Anomalous X-ray pulsars. School encyclopedia What are x-rays

This density approaches the density of matter inside atomic nuclei:

Only neutron stars can be so compact, compressed to such a high degree: their density is really close to nuclear. This conclusion is confirmed by the entire fifteen-year history of the study of pulsars. But what is the origin of the rapid rotation of neutron star-pulsars? It is undoubtedly caused by the strong compression of the star during its transformation from an "ordinary" star into a neutron one. Stars always have a rotation with one or another speed or period: the Sun, for example, rotates around its axis with a period of about a month. When a star contracts, its rotation speeds up. The same thing happens to her as to a dancer on ice: by pressing his hands to himself, the dancer accelerates his rotation. One of the basic laws of mechanics operates here - the law of conservation of angular momentum (or angular momentum). It follows from it that when the dimensions of a rotating body change, the speed of its rotation also changes; but the work remains unchanged

(which is - up to an insignificant numerical factor - the angular momentum). In this product, Q is the frequency of rotation of the body, M is its mass, R is the size of the body in the direction perpendicular to the axis of rotation, which coincides in the case of a spherical star. with its radius. At a constant mass, the product remains constant

And, therefore, with a decrease in the size of the body, the frequency of its rotation increases according to the law: (1.3)

Further contraction to the size of a neutron star,

means a decrease in radius by a factor of a thousand. Accordingly, the frequency of rotation should increase by a million times and its period should decrease by the same amount. Instead of, say, a month, the star now makes one revolution around its axis in just three seconds. A faster initial rotation gives even shorter periods. Nowadays, not only pulsars emitting in the radio range are known - they are called radio pulsars, but also X-ray pulsars emitting regular pulses of X-rays. They, too, turned out to be neutron stars; there is a lot in their physics that makes them similar to the bursters. But both radio pulsars and X-ray pulsars differ from bursters in one fundamental respect: they have very strong magnetic fields. It is magnetic fields - together with fast rotation - that create the effect of pulsations, although these fields act differently in radio pulsars and X-ray pulsars.

We will first talk about X-ray pulsars, whose emission mechanism is more or less clear, and then about radio pulsars, which have so far been studied to a much lesser extent, although they were discovered before X-ray pulsars and bursters.

X-ray pulsars

X-ray pulsars are close binary systems in which one of the stars is a neutron star and the other is a bright giant star. About two dozen of these objects are known. The first two X-ray pulsars - in the constellation of Hercules and in the constellations of Centaurus - were discovered in 1972 (three years before the discovery of bursters) with the help of the American research satellite "Uhuru". The pulsar in Hercules sends out pulses with a period of 1.24 s. This is the rotation period of a neutron star. There is one more period in the system - the neutron star and its companion rotate around their common center of gravity with a period of 1.7 days. The orbital period was determined in this case due to the (accidental) circumstance that, in its orbital motion, the "ordinary" star regularly appears on the line of sight connecting us and the neutron star, and therefore it obscures the x-ray source for a while. This is obviously possible when the plane of stellar orbits makes only a small angle with the line of sight. X-ray radiation stops for about 6 hours, then reappears, and so on every 1.7 days.

(By the way, observing X-ray eclipses for bursters before

hasn't been successful lately. And it was strange: if the orbits of double

systems are randomly oriented in space, it should be expected that from

more than three dozen bursters, at least a few have

planes of orbital motion approximately parallel to the line of sight

(like a pulsar in Hercules) so that an ordinary star can periodically

close the neutron star from us. Only in 1982, i.e. 7 years after

opening bursters, one example of an eclipsing burster was finally

discovered.) Long-term observations made it possible to establish one more -

the third is the period of the X-ray pulsar in Hercules: this period is

35 days, of which II days the source shines, and 24 days it does not. The reason of that

phenomenon remains unknown. A pulsar in the constellation Centaurus

pulsation period 4.8 s. The orbital period is 2.087

day, it is also found by x-ray eclipses. Long-term

changes similar to the 35-day period of the pulsar in the constellation Hercules in

this pulsar is not found. Companion of a neutron star in a binary system

This pulsar is a bright visible giant star with a mass of 10-20 Suns. In most cases, the companion of a neutron star in x-rays

pulsars is a bright blue giant star. In this they differ from

bursters that contain faint dwarf stars. But as in the bursters, in

these systems, it is possible for matter to flow from an ordinary star to

neutron star, and their radiation also arises due to heating

the surface of a neutron star by the flux of accreted matter. It's the same

physical mechanism of radiation, as in the case of the background (not flare)

burster radiation. Some of the X-ray pulsars have matter

flows to the neutron star in the form of a jet (as in bursters). Most

In the same cases, a giant star loses matter in the form of a stellar wind -

outgoing from its surface in all directions of the plasma flow, ionized

gas. (A phenomenon of this kind is also observed in the Sun, although the solar wind and

weaker - the Sun is not a giant, but a dwarf.) Part of the stellar wind plasma enters

in the vicinity of a neutron star, into the zone of dominance of its gravitation, where and

is captured by her.

However, when approaching the surface of a neutron star, charged

plasma particles begin to experience another force field

magnetic field of a neutron star-pulsar. The magnetic field is capable

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x-ray pulsar Kyiv, x-ray pulsar thermal imager
- a cosmic source of alternating X-ray radiation coming to Earth in the form of periodically repeating pulses.

• 1 Discovery history
• 2 Physical nature of X-ray pulsars
• 3 See also
• 4 Notes
• 5 Links

Discovery history

The discovery of X-ray pulsars as a separate phenomenon occurred in 1971 using data obtained by the first Uhuru X-ray orbital observatory. The first discovered X-ray pulsar Centaurus X-3 showed not only regular brightness pulsations with a period of about 4.8 seconds, but also a regular change in this period. Further studies have shown that the change in the period of pulsations in this system is associated with the Doppler effect when the source of pulsations moves along the orbit in the binary system. It is interesting to note that the GX 1+4 source, discovered in a stratospheric experiment conducted in October 1970 (an article about these measurements was submitted for publication after the publication of the result on the Cen X-3 source by the Uhuru observatory data group), and which regular brightness changes with a period of about 2.3 minutes were detected, also turned out to be a pulsar. However, the limited data of the stratospheric experiment did not allow us to make reliable statements about the strict regularity of the change in the brightness of this source; therefore, this source cannot be considered the first discovered X-ray pulsar.

Formally, for the first time, the radiation of a magnetized rotating neutron star (i.e., a pulsar) in the Crab Nebula was discovered back in 1963, i.e. even before the discovery of neutron stars in 1967 by A. Huish and J. Bell. However, the very short period of rotation of a neutron star in the Crab Nebula (about 33 ms) did not make it possible to detect X-ray pulsations at this frequency until 1969.

Physical nature of X-ray pulsars

X-ray pulsars can be divided into two large classes according to the source of energy that feeds the X-rays: accreting X-ray pulsars and single X-ray pulsars. The first are a binary system, one of the components of which is a neutron star, and the second is a star that either fills its Roche lobe, as a result of which matter flows from an ordinary star to a neutron one, or a giant star with a powerful stellar wind.

Neutron stars are stars with very small sizes (20-30 km in diameter) and extremely high densities exceeding the density of an atomic nucleus. It is believed that neutron stars appear as a result of supernova explosions. During a supernova explosion, the core of a normal star rapidly collapses, which then turns into a neutron star. During compression, due to the law of conservation of angular momentum, as well as conservation of the magnetic flux, there is a sharp increase in the speed of rotation and the magnetic field of the star. The fast rotation speed of a neutron star and extremely high magnetic fields (1012-1013 gauss) are the main conditions for the occurrence of the X-ray pulsar phenomenon.

The infalling matter forms an accretion disk around the neutron star. But in the immediate vicinity of a neutron star, it is destroyed: the movement of the plasma is greatly hindered across the magnetic field lines. Substance can no longer move in the plane of the disk, it moves along the lines of the field and falls on the surface of the neutron star in the region of the poles. As a result, the so-called accretion column is formed, the size of which is much smaller than the size of the star itself. Matter, hitting the solid surface of a neutron star, is strongly heated and begins to radiate in x-rays. Pulsations of radiation are connected with the fact that due to the rapid rotation of the star, the accretion column now disappears from the view of the observer, then reappears.

In terms of the physical picture, close relatives of X-ray pulsars are polars and intermediate polars. The difference between pulsars and polars is that a pulsar is a neutron star, while a polar is a white dwarf. Accordingly, they have lower magnetic fields and rotation speed.

As a neutron star ages, its field weakens, and an X-ray pulsar can become a burster.

Single X-ray pulsars are neutron stars whose X-ray emission results either from the emission of accelerated charged particles or from the simple cooling of their surfaces.

see also

• neutron star
• radio pulsar
• Pulsar
• Polars (cataclysmic variables)
• Intermediate polar

Notes

1. V. M. Lipunov. Astrophysics of neutron stars. - The science. - 1987. - S. 139.

Links

• Space physics. Little Encyclopedia, Moscow: Soviet Encyclopedia, 1986

x-ray pulsar anime, x-ray pulsar cues, x-ray pulsar sights, x-ray pulsar thermal imager

X-ray pulsar Information About

X-RAY PULSARS

- sources of alternating periodic. x-ray neutron stars with a strong magnetic field. field, radiating due to accretions. Magn. field on the surface R. p. ~ 10 11 -10 14 gauss. Luminosities most R. p. from 10 35 to 10 39 erg / s. Pulse periods R from 0.07 s to several thousand seconds. R. p. are included in close binary star systems (see. close binary stars) the second component to-rykh is a normal (non-degenerate) star, which supplies the substance necessary for the accretion and normal functioning of the R. p. of the Galaxy and those lying in its plane, and in low-mass binary systems belonging to the population of the II Galaxy and belonging to its spherical. component. R. p. also discovered in the Magellanic Clouds.

Rice. 1. Recording of the X-ray pulsar Centaur X-3, obtained from the Uhuru satellite on May 7, 1971. On the vertical axis - the number of readings over a time interval of 1 bin = 0.096 s, on the horizontal axis - time in bins.

Rice. Fig. 2. Long-term variability of X-ray emission from the Centaur-X-3 source (lower graph, N - number of readings, s -t). Characteristic X-ray eclipses are visible. The upper graph shows the changes in the period P, proving the motion of the pulsar around the center of mass of the binary system (A 1.387-10 -3).

At the beginning stage of X-ray research. objects were given names according to the constellations in which they are located. For example, Hercules X-1 means the first X-ray. brightness object in the constellation Hercules, Centaur X-3 - the third brightness in the constellation Centaur. R. p. in the Small Magellanic Cloud is designated as SMC X-1, in the Large Magellanic Cloud - LMC X-4 [often found in X-ray notation. sources letter X - from English. X-rays (X-rays)]. Detection from satellites of a large number of X-rays. other sources required. Astronomical coordinates). The numbers in the designation of sources discovered by the Ariel satellite (Great Britain) have a similar meaning, for example. A0535 + 26. Type designations GX1+4 refer to sources in the center. regions of the galaxy. The numbers correspond to the galactic coordinates l and b(in this case l = 1°, b=+4°). Other designations are also used. Thus, a flashing RP with a period of about 8 seconds discovered from the board of the Soviet AMS Venera-11, -12 in the Cone experiment was named FXP0520-66.

Variability of radiation of X-ray pulsars. short period x-ray variability radiation R. p. illustrates fig. 1, on Krom there is a record of the radiation of one of the first discovered R. p. - Centaur X-3 (May 1971, satellite "Uhuru"). Pulse repetition period P = 4.8 s

On fig. 2 shows a long period. variability R. n. Centaur X-3. Once in two days, R. p. periodically "disappears" (eclipsed) for 11 hours (lower. R. depends on the phase of the two-day period T= 2.087 days according to the harmonic law (upper graph): where is the change R, R 0- unperturbed value R, A - amplitude relative. changes Р, t0 corresponds to one of the moments when the period deviation is maximum. These two facts are interpreted unambiguously: R. p. enters a binary system with an orbital period equal to T."Disappearances" are explained by eclipses of R. p. Roche lobe. Periodic changes R are due to the Doppler effect during the orbital motion of the R. p. around the center of mass of the binary system. ,where i- the orbital inclination angle of the binary system (in this system is close to 90°), v- the speed of the orbital movement of R. p.; v sin i= 416 km/s, the orbital eccentricity is small. X-ray eclipses have been discovered in far from all binary systems with R. p.

Rice. 3. Simplified picture of accretion onto a magnetized neutron star in a binary system. The gas enters the star as in a geometrically thin disk, and M is the angular velocity of rotation and the magnetic moment of the neutron star. The conditions for plasma freezing into the magnetosphere are not favorable on its entire surface.

After the discovery of R. p. in its vicinity, a variable optical is usually quickly found. a star (the second component of a binary system), the brightness of which changes with a period equal to the orbital or half as long (see below). In addition, the spectral lines of the optical components experience Doppler shift, 2 t in filter AT(cm. astrophotometry). Part of the X-ray radiation is reflected by the star's atmosphere, but DOS. the share is absorbed by it and processed into optical. R. Part of the energy is spent on eff. heating of a substance on the surface, accompanied by the formation of m. n. induced. stellar wind. The second effect, called the ellipsoidal effect, is related to the fact that the shape of the star filling the Roche lobe differs noticeably from a spherical one. As a result, b. h. surface and two times - smaller. Such variability with a period half the orbital period is observed in binary systems where the luminosity of the optical. component is much higher than Rg. the luminosity of the R. p. In particular, it is precisely because of this variability that the normal component of the Centaur X-3 source was discovered.

Accretion onto a neutron star with a strong magnetic field. In close binary star systems, two basic systems are possible. types of accretion: disk and spherically symmetrical. Roche lobe), then the flowing substance has a mean. beats

Rice. 4. Pulse profiles of a number of X-ray pulsars. The energy intervals for which the data were obtained and the periods P are given.

Rice. 5. Energy dependence of the pulse profile for two X-ray pulsars.

Rice. 6. Spectra of a number of X-ray pulsars. The X-ray line of iron with hv6.5-7 keV is noticeable.

Free fall (with spherically symmetric accretion) is possible only at large distances R from a star. At a distance L m ~ 100-1000 km (radius of the magnetosphere), the pressure of the magnet. field of a neutron star is compared with the pressure of the accreting flow of matter ( - substance density) and stops it. In the zone R< R M the closed magnetosphere of a neutron star is formed (Fig. 3, a), near R M a shock wave arises, in which the plasma is cooled by the radiation of the RP due to Compton scattering. Due to the Rayleigh-Taylor instability, it becomes possible for plasma droplets to penetrate into the magnetosphere, where they are further crushed and frozen into the magnetic field. field. Magn. field-channelizes the flow of accreting plasma and directs it to the magnetic region. b). The zone, on which the substance falls, apparently, . The flux of matter falling onto the star, necessary to maintain the luminosity L x ~ 10 35 -10 39 erg/s, is equal to a year. More than a ton of matter falls per 1 cm 2 of the surface every second. Free fall speed is 0.4 With.

In R. p. with luminosity L x < 10 36 эрг/спадающие протоны и электроны тормозятся в атмосфере (образованной веществом,

Rice. 7. Period P (in s) as a function of time for a number of X-ray pulsars.

In R. The pressure of light) on incident electrons is capable of stopping the flow of accreting matter. Near the surface of a neutron star (at a height of less than 1 m), radiation-dominants can form. shock wave. If the luminosity of the R. p. exceeds 10 37 erg / s, then above the surface of a neutron star in the region of the magnetic. poles an accretion column is formed. critical luminosity, because from the sides it is held magnetically. field, not gravity. Moreover, if the magnetic Since the field of a neutron star exceeds 10 13 G, then at the base of the column the temperature of plasma and radiation reaches 10 10 K. At such temperatures, the processes of creation and annihilation of electron-positron pairs occur. Neutrinos produced in a reaction , take away the main share of luminosity. X-ray the luminosity (exceeding the critical one) is a small fraction of the neutrino luminosity, and the luminosities of SMC X-1 and LMC X-4 ~ 10 m erg / s, i.e., they are much higher than the critical one. These objects have, apparently, and mean. neutrino luminosity. The emitted neutrinos heat the interior of the neutron star and, being absorbed in the interior of the normal component of the binary system, make a small contribution to its optical. luminosity. The flux of accreting matter in such objects can reach (10 - 6 -10 - 5 )in year. In this case, a situation is possible when, during 10 6 -10 5 years of "work" of R. p., approx. 1substance, the stability limit for neutron stars will be exceeded, there will be gravitational collapse, accompanied by an explosion supernova rare type and education black hole. This can happen only with disk accretion, when the radiation pressure does not prevent accretion at large distances from the gravitating center.

Formation of pulse profiles and emission spectra of X-ray pulsars. P is equal to the rotation period of the neutron star. The presence of a strong magnet. fields can lead to radiation directivity. Depending on the ratio between the energy of photons hv, magnetic strength. fields H and plasma swarm T e both "pencil" and "knife" patterns can be formed. The most important parameter is the gyrofrequency (cyclotron frequency) of the electron. The degree of directivity is a f-tion of relations. The directivity pattern determines the shape of the pulse profile of R. p. 4. The shape of the profiles of many R. p. changes with increasing photon energy (Fig. 5).

The emission spectrum of a neutron star must be multicomponent. They emit a shock wave, an accretion column, the surface of a neutron star near the base of the column, and plasma flowing through the magnetosphere to the poles of a neutron star. This plasma absorbs the hard radiation of the column and re-radiates it in the "soft" X-ray. range both in the continuum (continuous spectrum) and in x-rays. lines (characteristic and resonant) of ions of heavy elements. If plasma flows on the magnetosphere of a high-luminosity RP do not cover its entire surface, then "windows" are formed, into which "hard" radiation freely escapes, while other directions are closed to it due to large optical radiation. thicknesses of plasma flows. The rotation of a neutron star should lead to radiation pulsations. This is another mechanism for the formation of the X-ray profile. The most important step in the study of R. p. was the discovery of a gyroline [spectral line due to cyclotron radiation (or absorption) of electrons] in the spectrum of R. p. Hercules X-1. The discovery of the gyroline gave the method of direct experimentation. hv H = 56 keV. According to the ratio hv H = 1,1 (H/10 11 G) keV, magnetic strength the field on the surface of this neutron star is 5*10 12 G.

Acceleration and deceleration of the rotation of neutron stars. Unlike radio pulsars (some of them, in particular pulsars in the Crab and Sails, radiate in X-ray. range) that radiate due to the rotational energy of a magnetized neutron star and increase their period with time; RPs that radiate due to accretion accelerate their rotation. Indeed, during disk accretion, the matter falling onto the magnetosphere has a noticeable sp. the moment of the amount of movement. Freezing into the magnet. field, the accreting plasma moves towards the surface of the star and transfers its angular momentum to it. As a result, the rotation of the star accelerates and the pulse repetition period decreases. This effect is characteristic of all R. p. (Fig. 7). However, sometimes slowdown is observed. This is possible if the rate of accretion or the direction of the moment of the amount of movement of the accreting matter changes. Among the mechanisms leading to an increase in the period, the so-called. propeller mechanism. It is assumed that R. A. Sunyaev.

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