun are considered tiny objects. Their diameter varies within 20 km. Due to this discrepancy between volume and mass, a neutron star is the owner of the strongest gravitational and magnetic fields. Just imagine: a teaspoon of the matter of a neutron luminary weighs as much as the entire Mount Everest!
degeneration
The incredibly high gravitational waves of a neutron star acting around it are the reason why matter cannot be in the form of individual atoms that begin to break down. The matter itself passes into a degenerate neutron, in which the structure of the neutrons themselves will not allow the star to pass into a singularity and then into a black hole. If the mass of degenerate matter begins to increase due to the addition to it, then the gravitational forces will be able to overcome the resistance of neutrons. Then nothing will prevent the destruction of the structure formed as a result of the collision of neutron stellar objects.
Mathematical model
Studying these celestial objects, scientists came to the conclusion that the density of a neutron star is comparable to the density of matter in the nucleus of an atom. Its performance ranges from 1015 kg/m³ to 1018 kg/m³. Thus, independent existence of electrons and protons is impossible. The matter of a star practically consists of only neutrons.
The created mathematical model demonstrates how powerful periodic gravitational interactions that arise between two neutron stars break through the thin shell of two stars and throw out a huge amount of radiation (energy and matter) into the space surrounding them. The process of rapprochement is very fast, literally in a fraction of a second. As a result of the collision, a toroidal ring of matter is formed with a newborn black hole in the center.
Importance
Modeling such events is essential. Thanks to them, scientists were able to understand how a neutron star and a black hole are formed, what happens when stars collide, how supernovae are born and die, and many other processes in outer space. All these events are the source of the appearance of the heaviest chemical elements in the Universe, even heavier than iron, unable to form in any other way. This speaks of the very important importance of neutron stars throughout the universe.
The rotation of a celestial object of enormous volume around its axis is amazing. Such a process causes a collapse, but with all this, the mass of a neutron star remains practically the same. If we imagine that the star will continue to shrink, then, according to the law of conservation of angular momentum, the angular velocity of rotation of the star will increase to incredible values. If it took about 10 days for a star to make a complete revolution, then as a result it will complete the same revolution in 10 milliseconds! These are incredible processes!
collapse development
Scientists are investigating such processes. Perhaps we will witness new discoveries, which so far seem fantastic to us! But what can be if we imagine the development of the collapse further? To make it easier to imagine, let's take a neutron star/earth pair and their gravitational radii for comparison. So, with continuous compression, a star can reach a state where neutrons begin to turn into hyperons. The radius of the celestial body will become so small that we will face a lump of a superplanetary body with the mass and gravitational field of a star. This can be compared to the fact that the earth became equal in size to a ping-pong ball, and the gravitational radius of our luminary, the Sun, would be equal to 1 km.
If we imagine that a small lump of stellar matter has the attraction of a huge star, then it is able to hold an entire planetary system near it. But the density of such a celestial body is too high. Rays of light gradually cease to penetrate through it, the body, as it were, goes out, it ceases to be visible to the eye. Only the gravitational field does not change, which warns that there is a gravitational hole here.
Discoveries and observations
For the first time from the merger of neutron stars were recorded quite recently: August 17th. Two years ago, a black hole merger was registered. This is such an important event in the field of astrophysics that observations were carried out simultaneously by 70 space observatories. Scientists were able to verify the correctness of the hypotheses about gamma-ray bursts, they were able to observe the synthesis of heavy elements described earlier by theorists.
Such widespread observation of the gamma-ray burst, gravitational waves and visible light made it possible to determine the region in the sky in which a significant event occurred, and the galaxy where these stars were. This is NGC 4993.
Of course, astronomers have been observing short ones for a long time. But until now, they could not say for sure about their origin. Behind the main theory was a version of the merger of neutron stars. Now she has been confirmed.
To describe a neutron star using the mathematical apparatus, scientists turn to the equation of state, which relates the density to the pressure of matter. However, there are a lot of such options, and scientists simply do not know which of the existing ones will be correct. It is hoped that gravitational observations will help resolve this issue. At the moment, the signal has not given an unambiguous answer, but it already helps to assess the shape of the star, which depends on the gravitational attraction to the second luminary (star).
The hypothesis of the existence of neutron stars was put forward by astronomers W. Baade and F. Zwicky immediately after the discovery of the neutron in 1932. But this hypothesis was confirmed by observations only after the discovery of pulsars in 1967.
Neutron stars are formed as a result of the gravitational collapse of normal stars with masses several times that of the Sun. The density of a neutron star is close to the density of an atomic nucleus, i.e. 100 million times higher than the density of ordinary matter. Therefore, with its huge mass, a neutron star has a radius of only approx. 10 km.
Due to the small radius of a neutron star, the force of gravity on its surface is extremely high: about 100 billion times higher than on Earth. This star is kept from collapse by the “degeneracy pressure” of dense neutron matter, which does not depend on its temperature. However, if the mass of the neutron star becomes greater than about 2 solar masses, then gravity will exceed this pressure and the star will not be able to withstand the collapse.
Neutron stars have a very strong magnetic field, reaching 10 12 -10 13 gauss on the surface (for comparison: the Earth has about 1 gauss). Two different types of celestial objects are associated with neutron stars.
Pulsars
(radio pulsars). These objects strictly regularly emit pulses of radio waves. The radiation mechanism is not completely clear, but it is believed that a rotating neutron star emits a radio beam in the direction associated with its magnetic field, the symmetry axis of which does not coincide with the axis of rotation of the star. Therefore, the rotation causes the rotation of the radio beam periodically sent to the Earth.
X-ray doubles.
Pulsating X-ray sources are also associated with neutron stars that are part of a binary system with a massive normal star. In such systems, gas from the surface of a normal star falls onto a neutron star, accelerating to tremendous speed. When hitting the surface of a neutron star, the gas releases 10-30% of its rest energy, while in nuclear reactions this figure does not even reach 1%. The surface of a neutron star heated to a high temperature becomes a source of X-rays. However, the fall of gas does not occur uniformly over the entire surface: the strong magnetic field of a neutron star captures the falling ionized gas and directs it to the magnetic poles, where it falls, like into a funnel. Therefore, only the regions of the poles become strongly heated, which on a rotating star become sources of X-ray pulses. Radio pulses from such a star no longer arrive, since radio waves are absorbed in the gas surrounding it.
Compound.
The density of a neutron star increases with depth. Under a layer of atmosphere only a few centimeters thick, there is a liquid metal shell several meters thick, and below - a solid crust kilometer thick. The substance of the bark resembles ordinary metal, but is much denser. In the outer part of the crust, it is mainly iron; the fraction of neutrons in its composition increases with depth. Where the density reaches approx. 4Ch 10 11 g/cm 3 , the proportion of neutrons increases so much that some of them are no longer part of the nuclei, but form a continuous medium. There, the substance looks like a "sea" of neutrons and electrons, in which the nuclei of atoms are interspersed. And at a density of approx. 2× 10 14 g/cm 3 (density of the atomic nucleus), individual nuclei disappear altogether and a continuous neutron "liquid" with an admixture of protons and electrons remains. Probably, neutrons and protons behave in this case as a superfluid liquid, similar to liquid helium and superconducting metals in terrestrial laboratories.
Often referred to as "dead" neutron stars are amazing objects. Their study in recent decades has become one of the most fascinating and rich in discoveries in astrophysics. Interest in neutron stars is due not only to the mystery of their structure, but also to their colossal density, and the strongest magnetic and gravitational fields. Matter there is in a special state resembling a huge atomic nucleus, and these conditions cannot be reproduced in terrestrial laboratories.
Birth at the tip of a pen
The discovery in 1932 of a new elementary particle, the neutron, made astrophysicists think about what role it could play in the evolution of stars. Two years later, it was suggested that supernova explosions are associated with the transformation of ordinary stars into neutron stars. Then the structure and parameters of the latter were calculated, and it became clear that if small stars (such as our Sun) turn into white dwarfs at the end of their evolution, then heavier ones become neutron ones. In August 1967, radio astronomers, while studying the scintillations of cosmic radio sources, discovered strange signals - very short, about 50 milliseconds long, radio emission pulses were recorded, repeating after a strictly defined time interval (of the order of one second). It was completely different from the usual chaotic picture of random irregular fluctuations in radio emission. After a thorough check of all the equipment, confidence came that the impulses were of extraterrestrial origin. It is difficult to surprise astronomers with objects that radiate with variable intensity, but in this case the period was so short, and the signals were so regular, that scientists seriously suggested that they could be messages from extraterrestrial civilizations.
Therefore, the first pulsar was named LGM-1 (from the English Little Green Men “Little Green Men”), although attempts to find any meaning in the received pulses ended in vain. Soon, 3 more pulsating radio sources were discovered. Their period again turned out to be much less than the characteristic oscillation and rotation times of all known astronomical objects. Due to the impulsive nature of the radiation, new objects began to be called pulsars. This discovery literally stirred up astronomy, and reports of the discovery of pulsars began to arrive from many radio observatories. After the discovery of a pulsar in the Crab Nebula, which arose due to a supernova explosion in 1054 (this star was visible during the day, as the Chinese, Arabs and North Americans mention in their annals), it became clear that pulsars are somehow connected with supernova explosions. .
Most likely, the signals came from the object left after the explosion. It took a long time before astrophysicists realized that pulsars were the rapidly rotating neutron stars they had been looking for.
crab nebula
The flash of this supernova (photo above), sparkling in the earth's sky brighter than Venus and visible even during the day, occurred in 1054 according to earth clocks. Almost 1,000 years is a very short time by cosmic standards, and yet, during this time, the most beautiful Crab Nebula managed to form from the remnants of the exploded star. This image is a composite of two images, one from the Hubble Space Telescope (shades of red) and the other from the Chandra X-ray telescope (blue). It is clearly seen that high-energy electrons emitting in the X-ray range lose their energy very quickly, so blue colors prevail only in the central part of the nebula.
Combining the two images helps to more accurately understand the mechanism of operation of this amazing space generator, which emits electromagnetic oscillations of the widest frequency range from gamma quanta to radio waves. Although most neutron stars have been detected by radio emission, they still emit the main amount of energy in the gamma and x-ray ranges. Neutron stars are born very hot, but they cool quite quickly, and already at a thousand years old have a surface temperature of about 1,000,000 K. Therefore, only young neutron stars shine in the X-ray range due to purely thermal radiation.
Pulsar physics
A pulsar is simply a huge magnetized top spinning around an axis that does not coincide with the axis of the magnet. If nothing fell on it and it did not emit anything, then its radio emission would have a rotation frequency and we would never hear it on Earth. But the fact is that this top has a colossal mass and high surface temperature, and the rotating magnetic field creates an electric field of enormous intensity, capable of accelerating protons and electrons to almost light speeds. Moreover, all these charged particles rushing around the pulsar are trapped in a trap from its colossal magnetic field. And only within a small solid angle near the magnetic axis, they can break free (neutron stars have the strongest magnetic fields in the Universe, reaching 10 10 10 14 gauss, for comparison: the terrestrial field is 1 gauss, the solar 1050 gauss) . It is these streams of charged particles that are the source of that radio emission, according to which pulsars were discovered, which later turned out to be neutron stars. Since the magnetic axis of a neutron star does not necessarily coincide with the axis of its rotation, when the star rotates, the stream of radio waves propagates in space like the beam of a flashing beacon - only for a moment cutting through the surrounding darkness.
X-ray images of the Crab Nebula pulsar in active (left) and normal (right) states
nearest neighbor
This pulsar is only 450 light-years from Earth and is a binary system of a neutron star and a white dwarf with an orbital period of 5.5 days. Soft X-rays received by the ROSAT satellite are emitted by polar caps PSR J0437-4715 heated up to two million degrees. In the process of its rapid rotation (the period of this pulsar is 5.75 milliseconds), it turns to the Earth with one or the other magnetic pole, as a result, the intensity of the gamma-ray flux changes by 33%. The bright object next to the small pulsar is a distant galaxy, which for some reason is actively glowing in the X-ray part of the spectrum.
Omnipotent gravity
According to modern evolutionary theory, massive stars end their lives in a colossal explosion that turns most of them into an expanding gaseous nebula. As a result, from the giant, many times larger than our Sun in size and mass, there remains a dense hot object about 20 km in size, with a thin atmosphere (made of hydrogen and heavier ions) and a gravitational field 100 billion times greater than the earth's. They called it a neutron star, believing that it consists mainly of neutrons. The substance of a neutron star is the densest form of matter (a teaspoon of such a supernucleus weighs about a billion tons). The very short period of signals emitted by pulsars was the first and most important argument in favor of the fact that these are neutron stars, which have a huge magnetic field and rotate at breakneck speed. Only dense and compact objects (only a few tens of kilometers in size) with a powerful gravitational field can withstand such a rotation speed without breaking into pieces due to the centrifugal forces of inertia.
A neutron star consists of a neutron liquid with an admixture of protons and electrons. "Nuclear liquid", very reminiscent of a substance from atomic nuclei, is 1014 times denser than ordinary water. This huge difference is quite understandable, because atoms are mostly empty space, in which light electrons flutter around a tiny heavy nucleus. The nucleus contains almost all the mass, since protons and neutrons are 2,000 times heavier than electrons. The extreme forces that occur during the formation of a neutron star compress the atoms so that the electrons pressed into the nuclei combine with protons to form neutrons. Thus, a star is born, almost entirely composed of neutrons. The superdense nuclear liquid, if brought to Earth, would explode like a nuclear bomb, but in a neutron star it is stable due to the enormous gravitational pressure. However, in the outer layers of a neutron star (as, indeed, of all stars), pressure and temperature drop, forming a solid crust about a kilometer thick. It is believed to consist mainly of iron nuclei.
Flash
The colossal X-ray flash of March 5, 1979, it turns out, occurred far beyond our Galaxy, in the Large Magellanic Cloud satellite of our Milky Way, located at a distance of 180 thousand light years from Earth. Joint processing of the gamma-ray burst on March 5, recorded by seven spacecraft, made it possible to accurately determine the position of this object, and today there is practically no doubt that it is located in the Magellanic Cloud.
The event that happened on this distant star 180 thousand years ago is hard to imagine, but it then flared up like as many as 10 supernovae, more than 10 times the luminosity of all the stars in our Galaxy. The bright dot in the upper part of the figure is the long and well-known SGR pulsar, and the irregular contour is the most probable position of the object that erupted on March 5, 1979.
Origin of the neutron star
A supernova explosion is simply the conversion of some of the gravitational energy into thermal energy. When the old star runs out of fuel and the thermonuclear reaction can no longer heat up its interior to the required temperature, a kind of collapse occurs - the gas cloud collapses onto its center of gravity. The energy released at the same time scatters the outer layers of the star in all directions, forming an expanding nebula. If the star is small, like our Sun, then a flash occurs and a white dwarf is formed. If the mass of the star is more than 10 times that of the Sun, then such a collapse leads to a supernova explosion and an ordinary neutron star is formed. If a supernova flares up in place of a very large star, with a mass of 2040 Solar, and a neutron star with a mass greater than three Suns is formed, then the process of gravitational compression becomes irreversible and a black hole is formed.
Internal structure
The hard crust of the outer layers of a neutron star is made up of heavy atomic nuclei arranged in a cubic lattice, with electrons flying freely between them, similar to Earth's metals, only much denser.
Open question
Although neutron stars have been intensively studied for about three decades, their internal structure is not known for certain. Moreover, there is no firm certainty that they really consist mainly of neutrons. As we move deeper into the star, pressure and density increase, and matter can be so compressed that it breaks up into quarks, the building blocks of protons and neutrons. According to modern quantum chromodynamics, quarks cannot exist in a free state, but are combined into inseparable "triples" and "twos". But, perhaps, at the boundary of the inner core of a neutron star, the situation changes and quarks break out of their confinement. To better understand the nature of a neutron star and exotic quark matter, astronomers need to determine the relationship between a star's mass and its radius (average density). By examining neutron stars with companions, one can accurately measure their mass, but determining the diameter is much more difficult. More recently, scientists using the capabilities of the XMM-Newton X-ray satellite have found a way to estimate the density of neutron stars based on gravitational redshift. The unusualness of neutron stars also lies in the fact that with a decrease in the mass of a star, its radius increases as a result, the most massive neutron stars have the smallest size.
Black Widow
The explosion of a supernova quite often informs a newborn pulsar of considerable speed. Such a flying star with a decent magnetic field of its own strongly perturbs the ionized gas that fills interstellar space. A kind of shock wave is formed, running ahead of the star and diverging in a wide cone after it. The combined optical (blue-green part) and X-ray (shades of red) image shows that here we are dealing not just with a luminous gas cloud, but with a huge flux of elementary particles emitted by this millisecond pulsar. The linear speed of the Black Widow is 1 million km/h, it rotates around its axis in 1.6 ms, it is already about a billion years old, and it has a companion star circling around the Widow with a period of 9.2 hours. The pulsar B1957 + 20 got its name for the simple reason that its most powerful radiation simply burns its neighbor, causing the gas that forms it to “boil” and evaporate. The red cigar-shaped cocoon behind the pulsar is the part of space where electrons and protons emitted by the neutron star emit soft gamma rays.
The result of computer simulation makes it possible to visualize, in a section, the processes occurring near a fast-flying pulsar. Rays diverging from a bright point this is a conditional image of that flow of radiant energy, as well as the flow of particles and antiparticles, which comes from a neutron star. The red outline on the border of the black space around the neutron star and the red glowing plasma puffs is the place where the stream of relativistic particles flying almost at the speed of light meets with the interstellar gas condensed by the shock wave. When decelerating sharply, the particles emit X-rays and, having lost their main energy, do not heat up the incident gas so much.
Convulsions of the giants
Pulsars are considered one of the early life stages of a neutron star. Thanks to their study, scientists learned about magnetic fields, and about the speed of rotation, and about the future fate of neutron stars. By constantly observing the behavior of a pulsar, one can determine exactly how much energy it loses, how much it slows down, and even when it ceases to exist, having slowed down enough to not be able to emit powerful radio waves. These studies confirmed many theoretical predictions about neutron stars.
Already by 1968, pulsars with a rotation period of 0.033 seconds to 2 seconds were discovered. The frequency of the radio pulsar pulses is maintained with amazing accuracy, and at first the stability of these signals was higher than the earth's atomic clock. And yet, with the progress in the field of time measurement for many pulsars, it was possible to register regular changes in their periods. Of course, these are extremely small changes, and only over millions of years can we expect a period to double. The ratio of the current rotation rate to the rotation deceleration is one way to estimate the age of a pulsar. Despite the astonishing stability of the radio signal, some pulsars sometimes experience so-called "disturbances". For a very short time interval (less than 2 minutes), the pulsar rotation speed increases by a significant amount, and then after some time returns to the value that was before the "violation". It is believed that the "violations" may be caused by a rearrangement of mass within the neutron star. But in any case, the exact mechanism is still unknown.
Thus, the Vela pulsar is subjected to large “violations” about once every 3 years, and this makes it a very interesting object for studying such phenomena.
magnetars
Some neutron stars, called SGR repetitive bursters, emit powerful bursts of "soft" gamma rays at irregular intervals. The amount of energy emitted by SGR during a typical flash, lasting a few tenths of a second, the Sun can radiate only for a whole year. Four known SGRs are within our Galaxy and only one is outside it. These incredible explosions of energy can be caused by starquakes, powerful versions of earthquakes, when the solid surface of neutron stars is torn apart and powerful streams of protons burst from their interiors, which, bogged down in a magnetic field, emit gamma and X-rays. Neutron stars were identified as sources of powerful gamma-ray bursts after a huge gamma-ray burst on March 5, 1979, when as much energy was thrown out in the first second as the sun emits in 1,000 years. Recent observations of one of today's most "active" neutron stars seem to support the theory that powerful bursts of gamma and X-rays are caused by starquakes.
In 1998, the well-known SGR suddenly woke up from its "slumber", which had not shown signs of activity for 20 years and splashed out almost as much energy as the gamma-ray flash on March 5, 1979. What struck the researchers most when observing this event was a sharp slowdown in the speed of rotation of the star, indicating its destruction. To explain powerful gamma and x-ray flares, a model of a magnetar, a neutron star with a superstrong magnetic field, was proposed. If a neutron star is born spinning very fast, then the combined effect of rotation and convection, which plays an important role in the first few seconds of the existence of a neutron star, can create a huge magnetic field through a complex process known as an “active dynamo” (the same way a field is created inside the Earth and the Sun). Theorists were amazed to discover that such a dynamo, operating in a hot, newborn neutron star, could create a magnetic field 10,000 times stronger than the normal field of pulsars. When the star cools down (after 10 or 20 seconds), convection and dynamo action stop, but this time is quite enough for the necessary field to appear.
The magnetic field of a rotating electrically conducting ball can be unstable, and a sharp restructuring of its structure can be accompanied by the release of colossal amounts of energy (a clear example of such instability is the periodic reversal of the Earth's magnetic poles). Similar things happen on the Sun, in explosive events called "solar flares." In a magnetar, the available magnetic energy is enormous, and this energy is quite enough for the power of such giant flares as March 5, 1979 and August 27, 1998. Such events inevitably cause a deep breakdown and changes in the structure of not only electric currents in the volume of a neutron star, but also its solid crust. Another mysterious type of object that emits powerful X-rays during periodic explosions are the so-called anomalous X-ray pulsars AXP. They differ from ordinary X-ray pulsars in that they emit only in the X-ray range. Scientists believe that SGR and AXP are life phases of the same class of objects, namely magnetars, or neutron stars, which emit soft gamma rays, drawing energy from the magnetic field. And although magnetars today remain the brainchild of theorists and there is not enough data confirming their existence, astronomers are stubbornly looking for the necessary evidence.
Candidates for Magnetars
Astronomers have already studied our own galaxy, the Milky Way, so thoroughly that it costs them nothing to draw a side view of it, marking the position of the most remarkable of the neutron stars on it.
Scientists believe that AXP and SGR are just two stages in the life of the same giant magnet a neutron star. For the first 10,000 years, a magnetar is an SGR pulsar, visible in ordinary light and giving repeated flashes of soft X-rays, and for the next millions of years, already as an anomalous AXP pulsar, it disappears from the visible range and puffs only in X-rays.
The strongest magnet
An analysis of the data obtained by the RXTE (Rossi X-ray Timing Explorer, NASA) satellite during observations of the unusual pulsar SGR 1806-20 showed that this source is the most powerful magnet known to date in the Universe. The magnitude of its field was determined not only on the basis of indirect data (on the slowing down of a pulsar), but also almost directly on the basis of measuring the rotation frequency of protons in the magnetic field of a neutron star. The magnetic field near the surface of this magnetar reaches 10 15 gauss. If it were, for example, in the orbit of the Moon, all magnetic information carriers on our Earth would be demagnetized. True, given that its mass is approximately equal to that of the Sun, this would no longer matter, because even if the Earth had not fallen on this neutron star, it would have revolved around it like crazy, making a complete revolution in just an hour.
Active dynamo
We all know that energy loves to change from one form to another. Electricity is easily converted into heat, and kinetic energy into potential energy. Huge convective flows of electrically conductive magma, plasma or nuclear matter, it turns out, can also convert their kinetic energy into something unusual, such as a magnetic field. The movement of large masses on a rotating star in the presence of a small initial magnetic field can lead to electric currents that create a field in the same direction as the original one. As a result, an avalanche-like growth of the own magnetic field of a rotating conductive object begins. The larger the field, the larger the currents, the larger the currents, the larger the field and all this is due to banal convective flows due to the fact that hot matter is lighter than cold, and therefore floats
Restless Neighborhood
The famous Chandra space observatory has discovered hundreds of objects (including in other galaxies), indicating that not all neutron stars are destined to live alone. Such objects are born in binary systems that survived the supernova explosion that created the neutron star. And sometimes it happens that single neutron stars in dense stellar regions such as globular clusters capture a companion. In this case, the neutron star will "steal" matter from its neighbor. And depending on how massive the star will keep her company, this "theft" will cause different consequences. The gas flowing from a companion with a mass less than that of our Sun, on such a "crumb" as a neutron star, will not be able to immediately fall due to its own too large angular momentum, so it creates a so-called accretion disk around it from the "stolen » matter. Friction during winding around a neutron star and compression in a gravitational field heats up the gas to millions of degrees, and it begins to emit X-rays. Another interesting phenomenon associated with neutron stars that have a low-mass companion is X-ray bursts (bursters). They usually last from a few seconds to several minutes and, at their maximum, give the star a luminosity nearly 100,000 times that of the Sun.
These outbursts are explained by the fact that when hydrogen and helium are transferred to a neutron star from a companion, they form a dense layer. Gradually, this layer becomes so dense and hot that a thermonuclear fusion reaction begins and a huge amount of energy is released. In terms of power, this is equivalent to the explosion of the entire nuclear arsenal of earthlings on every square centimeter of the surface of a neutron star within a minute. A completely different picture is observed if the neutron star has a massive companion. A giant star loses matter in the form of a stellar wind (a stream of ionized gas emanating from its surface), and the enormous gravity of a neutron star captures some of this matter for itself. But this is where the magnetic field comes into play, causing the falling matter to flow along lines of force toward the magnetic poles.
This means that X-rays are primarily generated at hot spots at the poles, and if the magnetic axis and the axis of rotation of the star do not coincide, then the brightness of the star turns out to be variable this is also a pulsar, but only X-ray. Neutron stars in X-ray pulsars have bright giant stars as companions. In bursters, the companions of neutron stars are low-mass stars of low brightness. The age of bright giants does not exceed a few tens of millions of years, while the age of faint dwarf stars can be billions of years, since the former consume their nuclear fuel much faster than the latter. It follows that bursters are old systems in which the magnetic field has weakened over time, while pulsars are relatively young, and therefore the magnetic fields in them are stronger. Maybe bursters once pulsated in the past, and pulsars have yet to flare in the future.
Pulsars with the shortest periods (less than 30 milliseconds), the so-called millisecond pulsars, are also associated with binary systems. Despite their rapid rotation, they are not the youngest, as one would expect, but the oldest.
They arise from binary systems, where an old, slowly rotating neutron star begins to absorb matter from its already aged companion (usually a red giant). Falling onto the surface of a neutron star, matter transfers rotational energy to it, causing it to spin faster and faster. This happens until the companion of the neutron star, almost freed from excess mass, becomes a white dwarf, and the pulsar comes to life and begins to rotate at a speed of hundreds of revolutions per second. However, astronomers have recently discovered a very unusual system where the companion of a millisecond pulsar is not a white dwarf, but a giant bloated red star. Scientists believe that they are observing this binary system just in the stage of "liberation" of the red star from excess weight and transformation into a white dwarf. If this hypothesis is wrong, then the companion star could be an ordinary globular cluster star accidentally captured by a pulsar. Almost all neutron stars that are currently known have been found either in X-ray binaries or as single pulsars.
And just recently, Hubble noticed in visible light a neutron star, which is not a component of a binary system and does not pulsate in the X-ray and radio range. This provides a unique opportunity to accurately determine its size and make adjustments to the understanding of the composition and structure of this bizarre class of burnt out, gravitationally compressed stars. This star was discovered for the first time as an X-ray source and emits in this range, not because it collects hydrogen gas as it moves through space, but because it is still young. Perhaps it is the remnant of one of the stars of the binary system. As a result of a supernova explosion, this binary system collapsed and the former neighbors began an independent journey through the Universe.
Baby eater of stars
As stones fall to the ground, so does a large star, releasing its mass bit by bit, gradually moving to a small and distant neighbor, which has a huge gravitational field near its surface. If the stars did not revolve around a common center of gravity, then the gas stream could simply flow, like a stream of water from a mug, onto a small neutron star. But since the stars circle in a round dance, the falling matter, before it reaches the surface, must lose most of its angular momentum. And here the mutual friction of particles moving along different trajectories and the interaction of the ionized plasma forming the accretion disk with the magnetic field of the pulsar help the process of falling matter to successfully end with an impact on the surface of a neutron star in the region of its magnetic poles.
Mystery 4U2127 Solved
This star has been fooling astronomers for more than 10 years, showing a strange slow variability in its parameters and flaring up differently each time. Only the latest research from the Chandra space observatory has made it possible to unravel the mysterious behavior of this object. It turned out that this is not one, but two neutron stars. Moreover, both of them have companions one star, similar to our Sun, the other to a small blue neighbor. Spatially, these pairs of stars are separated by a sufficiently large distance and live an independent life. But on the stellar sphere, they are projected almost to one point, which is why they were considered one object for so long. These four stars are located in the globular cluster M15 at a distance of 34 thousand light years.
Open question
In total, astronomers have discovered about 1,200 neutron stars to date. Of these, more than 1,000 are radio pulsars, and the rest are simply X-ray sources. Over the years of research, scientists have come to the conclusion that neutron stars are real originals. Some are very bright and calm, others periodically flare up and change with starquakes, and others exist in binary systems. These stars are among the most mysterious and elusive astronomical objects, combining the strongest gravitational and magnetic fields and extreme densities and energies. And each new discovery from their turbulent life provides scientists with unique information necessary for understanding the nature of Matter and the evolution of the Universe.
Universal standard
It is very difficult to send something outside the solar system, therefore, together with the Pioneer-10 and -11 spacecraft that went there 30 years ago, earthlings also sent messages to their brothers in mind. To draw something that will be understandable to the Extraterrestrial Mind, the task is not an easy one, moreover, it was also necessary to indicate the return address and the date of sending the letter... indicating the place and time of sending the message is ingenious. Discontinuous rays of various lengths, emanating from a point symbolizing the Sun, indicate the direction and distance to the nearest pulsars to the Earth, and the discontinuity of the line is nothing more than a binary designation of their period of revolution. The longest beam points to the center of our galaxy, the Milky Way. The frequency of the radio signal emitted by the hydrogen atom when changing the mutual orientation of the spins (direction of rotation) of the proton and electron is taken as the unit of time on the message.
The famous 21 cm or 1420 MHz should be known to all intelligent beings in the universe. According to these landmarks, pointing to the "radio beacons" of the Universe, it will be possible to find earthlings even after many millions of years, and by comparing the recorded frequency of pulsars with the current one, it will be possible to estimate when these man and woman blessed the first spacecraft that left the solar system.
Nikolay Andreev
