Ultra-compact X-ray binary star

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An ultra-compact X-ray double star (engl. Ultra Compact X-ray binary, UCXB) consists of a white dwarf or a helium star, the ground via a accretion on a neutron star transferred. These X-ray binary stars have an orbital period of less than 80 minutes, whereby the shortest known values ​​are only 11 minutes.

construction

UCXB are a subclass of X-ray binary stars with low mass and consist of a white dwarf or helium star that fills its Roche volume and loses matter to a compact star . Due to the short orbital period, it can only be a white dwarf or helium star , as no other star classes are known that fill their Roche limit volume with an orbital period of less than one hour. This assumption is supported by observations that the transferred matter either consists exclusively of helium or is a mixture of carbon and oxygen.

The transferred matter falls on a compact star due to the retention of the torque by an accretion disk . The compact star can be identified by its mass, its magnetic field, its period of rotation, its bursts due to the explosive ignition of nuclear fusions on its surface, as well as its quasi-periodic oscillations from the inner edge of the accretion disk. According to this, the compact star in all confirmed UCXBs is a neutron star and not a black hole .

The X-rays arise mainly from the conversion of potential energy . The matter accelerated by the gravitational field of the neutron star when it falls is slowed down by the viscosity in the accretion disk and when it hits a shock front above the surface of the neutron star, with the bremsstrahlung being emitted as X-rays. Here be luminosities of 10 to 37 erg / s reached, these values are quite low for X-ray binaries. The X-rays are variable in all Ultracompact X-ray binaries. These fluctuations are caused by

  • the temporary ignition of thermonuclear reactions on the surface of the neutron star in the form of bursts
  • Variations in the rate of mass accretion from companion to accretion disk
  • The magneto-rotation instability within the accretion disk changes the viscosity in the disk and thus the flow rate on the neutron star. This mechanism corresponds to that in dwarf novae and the soft X-ray transits.

Most of the Ultracompact X-ray binaries are transient sources that show the eruptions listed above. There is also a small group of persistent UCXBs that show only very small fluctuations in X-ray brightness over a period of decades. These double stars are in a state of high mass transfer, which means that the accretion disk is always ionized and cannot temporarily store any matter. This roughly corresponds to the behavior of the nova-like stars with the cataclysmic variables .

The mean mass transfer rates of 10 −11 solar masses per year are two orders of magnitude higher than would be expected for these binary stars. Therefore, the X-rays are likely to heat up the companion, causing it to expand and lose more mass to the neutron star than would be the case only due to the radiation of gravitational waves.

development

The neutron star in Ultracompact X-ray binaries is created by a supernova or an accretion induced collaps. In the first case, a binary star goes through one or two phases with a common envelope . The probability that the binary star system will be destroyed during the core collapse supernova or that the stars will merge during the common envelope phase is very high. Only a few UCXBs are likely to emerge from the second scenario. After that, a close binary star system arises from two white dwarfs, of which the massive component is a Mg-Ne white dwarf. Due to the mass transfer, it crosses the Chandrasekhar limit and collapses into a neutron star. The further development is dominated by a reduction in the distance between the two stars due to the emission of gravitational radiation .

One third of the ultracompact X-ray binaries have been found in globular clusters . Because of their high stellar density, these star clusters suggest an alternative path of development than in the galactic field. The UCXBs are formed here by dynamic capture of a star. A white dwarf, a neutron star or a main sequence star can be forced into an orbit around the neutron star. In all three cases, there is an unstable mass transfer and after a short time the companion appears as a degenerate nucleus that feeds the accretion disk around the neutron star at a relatively low transfer rate. In the phase of unstable mass transfer, no X-rays from the binary star system can be observed, since the electromagnetic radiation is absorbed by a dense circumstellar shell.

Due to the mass transfer in the ultracompact X-ray binaries, the rotation speed of the neutron star is accelerated; the UCXBs are considered to be a potential source of the millisecond pulsars . When the pulsar mechanism is switched on again, the radiation cone rotates in the orbital plane of the binary star system and hits the companion. The radiation flow of electromagnetic and particle radiation can be so intense that the companion is vaporized. These double stars in the phase of destruction of the companion are known as black widows . This creates a stellar wind from the companion, which transports not only matter but also torque from the binary star system and leads to an increase in the orbit radius. However, hardly any radio-loud millisecond pulsars are found in globular clusters, despite the discovery of a population of ultracompact X-ray binaries. Since the lifespan of UCXBs is quite short with about 10 8 years and the rate of formation of these interacting binary stars should have been roughly constant in the last billion years, a large number of millisecond pulsars should also be discovered as successors to the UCXBs.

Examples

  • SWIFT J1756.9–2508
  • 4U 1820-30
  • 4U 0513-40
  • 2S 0918-549
  • 4U 1543-624
  • 4U 1850-087
  • M 15 X-2
  • XTE J1807-294
  • 4U 1626-67
  • XTE J1751-305
  • XTE J0929-314
  • 4U 1916-05
  • NGC 6440 X-2

Individual evidence

  1. ^ LM van Haaften, R. Voss, and G. Nelemans: Late-time evolution of ultracompact X-ray binaries . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.6332 .
  2. ^ LM van Haaften, R. Voss, and G. Nelemans: Long-term luminosity behavior of 14 ultracompact X-ray binaries . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.0691 .
  3. ^ CO Heinke et al .: Galactic Ultracompact X-ray Binaries: Disk Stability and Evolution . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1303.5864v1 .
  4. ↑ Konstantin Pavlovskii and Natalia Ivanova: Ultra-compact X-ray binaries with high luminosity: a key for a new scenario . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.5162 .
  5. ↑ LM van Haaften, G. Nelemans, R. Voss, MA Wood, and J. Kuijpers: The evolution of ultracompact X-ray binaries . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1111.5978 .
  6. ^ TF Cartwright et al .: Galactic Ultracompact X-ray Binaries: Empirical Luminosities . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1303.5866v1 .
  7. ↑ LM van Haaften, G. Nelemans, R. Voss and PG Jonker: Formation of the planet orbiting the millisecond pulsar J1719-1438 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.6332 .
  8. ^ N. Ivanova: Low-Mass X-ray Binaries in Globular Clusters: Puzzles and Solutions . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.2203 .