X-ray pulsar

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An X-ray pulsar is a neutron star which , by means of rotation, generates a strictly periodically modulated signal in the range of X-rays with a period length of milliseconds to 1000 seconds. X-ray pulsars are divided into accretion-driven, rotation-driven and unusual X-ray pulsars according to the cause of the emitted electromagnetic radiation.

Accretion-driven X-ray pulsars

Akkretionsangetriebene X-ray pulsar (engl. Accretion powered X-ray pulsars) are always part of an X-ray double star . The observed period values ​​fall in the range from a few milliseconds to 1000 seconds. Extreme periods of up to a few hours have also been reported, but these have yet to be confirmed. According to the type of companion of the neutron star, the X-ray binary stars are divided into High Mass X-ray Binaries (HMXB), in which the companion is an early giant , or into Low Mass X-ray Binaries (LMXB) with an accompanying late main sequence star . In the case of the LMXBs, matter flows from the companion to the compact star by means of the Roche boundary flow , while the accretion of the HMXBs occurs via the stellar wind of the massive star. The matter heats up strongly, is ionized at the latest, and directed by the magnetic field of the pulsar to the magnetic poles. The X-rays are generated in the form of synchrotron radiation through the deflection of the charged particles in the magnetic field and as bremsstrahlung when they strike the magnetic poles on the surface of the neutron star. The periodicity of the radiation occurs at the rate of rotation of the pulsar because its magnetic poles deviate from the axis of rotation and become periodically visible or invisible. The period of rotation is (apparently) modulated by the orbit of the pulsar around its central star, because the light path to earth changes periodically.

Rotation-driven X-ray pulsars

Rotation powered X-ray pulsars are identical to radio pulsars . Examples are the Vela pulsar and the pulsar in the Crab Nebula , which in addition to pulsed radio radiation also emit periodically modulated optical, UV, X-ray and gamma radiation. In contrast to the long-wave emissions, the pulse profile in the X-ray range often shows a double maximum per rotation period. The periods observed for the rotation-driven X-ray pulsars are between 1.5 milliseconds and 5 seconds, with long-period pulsars only being weak sources in the X-ray range. The pulsed component of the X-ray radiation, like radio radiation, is generated by synchrotron radiation in the rotating magnetic field of the neutron star. Young rotation-driven X-ray pulsars still have a thermal and non-modulated component that is interpreted as black body radiation from the surface of the neutron star, which is several million degrees warm. The luminosity of pulsars decreases over time, with the maximum values ​​in the range of 10 34 -10 36 erg / s.

Rotation-driven X-ray pulsars transmit their signals permanently and with great temporal predictability. These properties can be used to determine the location in interplanetary space with an accuracy of five kilometers. The mode of operation corresponds to that of the Global Positioning System , in which the location can be calculated from the differences in transit time of the pulsed signals.

Unusual X-ray pulsars

Unusual X-ray pulsars (AXPs for short) are a small group of pulsating X-ray sources with periods of 2 to 12 seconds. They show strong bursts of 10 42 -10 44 ergs per eruption. Their spin-down age is around 10,000 to 100,000 years. Most AXPs repeat their eruptions every few years and are closely related to the soft gamma repeaters , whose eruptions and pulsations have been detected in the range of soft gamma radiation . There is probably no difference between the two groups, as some AXPs can also be detected in the area of ​​gamma radiation. Today they are interpreted as manifestations of magnetars . Magnetars are neutron stars with the strongest known magnetic flux densities of 10 14 –10 16 Gauss . The eruptions are interpreted as the energy released when magnetic flux lines short-circuit on the surface of the neutron stars.

A small group of the unusual X-ray pulsars, which are called low magnetic field magnetars , are characterized by a very low deceleration of the rotation with deceleration rates of 10 −13 s / s. The properties of these X-ray pulsars can also be described by rotating white dwarfs with magnetic field densities of 10 8 Gauss. In particular, the observed period jumps are more compatible with the assumption of a massive white dwarf than with a neutron star.

Relationship between accretion-driven and rotation-driven X-ray pulsars

It has long been assumed that accretion-driven X-ray pulsars are the forerunners of the rotation-driven millisecond pulsars . Through the accretion of matter, angular momentum is also transferred to the neutron star, which increases the rotational frequency. With IGR J18245–2452, a transition system has been observed that can be detected at times as a radio millisecond pulsar and at other times as an accretion-driven X-ray pulsar. The fact that matter is still temporarily accreted can be confirmed by means of thermonuclear bursts .

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists a little over 20 stars with the abbreviations XP , XPNG , XPR or XPRM , which means that only about 0.04% of all stars in this catalog belong to the X-ray pulsars class or a sub-category.

Examples

Individual evidence

  1. ^ Fulvio Melia: High-Energy Astrophysics . Princeton University Press, Princeton 2009, ISBN 978-0-691-14029-2 .
  2. ^ Walter HG Lewin , Jan van Paradijs , Edward PJ van den Heuvel: X-ray Binaries . Cambridge University Press, 1997, ISBN 978-0-521-59934-4 .
  3. ^ AGL Lyne, F. Graham-Smith: Pulsars . Johann Ambrosius Barth Verlag, Leipzig 1993, ISBN 3-335-00336-5 .
  4. Werner Becker, Mike G. Bernhardt, Axel Jessner: Autonomous Spacecraft Navigation With Pulsars . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.4842v1 .
  5. ^ Walter Lewin, Michael van der Klies: Compact Stellar X-ray Sources (Cambridge Astrophysics) . Cambridge University Press, Cambridge 2010, ISBN 978-0-521-15806-0 .
  6. K. Boshkayev, L. Izzo, Jorge A. Rueda, R. Ruffini: SGR 0418 + 5729, Swift J1822.3-1606, and 1E 2259 + 586 as massive fast rotating highly magnetized white dwarfs . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.5048v1 .
  7. A. Papitto et al .: Swinging between rotation and accretion power in a millisecond binary pulsar . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.3884v1 .
  8. Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved October 19, 2019 .