MOSCOW, August 28 - RIA Novosti. Scientists have discovered a record-heavy neutron star with twice the mass of the Sun, forcing them to reconsider a number of theories, in particular the theory that there may be "free" quarks inside the super-dense matter of neutron stars, according to a paper published Thursday in journal Nature.
A neutron star is the “corpse” of a star left behind after a supernova explosion. Its size does not exceed the size of a small city, but the density of the matter is 10-15 times higher than the density of an atomic nucleus - a “pinch” of the matter of a neutron star weighs more than 500 million tons.
Gravity “presses” electrons into protons, turning them into neutrons, which is why neutron stars get their name. Until recently, scientists believed that the mass of a neutron star could not 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 double star J1614-2230, three thousand light years from Earth, one of whose components is a neutron star and the other a white dwarf.
In this case, a neutron star is a pulsar, that is, a star emitting narrowly directed fluxes of radio emission; as a result of the star’s rotation, the radiation flux can be detected from the Earth’s surface using radio telescopes at different time intervals.
The white dwarf and neutron star rotate relative to each other. However, the speed of passage of a radio signal from the center of a neutron star is affected by the gravity of the white dwarf; it “slows down” it. Scientists, by measuring the time of arrival of radio signals on Earth, can accurately determine the mass of the object “responsible” for the signal delay.
"We are very lucky with this system. The rapidly spinning pulsar gives us a signal coming from an orbit that is perfectly positioned. Moreover, our white dwarf is quite large for stars of this type. This unique combination allows us to take full advantage of the Shapiro effect (gravitational delay of the signal) and simplifies measurements,” says one of the authors of the paper, 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 orbital plane. This makes it easier to accurately measure the masses of its constituent stars.
As a result, the mass of the pulsar turned out to be equal to 1.97 solar masses, which became 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 rather must interact with each other much more strongly than in ‘regular’ atomic nuclei,” explains the leader a group of astrophysicists working on this issue, Feryal Ozel from Arizona State University.
"It's amazing to me that something as simple as the mass of a neutron star can tell so much in different areas of physics and astronomy," Ransom says.
Astrophysicist Sergei Popov from the Sternberg State Astronomical Institute notes that studying neutron stars can provide vital information about the structure of matter.
“In terrestrial laboratories it is impossible to study matter with 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 find out what the maximum mass can be have a neutron star and not turn into a black hole,” Popov told RIA Novosti.
Neutron stars, often called “dead” stars, are amazing objects. Their study in recent decades has become one of the most fascinating and discovery-rich areas of astrophysics. Interest in neutron stars is due not only to the mystery of their structure, but also to their colossal density and strong magnetic and gravitational fields. The matter there is in a special state, reminiscent of a huge atomic nucleus, and these conditions cannot be reproduced in earthly laboratories.
Birth at the tip of a pen
The discovery of a new elementary particle, the neutron, in 1932 led astrophysicists to wonder what role it might 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 calculations were made of the structure and parameters of the latter, and it became clear that if small stars (like our Sun) at the end of their evolution turn into white dwarfs, then heavier ones become neutron ones. In August 1967, radio astronomers, while studying the flickering of cosmic radio sources, discovered strange signals: very short, lasting about 50 milliseconds, pulses of radio emission were recorded, repeated at a strictly defined time interval (about one second). This was completely different from the usual chaotic picture of random irregular fluctuations in radio emission. After a thorough check of all the equipment, we became confident that the pulses were of extraterrestrial origin. It is difficult for astronomers to be surprised by objects emitting 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 news 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 times of vibration and rotation of all known astronomical objects. Due to the pulsed nature of the radiation, new objects began to be called pulsars. This discovery literally shook up astronomy, and reports of pulsar detections began to come 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 related to supernova explosions .
Most likely, the signals came from an 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 for so long.
Crab Nebula
The outbreak 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 period of time by cosmic standards, and yet during this time the beautiful Crab Nebula managed to form from the remains of the exploding star. This image is a composition of two pictures: one of them was obtained by the Hubble Space Optical Telescope (shades of red), the other by the Chandra X-ray telescope (blue). It is clearly seen that high-energy electrons emitting in the X-ray range very quickly lose their energy, so blue colors prevail only in the central part of the nebula.
Combining two images helps to more accurately understand the mechanism of operation of this amazing cosmic generator, emitting electromagnetic oscillations of the widest frequency range - from gamma rays to radio waves. Although most neutron stars have been detected by radio emission, they emit the bulk of their energy in the gamma-ray and x-ray ranges. Neutron stars are born very hot, but cool quickly enough, and already at a thousand years of age they 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 rotational frequency and we would never hear it on Earth. But the fact is that this top has a colossal mass and a high surface temperature, and the rotating magnetic field creates a huge electric field, capable of accelerating protons and electrons almost to the speed of light. Moreover, all these charged particles rushing around the pulsar are trapped in its colossal magnetic field. And only within a small solid angle about 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 earth’s field is 1 gauss, the solar one 10 50 gauss) . It is these streams of charged particles that are the source of the radio emission from 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, a stream of radio waves propagates through space like the beam of a flashing beacon, only momentarily cutting through the surrounding darkness.
X-ray images of the Crab Nebula pulsar in its active (left) and normal (right) states
nearest neighbor
This pulsar is located 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. The soft X-ray radiation received by the ROSAT satellite is emitted by the polar ice caps PSR J0437-4715, which are heated to two million degrees. During its rapid rotation (the period of this pulsar is 5.75 milliseconds), it turns toward 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 that, for some reason, actively glows in the X-ray region of the spectrum.
Almighty Gravity
According to modern evolutionary theory, massive stars end their lives in a colossal explosion, turning most of them into an expanding nebula of gas. As a result, what remains from a giant many times larger than our Sun in size and mass is a dense hot object about 20 km in size, with a thin atmosphere (of hydrogen and heavier ions) and a gravitational field 100 billion times greater than that of the Earth. It was called a neutron star, believing that it consists mainly of neutrons. Neutron star matter 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, possessing a huge magnetic field and rotating 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 falling into pieces due to centrifugal inertial forces.
A neutron star consists of a neutron liquid mixed with protons and electrons. “Nuclear liquid,” which closely resembles the substance of atomic nuclei, is 1014 times denser than ordinary water. This huge difference is understandable, since atoms consist mostly of empty space, in which light electrons flit 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 generated by the formation of a neutron star compress the atoms so much that the electrons squeezed into the nuclei combine with protons to form neutrons. In this way, a star is born, consisting almost entirely of neutrons. The super-dense 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 flare of March 5, 1979, it turns out, occurred far beyond our Galaxy, in the Large Magellanic Cloud, a 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 quite accurately determine the position of this object, and the fact that it is located precisely in the Magellanic Cloud is now practically beyond doubt.
The event that happened on this distant star 180 thousand years ago is difficult to imagine, but it flashed then like 10 supernovae, more than 10 times the luminosity of all the stars in our Galaxy. The bright dot at the top of the figure is a long-known and well-known SGR pulsar, and the irregular outline is the most likely position of the object that flared up on March 5, 1979.
Origin of the neutron star
A supernova explosion is simply the conversion of part of the gravitational energy into heat. When an old star runs out of fuel and the thermonuclear reaction can no longer heat its interior to the required temperature, a collapse of the gas cloud occurs at its center of gravity. The energy released during this process scatters the outer layers of the star in all directions, forming an expanding nebula. If the star is small, like our Sun, then an outburst 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 erupts in the place of a very large star, with a mass of 20 x 40 solar, and a neutron star with a mass of more than three solar is formed, then the process of gravitational compression becomes irreversible and a black hole is formed.
Internal structure
The solid crust of the outer layers of a neutron star consists of heavy atomic nuclei arranged in a cubic lattice, with electrons flying freely between them, which is reminiscent of terrestrial metals, but 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 you move deeper into the star, pressure and density increase and matter can be so compressed that it breaks down 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 “threes” and “twos”. But perhaps, at the boundary of the inner core of a neutron star, the situation changes and the quarks break out of their confinement. To further understand the nature of a neutron star and exotic quark matter, astronomers need to determine the relationship between the star's mass and its radius (average density). By studying neutron stars with satellites, it is possible to measure their mass quite accurately, but determining their diameter is much more difficult. More recently, scientists using the XMM-Newton X-ray satellite have found a way to estimate the density of neutron stars based on gravitational redshift. Another unusual thing about neutron stars is that as the mass of the star decreases, its radius increases; as a result, the most massive neutron stars have the smallest size.
Black Widow
The explosion of a supernova quite often imparts considerable speed to a newborn pulsar. Such a flying star with a decent magnetic field of its own greatly disturbs the ionized gas filling interstellar space. A kind of shock wave is formed, running in front of the star and diverging into 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 stream 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 received its name for the simple reason that its 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 the electrons and protons emitted by the neutron star emit soft gamma rays.
The result of computer modeling makes it possible to very clearly, in cross-section, present the processes occurring near a fast-flying pulsar. The rays diverging from a bright point are a conventional image of the flow of radiant energy, as well as the flow of particles and antiparticles that emanates from a neutron star. The red outline at the border of the black space around the neutron star and the red glowing clouds of plasma is the place where the stream of relativistic particles flying almost at the speed of light meets the interstellar gas compacted by the shock wave. By braking sharply, the particles emit X-rays and, having lost most of their energy, no longer heat up the incident gas so much.
Cramp of the Giants
Pulsars are considered one of the early stages of the life of a neutron star. Thanks to their study, scientists learned about magnetic fields, the speed of rotation, and the future fate of neutron stars. By constantly monitoring the behavior of a pulsar, one can determine exactly how much energy it loses, how much it slows down, and even when it will cease to exist, having slowed down so much that it cannot emit powerful radio waves. These studies confirmed many theoretical predictions about neutron stars.
Already by 1968, pulsars with a rotation period from 0.033 seconds to 2 seconds were discovered. The periodicity 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 clocks. And yet, with progress in the field of time measurement, it was possible to register regular changes in their periods for many pulsars. Of course, these are extremely small changes, and only over millions of years can we expect the period to double. The ratio of the current rotation speed to the rotation deceleration is one of the ways to estimate the age of the pulsar. Despite the remarkable stability of the radio signal, some pulsars sometimes experience so-called "disturbances." In a very short time interval (less than 2 minutes), the rotation speed of the pulsar increases by a significant amount, and then after some time returns to the value that was before the “disturbance.” It is believed that the “disturbances” 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 undergoes large “disturbances” approximately every 3 years, and this makes it a very interesting object for studying such phenomena.
Magnetars
Some neutron stars, called repeating soft gamma ray burst sources (SGRs), emit powerful bursts of "soft" gamma rays at irregular intervals. The amount of energy emitted by an SGR in a typical flare lasting a few tenths of a second can only be emitted by the Sun in a whole year. Four known SGRs are located within our Galaxy and only one is outside it. These incredible bursts of energy can be caused by starquakes, powerful versions of earthquakes in which the solid surface of neutron stars is torn apart and powerful streams of protons burst from their depths, which, stuck in a magnetic field, emit gamma and X-ray radiation. Neutron stars were identified as sources of powerful gamma-ray bursts after the huge gamma-ray burst on March 5, 1979, released as much energy in the first second as the Sun emits in 1,000 years. Recent observations of one of the most active neutron stars currently appear to support the theory that irregular, powerful bursts of gamma-ray and X-ray radiation are caused by starquakes.
In 1998, the famous SGR suddenly woke up from its “slumber,” which had shown no signs of activity for 20 years and splashed out almost as much energy as the gamma-ray flare of March 5, 1979. What struck the researchers most when observing this event was the sharp slowdown in the speed of rotation of the star, indicating its destruction. To explain powerful gamma-ray and X-ray flares, a magnetar-neutron star model with a superstrong magnetic field was proposed. If a neutron star is born spinning very quickly, then the combined influence of rotation and convection, which plays an important role in the first few seconds of the neutron star's life, can create a huge magnetic field through a complex process known as an "active dynamo" (the same way the 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 (after 10 or 20 seconds), convection and the action of the dynamo stop, but this time is enough for the necessary field to arise.
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 transfer 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 to power such giant flares as March 5, 1979 and August 27, 1998. Such events inevitably cause deep disruption and changes in the structure of not only electrical currents in the volume of the neutron star, but also its solid crust. Another mysterious type of object that emits powerful X-ray radiation during periodic explosions is the so-called anomalous X-ray pulsarsAXP. 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 by drawing energy from a magnetic field. And although magnetars today remain the brainchild of theorists and there is not enough data confirming their existence, astronomers are persistently searching for the necessary evidence.
Magnetar candidates
Astronomers have already studied our home galaxy, the Milky Way, so thoroughly that it costs them nothing to depict its side view, indicating the position of the most remarkable of the neutron stars.
Scientists believe that AXP and SGR are simply two stages in the life of the same giant magnet neutron star. For the first 10,000 years, the magnetar is an SGR pulsar, visible in ordinary light and producing repeated bursts of soft X-ray radiation, and for the next millions of years it, like an anomalous AXP pulsar, disappears from the visible range and puffs only in the X-ray.
The strongest magnet
Analysis of data obtained by the RXTE satellite (Rossi X-ray Timing Explorer, NASA) 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 (from the slowing down of the pulsar), but also almost directly from measuring the rotation frequency of protons in the magnetic field of the 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 storage media on our Earth would be demagnetized. True, taking into account the fact that its mass is approximately equal to that of the Sun, this would no longer matter, since even if the Earth had not fallen on this neutron star, it would have been spinning around it like crazy, making a full revolution in just an hour.
Active dynamo
We all know that energy loves to change from one form to another. Electricity easily turns 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, for example, into 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 increase in the own magnetic field of a rotating current-conducting object begins. The greater the field, the greater the currents, the greater the currents, the greater the field and all this is due to banal convective flows, due to the fact that a hot substance is lighter than a cold one, and therefore floats up
Troubled neighborhood
The famous Chandra space observatory has discovered hundreds of objects (including in other galaxies), indicating that not all neutron stars are destined to lead a solitary life. 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 is to accompany it, this “theft” will cause different consequences. Gas flowing from a companion with a mass less than that of our Sun onto such a “crumb” as a neutron star cannot immediately fall due to its own angular momentum being too large, so it creates a so-called accretion disk around it from the “stolen » matter. Friction as it wraps around the neutron star and compression in the gravitational field heats 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. They usually last from several seconds to several minutes and at maximum give the star a luminosity almost 100 thousand times greater than the luminosity of the Sun.
These flares are explained by the fact that when hydrogen and helium are transferred to the neutron star from the 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. The giant star loses matter in the form of stellar wind (a stream of ionized gas emanating from its surface), and the enormous gravity of the neutron star captures some of this matter. But here the magnetic field comes into its own, causing the falling matter to flow along the lines of force towards the magnetic poles.
This means that X-ray radiation is primarily generated at hot spots at the poles, and if the magnetic axis and the rotation axis of the star do not coincide, then the brightness of the star turns out to be variable - it is also a pulsar, but only an X-ray one. Neutron stars in X-ray pulsars have bright giant stars as companions. In bursters, the companions of neutron stars are faint, low-mass stars. The age of bright giants does not exceed several tens of millions of years, while the age of faint dwarf stars can be billions of years old, 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. Perhaps bursters pulsated at some point in the past, but pulsars are yet to burst 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 turn out to be 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 also aged companion (usually a red giant). As matter falls onto the surface of a neutron star, it transfers rotational energy to it, causing it to spin faster and faster. This happens until the neutron star's companion, almost freed of 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, recently astronomers 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 at the stage of “liberating” the red star from excess weight and turning into a white dwarf. If this hypothesis is incorrect, then the companion star could be an ordinary globular cluster star accidentally captured by a pulsar. Almost all neutron stars that are currently known are found either in X-ray binaries or as single pulsars.
And 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 ideas about the composition and structure of this bizarre class of burnt-out, gravitationally compressed stars. This star was first discovered 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. It may be the remnant of one of the stars in 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 star eater
Just as stones fall to the ground, so a large star, releasing bits of its mass, gradually moves 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 swirl in a circle, the falling matter must lose most of its angular momentum before it reaches the surface. 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 matter fall to successfully end with an impact on the surface of the neutron star in the region of its magnetic poles.
Riddle 4U2127 solved
This star has been fooling astronomers for more than 10 years, showing 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 these were not one, but two neutron stars. Moreover, both of them have companions: one star is similar to our Sun, the other is like a small blue neighbor. Spatially, these pairs of stars are separated by a fairly large distance and live an independent life. But on the stellar sphere they are projected to almost the same 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 every new discovery from their turbulent life gives scientists unique information necessary to understand the nature of Matter and the evolution of the Universe.
Universal standard
It is very difficult to send something outside the solar system, so together with the Pioneer 10 and 11 spacecraft that headed there 30 years ago, earthlings also sent messages to their brothers in mind. To draw something that would be understandable to the Extraterrestrial Mind is not an easy task; moreover, it was also necessary to indicate the return address and the date of sending the letter... How clearly the artists were able to do all this is difficult for a person to understand, but the very idea of using radio pulsars for indicating the place and time of sending the message is brilliant. Intermittent rays of various lengths emanating from a point symbolizing the Sun indicate the direction and distance to the pulsars closest to the Earth, and the intermittency 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 Milky Way. The frequency of the radio signal emitted by a hydrogen atom when the mutual orientation of the spins (direction of rotation) of the proton and electron changes is taken as the unit of time in the message.
The famous 21 cm or 1420 MHz should be known to all intelligent beings in the Universe. Using 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 flight of the first spaceship that left the solar system.
Nikolay Andreev
They were predicted in the early 30s. XX century Soviet physicist L. D. Landau, astronomers V. Baade and F. Zwicky. In 1967, pulsars were discovered, which by 1977 were finally identified with neutron stars.
Neutron stars are formed as a result of a supernova explosion at the last stage of the evolution of a high-mass star.
If the mass of the supernova remnant (i.e., what remains after the shell is ejected) is greater than 1.4 M☉ , but less than 2.5 M☉, then its compression continues after the explosion until the density reaches nuclear values. This will lead to the fact that electrons will be “pressed” into the nuclei, and a substance consisting of only neutrons will be formed. A neutron star appears.
The radii of neutron stars, like the radii of white dwarfs, decrease with increasing mass. So, a neutron star with a mass of 1.4 M☉ (the minimum mass of a neutron star) has a radius of 100-200 km, and with a mass of 2.5 M☉ (maximum mass) - only 10-12 km. Material from the site
A schematic section of a neutron star is shown in Figure 86. The outer layers of the star (Figure 86, III) consist of iron, forming a hard crust. At a depth of approximately 1 km, a solid crust of iron with an admixture of neutrons begins (Fig. 86), which turns into a liquid superfluid and superconducting core (Fig. 86, I). At masses close to the limit (2.5-2.7 M☉), heavier elementary particles (hyperons) appear in the central regions of the neutron star.
Neutron star density
The density of matter in a neutron star is comparable to the density of matter in the atomic nucleus: it reaches 10 15 -10 18 kg/m 3. At such densities, the independent existence of electrons and protons is impossible, and the matter of the star consists almost entirely of neutrons.
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The pulsars 4U 0115+63 and V 0332+53 belong to a special type of source - flaring (or transient) X-ray pulsars. They either glow faintly in the X-ray range, or flash brightly, or even disappear completely. By the way pulsars transition from one state to another, one can judge their magnetic fields and the temperatures of the surrounding matter. The values of these parameters are so high that they cannot be obtained and measured directly in earthly laboratories.
The name of the pulsar begins with a letter, which indicates the first observatory that found it, and then there are numbers - the coordinates of the pulsar. “V” is the Vela 5B satellite, an American military satellite designed to monitor the territory of the USSR. “4U,” in turn, stands for “4th catalog of UHURU,” the first dedicated X-ray observatory in orbit. And when the first pulsar was discovered, it was initially called LGM-1, from “little green men”: it sent radio pulses at regular intervals, and researchers decided that it could be a signal from intelligent civilizations.
An X-ray pulsar is a rapidly rotating neutron star with a strong magnetic field. A neutron star can form a pair with an ordinary star and pull its gas onto itself - astrophysicists call this accretion. The gas spirals around the neutron star, forming an accretion disk, and is decelerated at the boundary of the neutron star’s magnetosphere. In this case, the substance penetrates a little inside the magnetosphere, “frozen” into it and flows along magnetic lines to the poles. Falling on the magnetic poles, it heats up to hundreds of millions of degrees and emits in the X-ray range. Since the magnetic axis of a neutron star is at an angle to the rotation axis, the X-rays rotate like the rays of a lighthouse and “from the shore” appear as repeating signals with a period from thousandths of a second to several minutes.
A neutron star is one of the possible remnants of a supernova explosion. At the end of the evolution of some stars, their matter is compressed so much due to gravity that electrons actually merge with protons and form neutrons. The magnetic field of a neutron star can exceed the maximum achievable on Earth by tens of billions of times.
For an X-ray pulsar to be observed in a system of two stars, matter must flow from an ordinary star to a neutron star. An ordinary star can be a giant or a supergiant and have a powerful stellar wind, that is, eject a lot of matter into space. Or it could be a small star like the Sun that has filled its Roche lobe—the region beyond which matter is no longer held by the star's gravitational pull and is pulled by the neutron star's gravity.
The X-ray pulsars 4U 0115+63 and V 0332+53 emit so erratically (i.e., they exhibit bursts of radiation) because each of them has a rather unusual companion star - a class Be star. The Be star rotates around its axis so quickly that from time to time its “skirt lifts” - a gas disk forms and grows along the equator - and the star fills the Roche lobe. Gas begins to rapidly accrete onto the neutron star, the intensity of its radiation increases sharply, and a flare occurs. Gradually, the “skirt” wears out, the accretion disk is depleted, and matter can no longer fall onto the neutron star due to the influence of the magnetic field and centrifugal forces. The so-called “propeller effect” occurs. In this regime, accretion does not occur and the X-ray source disappears.
Using the X-ray telescope on the Swift space observatory, Russian scientists were able to measure the threshold radiation intensity, that is, the luminosity below which the pulsar goes into “propeller mode.” This value depends on the magnetic field and the rotation period of the pulsar. The rotation period of the sources under study is known by measuring the arrival time of the pulses they emit - 3.6 sec for 4U 0115+63 and 4.3 sec for V 0332+53, which made it possible to calculate the magnetic field strength. The results coincided with the values obtained by other methods. However, the luminosity of the pulsars did not drop by 400 times, as expected, but only by 200 times. The authors suggested that either the surface of the neutron star, heated by the flare, cools and thereby serves as an additional source of radiation, or the propeller effect cannot completely block the flow of matter from an ordinary star and there are other “leakage” channels.
The transition to propeller mode is very difficult to detect, since in this mode the pulsar emits almost no radiation. During previous flares of sources 4U 0115+63 and V 0332+53, there was already an attempt to catch this transition, but due to the low sensitivity of the instruments available at that time, the “off state” could not be detected. Reliable confirmation that these pulsars actually “turn off” has only now been received. Moreover, it is shown that information about the transition to the “propeller mode” can be used to determine the strength and structure of the magnetic field of neutron stars.
Alexander Lutovinov, professor of the Russian Academy of Sciences, Doctor of Physics and Mathematics, head of the laboratory at the Space Research Institute of the Russian Academy of Sciences and teacher at MIPT explains: “One of the fundamental issues in the formation and evolution of neutron stars is the structure of their magnetic fields. During the research, we determined for two neutron stars the dipole component of the magnetic field, which is precisely responsible for the propeller effect. We showed that this independently obtained value can be compared with the magnetic field value already known from measurements of cyclotron lines, and thus estimate the contribution of other higher order components that enter into the field structure.” The measurement results, calculations and conclusions were published in the journal
Astrophysicists have filmed the very quickly fading radiation of pulsars after powerful flares - the transition to the so-called propeller mode. The phenomenon, theoretically predicted more than forty years ago, was reliably recorded for the first time.
An international team of astrophysicists, which included Russian scientists from the Space Research Institute of the Russian Academy of Sciences, MIPT and the Pulkovo Observatory of the Russian Academy of Sciences, filmed the very quickly fading radiation of pulsars after powerful flares - the transition to the so-called propeller mode. Theoretical predictions of this effect were made more than forty years ago, but only now this phenomenon was reliably recorded for the first time for the X-ray pulsars 4U 0115+63 and V 0332+53. The measurement results, calculations and conclusions were published in the journal Astronomy & Astrophysics.
The pulsars 4U 0115+63 and V 0332+53 belong to a special type of source - flaring (or transient) X-ray pulsars. They either glow faintly in the X-ray range, or flash brightly, or even disappear completely. By the way pulsars transition from one state to another, one can judge their magnetic fields and the temperatures of the surrounding matter. The values of these parameters are so high that they cannot be obtained and measured directly in earthly laboratories.
The name of the pulsar begins with a letter, which indicates the first observatory that found it, and then there are numbers - the coordinates of the pulsar. "V" is the Vela 5B satellite, an American military satellite designed to monitor the territory of the USSR. “4U,” in turn, stands for “4th catalog of UHURU,” the first dedicated X-ray observatory in orbit. And when the first pulsar was discovered, it was initially called LGM-1, from “little green men”: it sent radio pulses at regular intervals, and researchers decided that it could be a signal from intelligent civilizations.
An X-ray pulsar is a rapidly rotating neutron star with a strong magnetic field. A neutron star can form a pair with an ordinary star and pull its gas onto itself - astrophysicists call this accretion. The gas spirals around the neutron star, forming an accretion disk, and is decelerated at the boundary of the neutron star’s magnetosphere. In this case, the substance penetrates a little inside the magnetosphere, “frozen” into it and flows along magnetic lines to the poles. Falling on the magnetic poles, it heats up to hundreds of millions of degrees and emits in the X-ray range. Since the magnetic axis of a neutron star is at an angle to the rotation axis, the X-rays rotate like the rays of a lighthouse and “from the shore” appear as repeating signals with a period from thousandths of a second to several minutes.
A neutron star is one of the possible remnants of a supernova explosion. At the end of the evolution of some stars, their matter is compressed so much due to gravity that electrons actually merge with protons and form neutrons. The magnetic field of a neutron star can exceed the maximum achievable on Earth by tens of billions of times.
For an X-ray pulsar to be observed in a system of two stars, matter must flow from an ordinary star to a neutron star. An ordinary star can be a giant or a supergiant and have a powerful stellar wind, that is, eject a lot of matter into space. Or it could be a small star like the Sun that has filled its Roche lobe - the region beyond which matter is no longer held by the star's gravitational force and is pulled by the gravity of the neutron star.
The X-ray pulsars 4U 0115+63 and V 0332+53 emit so erratically (i.e., they exhibit bursts of radiation) because each of them has a rather unusual companion star - a class Be star. The Be star rotates around its axis so quickly that from time to time its skirt “lifts” - a gas disk forms and grows along the equator - and the star fills the Roche lobe. Gas begins to rapidly accrete onto the neutron star, the intensity of its radiation increases sharply, and a flare occurs. Gradually, the “skirt” wears out, the accretion disk is depleted, and matter can no longer fall onto the neutron star due to the influence of the magnetic field and centrifugal forces. The so-called “propeller effect” occurs. In this regime, accretion does not occur and the X-ray source disappears.
In astronomy, the term “luminosity” is used, that is, the total energy emitted by a celestial body per unit time. The threshold luminosity for source 4U 0115+63 is shown in red. For another source (V 0332+53) a similar picture is observed. Where the blue lines are drawn, the distance between the pulsar and the optical star is minimal. In this position, the accretion mode can be temporarily resumed if there is a sufficient amount of matter, which is clearly visible in the figure.
Using the X-ray telescope on the Swift space observatory, Russian scientists were able to measure the threshold radiation intensity, that is, the luminosity below which the pulsar goes into “propeller mode.” This value depends on the magnetic field and the rotation period of the pulsar. The rotation period of the sources under study is known by measuring the arrival time of the pulses they emit - 3.6 sec for 4U 0115+63 and 4.3 sec for V 0332+53, which made it possible to calculate the magnetic field strength. The results coincided with the values obtained by other methods. However, the luminosity of the pulsars did not drop by 400 times, as expected, but only by 200 times. The authors suggested that either the surface of the neutron star, heated by the flare, cools and thereby serves as an additional source of radiation, or the propeller effect cannot completely block the flow of matter from an ordinary star and there are other “leakage” channels.
The transition to propeller mode is very difficult to detect, since in this mode the pulsar emits almost no radiation. During previous flares of sources 4U 0115+63 and V 0332+53, there was already an attempt to catch this transition, but due to the low sensitivity of the instruments available at that time, the “off state” could not be detected. Reliable confirmation that these pulsars actually “turn off” has only now been received. Moreover, it is shown that information about the transition to the “propeller mode” can be used to determine the strength and structure of the magnetic field of neutron stars.
Alexander Lutovinov, professor of the Russian Academy of Sciences, Doctor of Physical and Mathematical Sciences, head of the laboratory at the Space Research Institute of the Russian Academy of Sciences and teacher at MIPT explains:
“One of the fundamental questions about the formation and evolution of neutron stars is the structure of their magnetic fields. During the research, we determined for two neutron stars the dipole component of the magnetic field, which is precisely responsible for the propeller effect. We showed that this independently obtained value can be compared with the magnetic field value already known from measurements of cyclotron lines, and thus estimate the contribution of other higher order components that enter into the field structure.”
