Soft gamma repeater

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Soft gamma repeaters ( SGR abbreviation ) are a class of compact stars with outbreaks and flares in the area of ​​hard X-ray or soft gamma radiation , which are repeated over a period of days to decades. In contrast to other isolated neutron stars , the energy emitted is not generated from the rotational energy .

properties

The distances to the soft gamma repeaters are generally not known. From the spatial distribution in the sky, the distances have been estimated at an average of several kiloparsecs , since the SGR are in the galactic plane . Only some SGR have been observed in or near the center of supernova remnants and their distance can be determined independently. Because of their proximity to supernova remnants, soft gamma repeaters are likely to be young neutron stars that were formed by a gravitational collapse of a massive star.

The rotation periods of the SGR are in the range of 2 to 12 seconds and slow down their rotation by 10 −10 to 10 −12 s / s. The deceleration is not uniform, but, as with pulsars , there are period jumps . However, the soft gamma repeaters also show negative period jumps in which the period slows down in a very short period of time.

About half of all SGR can be detected in the X-ray range outside of the eruptions with very low luminosities of 10 35 to 10 36 erg / s . The rest X-ray spectrum can be described as a soft black body radiation with a superimposed Power Law component . In addition, there can be a small portion of a pulsed hard X-ray spectrum with energies of up to one MeV .

Some soft gamma repeaters could be detected as weak optical sources in the near infrared , which are also pulsed. The optical radiation can be correlated or anticorrelated to the X-ray or gamma radiation. Some SGRs were also found in the mid-infrared. This radiation is interpreted as the result of a disk of debris around the compact star.

Outbreaks and flares

All soft gamma repeaters show short flares with a maximum luminosity of 10 38 to 10 42 erg / s and a duration of 0.01 to one second with a median of 0.1 s. They usually consist of one or a few pulses with a steep rise and a slow fall. A flare can occur individually or as a group of 10 to 100 over a period of a few hours. The permanent radiation also increases slightly during a flare series. The emitted radiation corresponds to a hard bremsstrahlung spectrum of 30 to 40 keV.

Large outbreaks have luminosity of 10 44 to 10 46 erg / s. They start with a short hard spike and a slowly decaying fade with superimposed pulsating radiation. The beginning of the eruption is interpreted as an emission from a relativistically expanding plasma of electrons and positrons , while the pulsating radiation is viewed as a ball of fire captured by a magnetic field that rotates around the compact star. The duration of a large outbreak is a few tens of seconds. Sometimes short precursors are observed before the actual outbreak and an afterglow in the area of radio radiation and soft gamma radiation.

Quasi-periodic oscillations in the range from 20 to 150 Hz were detected in the descending range of large bursts. For some SGRs, radio emissions can be observed for a period of several months after the outbreaks. The radio emission shows an anisotropic emission of relativistically accelerated matter with a mass of 10 24 to 10 25 grams in the form of clubs.

Very rarely are weak bursts observed with energies and durations between the large bursts and the flares.

Unusual X-ray pulsars

The unusual X-ray pulsars (anomalous X-ray pulsar, AXP for short) have historically been differentiated from the soft gamma repeaters, as the former were discovered in the area of ​​X-rays, while gamma-ray detectors provided the first observations of soft gamma repeaters. Today it is assumed that the two star classes do not differ in their properties. The two classes are combined as SGRs / AXPs.

Magnetar model

The standard model for soft gamma repeaters and unusual X-ray pulsars is based on a neutron star with a high magnetic flux density , the magnetars . The emitted radiation is extracted from a magnetic field of 10 13 to 10 15 Gauss . This assumption is based on the fact that

  • the energy stored in the rotation in neutron stars is insufficient for the electromagnetic radiation emitted in large eruptions
  • the AXP / SGR cannot be observed in binary star systems and thus accretion as with the X-ray binary stars can be excluded
  • the thermal energy is insufficient for 10,000 year old neutron stars. This is likely to be the middle age for soft gamma repeaters.

The reasoning for the high magnetic flux density is based on the rapid rise in the large bursts, which corresponds to the propagation speed of an Alfvén wave around a neutron star. The second line of argument relates to the observation that the Eddington luminosity is exceeded by several orders of magnitude in the eruptions and that other known emission mechanisms can thus be excluded.

A strong magnetic field can be generated when the neutron star is born if, due to the rapid onset of neutrino cooling, turbulent convection occurs and thus creates a dynamo effect. The magnetic field is subsequently frozen in the cooling crust. Alternatively, the magnetic field could be enhanced from the original magnetic field of a high mass star during gravitational collapse. Young magnetars are likely to be quickly decelerated to periods of rotation of a few seconds within a period of a few 1,000 years due to dipole radiation and a stellar wind made up of electrically charged particles that interact with the magnetic field.

The magnetic field deforms the crust of the neutron star and generates the persistent X-rays in the resting phases through low seismic activities. The emission during the flares and large bursts is less well understood. Either magnetic short circuits occur in the field lines, as a result of which the magnetic energy is converted into thermal energy. Alternatively, the crust of the neutron star could break open due to magnetic stress, releasing energy similar to an earthquake.

The traditional magnetar model is being challenged by AXPs / SGRs SGR 0418 + 5729 and Swift J1822–1606. Their magnetic field, derived from the measured period slowdowns, is in the range of 10 12 G. This value corresponds to that of normal radio pulsars. However, neither pulsars show the behavior of soft gamma repeaters, nor was pulsed radio radiation ever observed with AXPs / SGRs. However, in May 2008 the X-ray pulsar PSR J1846-0258 showed four short bursts that are similar, but not identical, to the flares of soft gamma repeaters.

Alternative models

In addition to the magnetar model, alternative hypotheses have been developed to explain some unusual SGRs / AXPs or to replace the magnetar model:

  • Massive white dwarfs with strong magnetic fields and emission mechanisms similar to the magnetar model
  • By accretion in binary star systems with a neutron star, which has an unusually stiff crust
  • White dwarf pulsar, where white dwarfs act like classic pulsars with a strong magnetic field
  • As an interaction with a fall baking disc
  • As a quark star with a strong magnetic field

Examples

Individual evidence

  1. ↑ Sandro Mereghetti: Pulsars and magnetars . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1304.4825v1 .
  2. ↑ JA Kennea et al .: Swift Discovery of a New Soft Gamma Repeater, SGR J1745-29, near Sagittarius A * . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1305.2128v2 .
  3. ↑ David Murphy et al .: Detecting Long-Duration Narrow-Band Gravitational Wave Transients Associated with Soft Gamma Repeater Quasi-Periodic Oscillations . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1302.3915v1 .
  4. ↑ JG Coelho, M. Malheiro: Similarities of SGRs with low magnetic field and white dwarf pulsars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.2892v1 .
  5. ↑ Sirin Caliskan, Unal Ertan: On the X-ray Outbursts of Transient Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1209.1015v1 .
  6. ↑ Zachary Prieskorn, Philip Kaaret: Burst Fluence Distributions of Soft Gamma Repeaters 1806-1820 and 1900 + 14 in the RXTE PCA era . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.0045v1 .
  7. ↑ Michael Gabler, Pablo Cerdá-Durán, Nikolaos Stergioulas, José A. Font, Ewald Müller: Imprints of superfluidity on magneto-elastic QPOs of SGRs . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1304.3566v1 .
  8. ↑ JG Coelho, M. Malheiro: Magnetic dipole moment of SGRs and AXPs as white dwarf pulsars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.6078v1 .
  9. ↑ Kelsey Hoffman, Jeremy Heyl: Mechanical Properties of Non-Accreting Neutron Star Crusts . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1208.3258v1 .
  10. ^ H. Tong, RX Xu, LM Song, GJ Qiao: Wind braking of magnetars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.1626v1 .
  11. ↑ Jeremy S. Heyl, Ramandeep Gill: Magnetic Reconnection Instabilities in Soft-Gamma Repeaters . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1302.5715v1 .
  12. ↑ R. Turolla, P. Esposito: low-magnetic-field magnetars . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1303.6052v1 .
  13. ↑ JA Rueda, K. Boshkayev, L. Izzo, R. Ruffini, P. Loren Aguilar, B. Kulebi, G. Aznar Siguan, E. Garcia Berro: A white dwarf merger as progenitor of the anomalous X-ray pulsar 4U 0142 +61? In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1306.5936v1 .
  14. ↑ J.Wang, H.-K. Chang: Can SGRs / AXPs originate from neutron star binaries? In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.4766v1 .
  15. ↑ JG Coelho, M. Malheiro: Magnetic dipole moment of SGRs and AXPs as white dwarf pulsars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.6078v1 .
  16. ↑ XW Liu, RX Xu, GJ Qiao, JL Han, H. tong: Braking PSR J1734-3333 by a possible fall-back disk . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.4185v1 .
  17. ↑ Guojun Qiao, Xiongwei Liu, Renxin Xu, Yuanjie Du, Jinlin Han, Hao Tong, Hongguang Wang: AXPs & SGRs: Magnetar or Quarctar? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.3298v1 .