X-ray binary

An X-ray binary star is a binary star system that emits X-rays due to the conversion of gravitational energy into electromagnetic radiation. The energy produced by accretion, the flow of matter from one of the two stars to its compact partner. The compact star, it may 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 ways:

• As a stellar wind from the companion, who falls into the domain of attraction of the compact star. Such stellar winds are frequently found in main sequence stars and giant high mass.
• For stars that exceed the Roche limit, matter flows over the Lagrange point to the compact partner. Such a flow of matter can last several hundred million years.

Due to conservation of angular momentum, the material falls directly onto the compact partner, but first forms an accretion disk around the degenerate star. If in addition a magnetic field before, so it depends on its strength, how much the accretion disk is deformed. Due to the high heat in the accretion disk, the local matter is ionized and carries a charge per particle. This charge affects movement within the accretion a current which forms a magnetic field and, therefore, coupled with the magnetic field of the accreting object. If the magnetic field of the accreting object decreases, the accretion disk is largely flat. The stronger the magnetic field is, the greater is the value measured by accreting object radius from which the magnetic field for accreting object back tears the surrounding matter due to coupling of the accretion disk and along the magnetic field lines to the poles leads up. Therefore accreting objects with strong magnetic fields have no accretion disk. Suppose that there would now be a accretion before, the movement of the particles leads to Keppler'sche friction within the plate and heated to this, which emitted X-rays at respective temperatures as a heat radiation. Exceeds the matter transferred to the surface of the white dwarf or neutron star, this leads to heating of the crust, which also emits X-rays.

Classification according to the compact star

White dwarf as a partner

If the mass receiver in a double star system a white dwarf, as soft X-ray radiation is emitted. It is in the indication of the hardness of the relationship between low-energy to higher energy X-rays. Cause of the soft X-ray radiation is typically 10,000 km with the much larger diameter of the white dwarf in comparison to that of a neutron star or black hole, so that the case due to the lower gravitational field less energy is released. We call such systems as cataclysmic variables. If the white dwarf via a magnetic field, the accretion disc is partially or completely suppressed, and the binary star system belongs to the group of polar or DQ Herculis stars. They exhibit a strong degree of polarization in their optical radiation. If the white dwarf via a magnetic field is too weak to influence the flow of matter, so X-ray radiation is released when matter is transferred from the accretion disk onto the white dwarf. This happens cyclically in dwarf novae.

Neutron star as a partner

Is the partner a neutron star or a magnetar, the matter is greatly accelerated in the fall by the gravitational field and sets the energy generated on impact with the surface of the neutron star free. Since the material is present as a plasma in the accretion disk, it is subject to the forces of the magnetic field of the neutron star whose magnetic field strength can reach up to 1011 Tesla or Gauss in 1015. The ionized material follows the magnetic field lines and thus falls to the magnetic poles to the star surface. Due to the large gravitational potential, the material it reaches speeds of up to 100,000 km / s, which corresponds to 30% of the speed of light. The impact area has reached a lower area of ​​a few kilometers in diameter and there are temperatures of 100 million degrees Kelvin. Most of the energy is emitted as X-rays. The corresponding capacity is up to 10,000 solar luminosities. A solar luminosity corresponds to the light radiated by the sun in the entire spectral energy. Due to the rotation of the star and the neutron shadowing by the flowing stream of material, the X-ray is temporarily emitted only in the direction of the earth. Therefore, the X-ray binaries are called neutron stars and strong magnetic fields and x-ray pulsars.

An example of an X-ray Pulsar Hercules X 1 at a distance of 15,000 light years. It was discovered in 1971 by the Uhuru satellite. Meanwhile, more than 1,000 such systems are known in the Milky Way. Another example is Centaur X 3, the first Röntgenpulsar discovered.

Another effect is the transfer of torque through the inflowing matter to the neutron star. This means of giving the rotation frequencies of up to several thousand hertz. This corresponds to a rotation of the neutron star per millisecond. X-ray binaries are thus the birthplaces for the born-again Millisekundenpulsare. It has been observed that increases rapidly during outbreaks, so periods of intense mass accretion, the rotation frequency.

Black hole as a partner

Because of the lack of a surface is created in black holes, the X-rays only in the accretion disk. The temperature rises to the inner edge of the disk to go where it reaches values ​​that lead to the emission of intense X-ray radiation. Since black holes have no magnetic field, the plasma from the Akkretionscheibe falls through a transition layer into the black hole. The transition layer is in the plane of the accretion disk. The X-ray varies with variations in the second non-periodic and the millisecond range, which are referred to as quasi-periodic oscillations. This radiation pattern is in the context of an astronomical observation, the most important evidence of the existence of a black hole in an X-ray binary star.

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 for the companion

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

If the mass of the companion to the compact star in fewer than two solar masses, it is referred to as a low-mass X-ray binary star. The star transfers mass via the Lagrangian point to the compact star, the orbital period of the binary system of fractions of days ranging up to several days. The companion is located either near the main sequence, is a white dwarf or a developed helium star. Red giants in symbiotic X-ray stars are extremely rare. The companions are difficult to observe, as in the optical dominates the accretion disk. The main sequence companion arise in binary stars, in which the massive star has undergone a core collapse or hydrodynamic supernova. The white dwarfs or helium stars revolve primarily a compact star, which is caused by an accretion - induced collapse or an evolution. LXMB be observed at high galactic latitudes and distances from the galactic plane. Because of the compact star, a neutron star or black hole, is the product of a massive star with more than eight solar masses should LXMB actually be found along the galactic plane. Probably the supernova explosion was asymmetric and has the double star system at the birth of the compact star given a high proper motion.

X-ray binaries with intermediate-mass companions and the spectral type A or F are quite rare. The reason is that periods are very short with a strong stellar wind as HMXB and mass transfer as in LMXB about the Roche limit is not stable. Because of the compact star is more massive than the donor shortens the path axis, which increases mass transfer enhanced. As a result, the periods with sufficiently strong mass transfer are quite short. In addition, in the case of X-ray radiation is produced Rochegrenzfluß on impact with the compact star and the inner edge of the accretion disk, but the X-rays is due to the high mass transfer rates are often absorbed by zirkumstellares material again.

Classification

X-ray binaries to the variability of their radiation are divided into partially overlapping classes according to the spectrum of the cause and type:

• Soft X -ray transient ( SXT, dt 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-rays 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 area. In the outbreaks increasingly matter falling onto the compact star. The outbreak mechanism is probably an instability in the accretion disk around the compact star as the dwarf novae. The SXT be referred to as X- Nova.

• Symbiotic X -ray binaries (German Symbiotic X-ray binaries ) have as a companion of the compact star, a red giant, located either on the Red Giant Branch or Asymptotic Giant Branch. The transfer of matter to the compact star is done with the developed companions mostly on stellar winds. The slow rotation periods of the neutron stars in these X-ray binaries of up to 18,000 seconds, only be a consequence of a spherically -symmetric accretion without the presence of an accretion disk, which is why the X-ray luminosity of 1036 erg per second does not exceed, in a typical accretion rate of only 10-13 solar masses per year. Due to the radii of the red giant, the symbiotic X-ray binaries have the longest known orbital periods of up to 30,000 days. The X-rays produced by the incident on a neutron star, or as a result of a thermonuclear runaway in Symbiotic novae.
• Super Soft ( X -ray) Sources ( SSS, dt Super Soft X-ray sources ) Send predominantly X-rays with energies from 0.09 to 2.5 keV. It is predominantly white dwarfs with continuous hydrogen burning on their surface. Most SSS occur in close binary systems, if is continuously accreted enough matter from the companion. This can result in polars, his VY Scl stars and symbiotic stars the case. There are also temporary Super Soft X-ray sources such as novae and dwarf novae. Also among the SSS without being necessarily embedded in a binary star system, single white dwarfs on their over-cooling paths. This exposed core of a star radiates developed at the beginning of soft X-rays as heat radiation. These young white dwarfs are still partially the central stars of planetary nebulae.
• Be / X -ray binaries ( BeXRB, dt Be - X-ray binaries ) consist of a compact star and a Be star, which ejects at times due to rapid rotation and pulsations matter which build up as an equatorial gas ring around the early star. Does the compact star, usually a neutron star, through this ring as an outbreak is generated in the field of X-ray radiation on accretion.
• Supergiant X -ray binaries ( SGXB, dt supergiant X-ray binaries ) have a supergiant as a companion of a compact star. Characteristic of the supergiant is 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 The compact star in SGXBs is a neutron star in a close orbit and due to the strong mass incidence are the SGXB bright objects on X-ray sky.
• Supergiant Fast X -ray Transients ( SFXT, dt supergiant X-ray stars with rapid bursts ) have an OB - supergiant as a companion of a neutron star. This group of X-ray binaries shows rapid increases of the X-ray brightness during outbursts, the maximum brightness is reached within minutes. The outbreaks only last for a few hours while the X-ray brightness temporarily rises by up to 10,000 times in relation to the rest brightness. These outbreaks could be the result of clumps in the stellar wind of early supergiants, a passage of the neutron star by a ring of matter in the equatorial plane of the OB supergiant or a magnetic propeller of the pulsar,
• X -ray Burster (German X-ray burster ) show a sudden increase in X-rays due to an explosive ignition of thermonuclear reactions on the surface of a neutron star in an X-ray binary star. Wherein ignition of the burst accreted hydrogen, helium and possibly the carbon, in the state of degeneration occurs. Therefore, the heating does not lead to a cooling expansion and the thermonuclear reactions capture the entire envelope around the neutron star within a split second. The burst lasts from several seconds to hours, with the distance between the bursts is located in a binary star system in the range of days. The X -ray Burster correspond to the classical novae, where there is a thermonuclear runaway on the surface of a white dwarf in a close binary system.
• X -ray pulsars (German Röntgenpulsare ) show a periodic variability of X-ray radiation on 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 from to 1012 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. Above the poles to form a shock wave in the next bremsstrahlung and cyclotron radiation is emitted. If the axis of the magnetic field towards the axis of rotation inclined, so there is a modulation of the X-rays, because the heated to several million Kelvin magnetic poles only intermittently emit radiation toward Earth.
• Accreting Millisecond X - Ray Pulsars are a rare group of LMXB and the immediate precursors of Millisekundenpulsaren. With them also of angular momentum to the neutron star is transferred through the accretion of matter from a companion besides matter. This leads to an increase in the rotational frequency, and since the material of the magnetic field lines in a pulsed emission of X-rays, which results in shock fronts on the magnetic poles. If mass transfer is completed, the neutron star appears as a rapidly rotating pulsar, a recycled millisecond pulsar. As a companion to the AMXP brown dwarfs, white dwarfs, helium stars and red dwarfs have been identified that orbit the neutron star in narrow lanes with circulation periods of between 50 minutes and 20 hours.
• Microquasare are binaries with a neutron star or black hole, which one or two relativistic jets ejects and it appears like a small version of a quasar. In quasars, supermassive black hole at the center of a galaxy accretes matter, emitting up to one hundred times luminosity of the Milky Way. The jets can usually only be detected in the radio band. If a jet of a microquasar is directed towards Earth could this appear as an ultra-bright X-ray source. Microquasars are aligned with the observer Jets are also referred to as Mikroblazare
• Ultraluminous X - ray sources ( ULX, dt ultra -luminous X-ray sources ) are X-ray sources with a luminosity of more than 1039erg / s, exceeding the Eddington limit assuming an isotropic emission. They have so far been detected only outside the Milky Way. Because of the rapid variability of the ULX is likely to be accreting black holes in a close binary system. The X-ray sources are often embedded in extended emission nebula, which are expanding at a rate on the order of 100 km / s. The luminosity of this class of X-ray binaries is so high that it either is medium weight black holes with masses 100-10000 solar masses or a stellar black hole with a non- isotropic emission of X-rays.
• Ultra Compact X -ray binaries ( UCXB ​​, dt Ultra-compact X-ray binaries ) consist of a white dwarf and sdB star and a neutron star with an orbital period of less than one hour. The neutron star accretes helium- rich matter and rotates with periods of fractions of a second. Therefore, the 30 known in the Milky Way UCXB ​​regarded as potential precursors of Millisekundenpulsaren.
• Low- luminosity X -ray transients (Eng. temporary X-ray stars with low luminosity ) are binaries with a compact star ( black hole or neutron star ) with an X-ray luminosity of 1034-1036 erg / s in the range of 2 to 10 keV. The luminosity is about 2 to 5 orders of magnitude below the normal X-ray binaries. The accretion rate of the compact star is in the forefront of 10-13 solar masses per year and requires an unusual companion in the double star system. It could be either helium star or planetary body. However, also show some luminous low mass X -ray binaries phases with such a low accretion rate. Another name for these X-ray binaries with low luminosity is very- faint X -ray binary transients. The compact star in this double star systems is in most cases a neutron star because of the detection of type I bursts.

Influence of X-ray radiation on the companion

The X-ray radiation hits the atmosphere of the companion and the X-ray source heats the facing page in close binary systems. This reflection effect leads to a change in the spectrum and the brightness periodically with the orbital period of the binary system. Therefore, the reflection effect is used for optical identification of the X-ray source, since the positioning accuracy of X-ray sources is usually only of the order of minutes of arc.

X-ray binaries in globular clusters

Compared with the galactic field occur in globular cluster X-ray binaries on unusually frequent. It involves cataclysmic variables, LMXB ( low mass X-ray binaries ) and their successors, the Millisekundenpulsare. The cause of the frequency is in the large stellar density assumed in these clusters, which are up to 1000 stars per Kubikparsec compared to less than 1 star per Kubikparsec in the galactic field. According frequently occurs in globular clusters to close encounters between stars with the possibility of formation of a close binary system by Gezeiteneinfang, mass exchange in a close binary system and by collisions. Based on the stellar mass density of LMXB is a factor of 100 greater than in the general galactic field. The density of X-ray binaries increases with metallicity. The correlation between the number of X-ray binaries with the content of heavy elements is consistent with the increase in red giants in the globular clusters. Since red giants a larger cross -section than all the other occurring in globular cluster star species, it is also likely to collisions and Gezeiteneinfängen that can lead to the formation of an X-ray binary star.

Bursts

The term describes a sharp increase of the bursts of X-rays for a short time connected to a slower drop. The bursts can be distinguished between the type II, which is attributed to an increase in the accretion rate, and the 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 super burst.

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

In the type I bursts the accreted matter is compressed to the surface of the neutron star until it is degenerate and it comes to nuclear reactions such as the hydrogen burning and helium burning. The duration of the normal type I bursts is a few minutes with an increase within a few seconds and the cyclic distance between the bursts in a few hours. The distance between the super burst is closer to months to years. It is believed that in the super bursts the ashes of the nuclear reactions of the normal type I bursts and ignites, there is a fusion of carbon. The type I bursts correspond to novae in the cataclysmic binary stars. In the cooling phase of a type I bursts shows a characteristic for each ray binary gradient, which suggests that getting the entire surface of the neutron star X-ray radiation emitted by the end of the thermonuclear reactions. Is the distance to the known double-star, the radii and the mass of the neutron stars can be estimated. Calculated values ​​with masses of about 1.5 solar masses and radii of less than 10 kilometers close to the parameters specified in other ways.

In Type I bursts the X-ray radiation in the course of the outbreak is getting softer. This is attributed to a reduction in temperature due to expansion of the photo- sphere at the eruptions. Type I bursters occur in contrast to the type II flares only in X-ray binaries with low mass. Since the type I outbursts need a supply of fresh material akkretiertem they mostly occur during Akkretionsphasen who have already increased the X-rays. Therefore, the X-ray novae and soft X -ray Transients I produce eruptions with a neutron star, most type.

Quasi- Periodic Oscillations

In a Fourier analysis of the X-rays show up at almost all X-ray binaries certain frequency ranges with a higher intensity. This phenomenon is referred to as quasi-periodic oscillations ( QPO ). The QPO are individual for each binary star system in the range of a few hertz to kilohertz and change with the outbreak status, the ratio of hard to soft X-rays and the intensity of X-rays. Quasi- periodic oscillations are observed in both neutron stars, black holes candidates as well as accreting white dwarfs as a star and seem to be associated with the accretion disk. Most hypotheses suggest the QPO as a preferred orbit in the accretion disk, but it could also be oscillations in the accretion disk. QPOs are used under the assumption of a relationship with the smallest possible orbit around the compact star to limit the mass of black holes and the equation of state of relativistic degenerate matter in the interior of neutron stars. The QPOs could be caused by the Lense- Thirring effect, if the rotation axis of the accretion disk and the rotation axis of the compact neutron star differ by at least 15 ° from each other. 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 eclipsing X-ray binaries with low mass.

Alternatively, the QPOs may also be the result of a non-symmetrical shape of the accretion which leads to vibrations in the disc. A similar phenomenon is known as Zwergnovaoszillation or as Superhump in cataclysmic variables. In cases where a slight deviation from the axial symmetry and is this in a resonant relationship with the orbital period of the binary system, then amplifies the asymmetry and can cause quasi-periodic intensity variations.