X-ray binary star

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Artist's impression of an X-ray binary star with accretion disk and jet

An X-ray double star (engl. X-ray binary, XRB) is a binary system with more pronounced X-ray luminosity . The accretion of matter from a companion star onto a compact star creates a characteristic glow in the high-energy range of electromagnetic radiation . The compact object can be a white dwarf , a neutron star or a black hole .

The flow of matter

The flow of matter onto the compact star can occur in two variants:

  • As a star wind from the companion who gets into the area of ​​attraction of the compact star. Such stellar winds are often found in main sequence stars and giants of high mass.
  • For stars that cross the Roche limit, matter flows over the Lagrange point to the compact partner. Such a flow of matter can last for several hundred million years.

Due to the conservation of angular momentum , the material does not fall directly onto the compact partner, but first forms an accretion disk around the degenerate star. If there is also a magnetic field, its strength depends on how much the accretion disc is deformed. Due to the high heat in the accretion disc, the matter there is ionized and carries a charge for each particle. When moving within the accretion disk, this charge causes a current that creates a magnetic field and therefore couples it to the magnetic field of the accretion object. If the magnetic field of the accretion object is weak, the accretion disk is largely flat. The stronger the magnetic field, the larger the radius measured from the accretion object, from which the magnetic field towards the accretion object tears the surrounding matter out of the accretion disk due to the coupling and leads along the magnetic field lines to the poles. Therefore, accretion objects with strong magnetic fields do not have an accretion disk. Assuming that there is now an accretion disk, the Keplerian movement of the particles leads to friction within the disk and heats it up, as a result of which X- rays are emitted as thermal radiation at corresponding temperatures . If the transferred matter hits the surface of the white dwarf or neutron star , this leads to a warming of the crust, which also emits X-rays.

Classification according to the compact star

White dwarf as a partner

If the mass recipient in the binary star system is a white dwarf, soft X-rays are emitted. The indication of the hardness is the ratio between lower-energy and higher-energy X-rays. The cause of the soft X-ray radiation is the typically 10,000 km much larger diameter of the white dwarf compared to that of a neutron star or black hole, so that less energy is released when it falls due to the lower gravitational field . Such systems are called cataclysmic variables . If the white dwarf has a magnetic field, the accretion disk is partially or completely suppressed, and the binary star system belongs to the group of polar or DQ Herculis stars . They show a strong degree of polarization in their optical radiation. If the white dwarf has a magnetic field that is too weak to influence the flow of matter, X-rays are released when matter is transferred from the accretion disk to the white dwarf. With dwarf novae this happens cyclically.

Neutron star as a partner

If the partner is a neutron star or a magnetar , the matter is strongly accelerated by the gravitational field when it falls and releases the energy gained when it hits the surface of the neutron star. Since the material in the accretion disk is in the form of plasma , it is subject to the forces of the magnetic field of the neutron star, whose magnetic field strength can reach up to 10 11  Tesla or 10 15  Gauss . The ionized material follows the magnetic field lines and therefore crashes at the magnetic poles onto the star's surface. Due to the great gravitational potential, the material reaches speeds of up to 100,000 km / s, which is 30% of the speed of light . The impact area has a small area of ​​a few kilometers in diameter and temperatures of 100 million Kelvin are reached there. Most of the energy is emitted as X-rays. The associated power is up to 10,000 solar luminosities. A solar luminosity corresponds to the energy emitted by the sun in the entire spectral range . Due to the rotation of the neutron star and the shadowing by the inflowing material flow, the X-ray radiation is only temporarily emitted in the direction of the earth. That is why the X-ray binary stars with neutron stars and strong magnetic fields are also called X-ray pulsars.

An example of an X-ray pulsar is Hercules X-1 at a distance of 15,000 light years. It was discovered in 1971 by the Uhuru satellite . Over 1,000 such systems are now known in the Milky Way . Another example is Centaurus X-3 , the first X-ray pulsar to be discovered.

Another effect is the transfer of torque through the inflowing matter to the neutron star. This accelerates it to rotational frequencies of up to a few thousand Hertz . This corresponds to one rotation of the neutron star per millisecond. X-ray binary stars are thus the birthplaces for the reborn millisecond pulsars . It has been observed that during outbreaks, i.e. phases of intense mass accretion, the frequency of rotation increases rapidly.

Black hole as a partner

Artist's impression of Cygnus X-1

Due to the lack of a surface, the X-rays in black holes are generated exclusively in the accretion disk. The temperature rises towards the inner edge of the pane and there reaches values ​​which lead to the emission of intensive X-rays. Since black holes have no magnetic field, the plasma falls from the accretion disk through a transition layer into the black hole. The transition layer lies in the plane of the accretion disk. The X-ray radiation fluctuates with non-periodic variations in the seconds and milliseconds range, which are referred to as quasi-periodic oscillations . This radiation pattern is the most important indication of the presence of a black hole in an X-ray binary star in the context of an astronomical observation.

The best candidate for an X-ray binary star with a black hole as the primary star is the X-ray source Cygnus X-1 at a distance of about 6,000 light years.

Classification according to the companion

HMXB (High mass X-ray binaries)

If a star with a mass of more than ten solar masses in a binary star system runs around the common center of gravity with a compact companion, then it is either a Be star , an O star or a blue supergiant . The gas is transferred to the compact star by means of a stellar wind or, in the case of the Be stars, is accreted when passing through a circumstellar gas disk. The period of rotation ranges from a few days to thousands of days. The tracks are often elliptical. In the optical, the light of the massive star dominates.

LMXB (Low mass X-ray binaries)

If the mass of the companion of the compact star is less than two solar masses, it is referred to as a low-mass X-ray binary star. The star transfers mass via the Lagrange point to the compact star, whereby the orbital period of the binary star system ranges from fractions of days to a few days. The companion is either near the main sequence , is a white dwarf or an evolved helium star. Red giants in symbiotic X-ray stars are extremely rare. The companions are difficult to observe, as the accretion disk dominates in the optical . The main sequence companions arise in binary stars in which the massive star has passed through a core collapse or hydrodynamic supernova . The white dwarfs or helium stars predominantly orbit a compact star that was created by an accretion or evolution-induced collapse. LMXB are observed at high galactic latitudes and distances from the galactic plane. Since the compact star, a neutron star or black hole, emerged from a massive star with more than eight solar masses, the LMXB should actually be found along the galactic plane. The supernova explosion was probably asymmetrical and gave the binary star system a high degree of proper motion when the compact star was born .

IMXB (Intermediate mass X-ray binaries)

X-ray binary stars with companions of medium mass and the spectral type A or F are observed quite rarely. The reason for this is that phases with strong stellar winds as with HMXB are very short and a mass transfer as with LMXB across the Roche border is not stable. Because the compact star is more massive than the donor, the orbital axis is shortened , which increases the mass transfer. As a result, the periods with sufficiently strong mass transfer are quite short. In addition, in the case of Roche boundary flux, X-rays are generated when they hit the compact star and at the inner edge of the accretion disk , but the X-rays are often reabsorbed by circumstellar material due to the high mass transfer rates.

classification

X-ray binary stars are divided into partly overlapping classes according to the spectrum, the cause and the type of variability of their radiation:

  • Soft X-ray transient (SXT, German temporary soft X-ray sources) consist of a compact star, a neutron star or a black hole and a red dwarf star . Most of the time, the X-ray radiation is below the detection limit and increases with a cycle of years to decades by more than a factor of 1000 in the optical and X-ray range. During the eruptions, more matter falls on the compact star. The outbreak mechanism is likely to be an instability in the accretion disk around the compact star, as in the dwarf novae . The SXT are also known as X-ray nova.
Fictional representation of a neutron star with a red giant (NASA)
  • Symbiotic X-ray binaries have a red giant as a companion of the compact star, which is either on the red giant branch or the asymptotic giant branch . The transfer of matter to the more compact star takes place with the developed companions mostly via stellar winds. The slow rotation periods of the neutron stars in these X-ray binary stars of up to 18,000 seconds can only be the result of a spherically symmetrical accretion without the presence of an accretion disk, which is why the X-ray luminositydoes not exceed10 36 ergs per second with a typical accretion rate of only 10 −13 solar masses per year . Due to the radii of the red giants, the symbiotic X-ray binary stars have the longest known orbital periods of up to 30,000 days. The X-rays arise from the incidence of a neutron star or as a result of a thermonuclear runaway in symbiotic novae .
  • Super soft X- ray sources (SSS) mainly emit X-rays with energies between 0.09 and 2.5 keV . They are mostly white dwarfs with continuous hydrogen burning on their surface. Most SSS occur in close binary star systems when enough matter is continuously accreted from the companion. This can be the case with polar , VY-Scl, and symbiotic stars. There are also temporary super-soft X-ray sources such as novae and dwarf novae . The SSS also includes individual white dwarfs on their supercooling orbits without necessarily being embedded in a binary star system. This exposed core of a developed star initially emits soft X-rays as thermal radiation . Some of these young white dwarfs are still the central stars of planetary nebulae .
  • Be / X-ray binaries (BeXRB, dt. Be X-ray binary stars) consist of a compact star and a Be star , which at times ejects matter due to rapid rotation and pulsations , which forms an equatorial gas ring around the early star. If the compact star, usually a neutron star, runs through this ring, accretion causes an outbreak in the range of the X-rays.
  • Supergiant X-ray binaries (SGXB, German supergiant X-ray binary stars) have a supergiant as a companion to a compact star. A strong stellar wind with mass loss rates between 10 −8 and 10 −6 solar masses per year at speeds of the outflowing gas of up to 2,000 km / s is characteristic of the supergiants . The compact star in SGXBs is a neutron star in a narrow orbit , and due to the strong mass incidence, the SGXBs are bright objects in the X-ray sky.
  • Supergiant Fast X-ray Transients (SFXT, German supergiant X-ray stars with rapid bursts) have an OB supergiant as a companion to a neutron star. This group of X-ray binary stars shows rapid increases in X-ray brightness during bursts, with maximum brightness being reached within minutes. The outbreaks only last a few hours, while the X-ray brightness increases temporarily by up to 10,000 times compared to the normal brightness. These eruptions could be the result of clumps in the stellar wind of the early supergiant, a passage of the neutron star through a ring of matter in the equatorial plane of the OB supergiant or a magnetic propeller of the pulsar,
  • X-ray bursters show a sudden increase in X-ray radiation due to an explosive ignition of thermonuclear reactions on the surface of a neutron star in an X-ray binary star. During the burst, the accreted hydrogen , helium and possibly carbon are ignited in the state of degeneration . Therefore, the heating does not lead to a cooling expansion, and the thermonuclear reactions cover the entire envelope around the neutron star within fractions of a second. The burst lasts between a few seconds and hours, with the interval between the bursts in a binary star system being in the range of days. The X-ray Burster correspond to the classic Novae , in which there is a thermonuclear runaway on the surface of a white dwarf in a close binary star system.
  • X-ray pulsars show a periodic variability of the X-ray radiation in the order of seconds to minutes and are among the brightest X-ray sources in the sky. This is the result of a strong magnetic field of the neutron star of up to 10 12 Gauss , derived from the cyclotron lines in the X-ray spectrum. Due to the magnetic field, the accreted matter moves along the magnetic field lines and hits the magnetic poles of the neutron star. A shock wave forms above the poles, in whichcyclotron radiation is emittedin addition to bremsstrahlung . If the axis of the magnetic field isinclined tothe axis of rotation ,the X-ray radiation is modulated because themagnetic poles, which are heatedto several million Kelvin, only emit radiation towards earth at times.
  • Accreting Millisecond X-Ray Pulsars are a rare group of LMXB and the immediate precursors of Millisecond pulsars . With them, through the accretion of matter from a companion, not only matter but also angular momentum is transferred to the neutron star. This leads to an increase in the frequency of rotation and to a pulsed emission of X-rays, which is created in shock fronts over the magnetic poles. When the matter transfer stops, the neutron star appears as a rapidly rotating pulsar, a recycled millisecond pulsar. Brown dwarfs , white dwarfs , helium stars and red dwarfs , which orbit the neutron star in narrow orbits with orbits between 50 minutes and 20 hours, have been identifiedas companions of the AMXP.
  • Microquasars are double stars with a neutron star or black hole thatejectsone or two relativistic jets and appears like a small version of a quasar . In quasars, a supermassive black hole accretes matter in the center of a galaxy and emits up to a hundred times the luminosity of the Milky Way. The jets can usually onlybe detectedin the radio range . When a microquasar jet is aimed precisely at Earth, it could appear as an ultra-bright X-ray source. Microquasars with aligned to the observer Jets are also called Mikroblazare referred
  • Ultraluminous X-ray sources (ULX, dt. Ultra-luminous X-ray sources ) are X-ray sources with a luminosity of more than 10 39 erg / s, which, assuming isotropic emission,exceedthe Eddington limit. So far, they have only been detected outside the Milky Way. Due to the rapid variability of the ULX, they are likely accretive black holes in a close binary star system . The X-ray sources are oftenembeddedin extensive emission nebulae that expand at a speed of the order of 100 km / s. The luminosity of this class of X-ray binary stars is so high that they are either medium-weight black holes with masses between 100 and 10,000 solar masses or a stellar black hole with a non- isotropic emission of the X-rays.
  • Ultracompact X-ray binaries (UCXB, dt. Ultracompact X-ray binary stars) consist of a white dwarf or sdB star and a neutron star with an orbital period of less than an hour. The neutron star accretes helium-rich matter and rotates with periods of fractions of a second. The 30 UCXB known in the Milky Way are therefore considered to be potential precursors of millisecond pulsars .
  • Low-luminosity X-ray transients (dt. Temporary X-ray stars with low luminosity) are double stars with a compact star (black hole or neutron star) with an X-ray luminosity of 10 34 to 10 36 erg / s in the range of 2 to 10 keV. The luminosity is 2 to 5 orders of magnitude below that of normal X-ray binary stars. The accretion rate of the compact star is at its peak at 10 −13 solar masses per year and requires an unusual companion in the binary star system. It could be helium stars or planetary bodies. However, some luminous low mass X-ray binaries also show phases with such a low accretion rate. Another name for these X-ray binary stars with low luminosity is very-faint X-ray binary transients . The compact star in these binary star systems is in most cases a neutron star because of the detection of type I bursts.

Occurrence in star catalogs

The General Catalog of Variable Stars lists a large number of sub-categories of X-ray binary stars. In total, despite the many sub-categories, it is only a little over 100 stars. Thus, a little over 0.2% of all stars in this catalog can be counted as X-ray binary stars.

Influence of X-rays on the companion

The X-ray radiation hits the companion's atmosphere and heats the side facing the X-ray source in close binary star systems. This reflection effect leads to a change in the spectrum and the brightness periodically with the period of rotation of the binary star system. The reflection effect is therefore used for optical identification of the X-ray source, since the positional accuracy of X-ray sources is usually only in the order of magnitude of minutes of arc .

X-ray binary stars in globular clusters

Compared to the galactic field , X-ray binary stars appear unusually frequently in globular clusters . These are cataclysmic variables , LMXB (low-mass X-ray binary stars) and their successors, the millisecond pulsars . The cause of the abundance is suspected to be the high star density in these star clusters, which are up to 1000 stars per cubic parsec compared to less than 1 star per cubic parsec in the galactic field. Accordingly, close encounters between stars in globular clusters often occur with the possibility of the formation of a close binary star system through tidal capture, mass exchange in a close binary star system and through collisions. In relation to the stellar mass, the density of LMXB is a factor of 100 greater than in the general galactic field. The density of X-ray binary stars increases with the metallicity . The correlation between the number of X-ray binary stars and the content of heavy elements agrees with the increase in red giants in the globular clusters. Since red giants have a larger cross-section than all other types of stars found in globular clusters, there are also more frequent collisions and tidal traps, which can lead to the formation of an X-ray binary star.

Bursts

The term bursts describes a strong increase in X-ray radiation for a short period of time combined with a slower decrease. A distinction is made between type II, which is attributed to an increase in the accretion rate, and type I, which is the result of thermonuclear reactions on the surface of neutron stars. The type I bursts are further split into normal bursts and superbursts.

The type II bursts are the result of a bistability of the accretion rate in the accretion disk around the compact star. This corresponds to the dwarf nova outbursts in cataclysmic binary star systems , in which a white dwarf receives the flow of matter instead of a neutron star or black hole in the X-ray binary stars.

In the case of type I bursts, the accreted matter on the surface of the neutron star is compressed until it is degenerate and nuclear reactions such as hydrogen burning and helium burning occur . The duration of the normal type I bursts is a few minutes with an increase within a few seconds and the cyclical interval between the bursts is a few hours. The interval between the superbursts is more like months to years. It is believed that during the Superbursts, the ashes of the nuclear reactions of the normal Type I bursts ignite and a fusion of carbon occurs. The type I bursts correspond to the novae in the cataclysmic binary stars. In the cooling phase of a type I burst, a characteristic curve for every X-ray binary star emerges, which suggests that the entire surface of the neutron star always emits X-rays after the end of the thermonuclear reactions. If the distance to the double star is known, the radii and mass of the neutron stars can be estimated. With masses of around 1.5 solar masses and radii of less than 10 kilometers, the calculated values ​​are close to the parameters determined in other ways.

In the case of type I bursts, the X-rays become increasingly softer as the outbreak progresses. This is attributed to a decrease in temperature due to an expansion of the photosphere during the eruptions. In contrast to type II eruptions, type I bursts only occur in X-ray binary stars with low mass. Since the type I outbreaks require a replenishment of freshly accreted material, they usually occur during accretion phases that have already increased the X-ray radiation. Therefore, the X-ray novae and soft X-ray transients with a neutron star produce most of the Type I eruptions.

Quasi-periodic oscillations

In a Fourier analysis of the X-rays, almost all X-ray binary stars show certain frequency ranges with a higher intensity. This phenomenon is known as quasi-periodic oscillations (QPO). The QPO are individually for each binary star system in the range from a few Hertz to Kilohertz and change with the outbreak status, the ratio of hard to soft X-rays and the intensity of the X-rays. Quasi-periodic oscillations are observed in neutron stars , candidates for black holes as well as in white dwarfs as accretion stars and seem to be related to the accretion disk . Most hypotheses suggest that the QPO is a preferred orbit in the accretion disk, but it could also be vibrations in the accretion disk. Assuming a relationship to the smallest possible orbit around the compact star, QPOs are used to limit the mass of black holes and the equation of state of relativistic-degenerate matter inside neutron stars. The QPOs could be caused by the lense thirring effect if the axis of rotation of the accretion disk and the axis of rotation of the compact neutron star deviate from each other by at least 15 °. The resulting precession of the accretion disk should lead to a modulation of the X-ray radiation with the precession period, which is also observed in some low-mass X-ray binary stars that can be eclipsed.

Alternatively, the QPOs could also be the result of a non-symmetrical shape of the accretion disk, which leads to vibrations in the disk. A similar phenomenon is known as dwarf nova oscillation or superhump in the cataclysmic variables. If there is a slight deviation from the axial symmetry and this is in a resonance relationship to the period of rotation of the binary star system, then the asymmetry increases and can lead to quasi-periodic fluctuations in intensity.

See also

  • Gravitational energy as an energy source for the observed processes

Individual evidence

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