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Utilisateur:Nupar/Bac à sable perso

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Description modifier

Comme les étoiles à neutrons, les magnétars ont un diamètre d'environ 20 km et ont une masse bien supérieur à celle du Soleil. La densité intérieure est telle qu'un dé à coudre rempli de sa matière aurait une masse de près de 100 millions de tonnes[1]. Les magnétars sont différents des étoiles à neutrons car ils ont un champ magnétique plus puissant, une rotation plus lente, la plupart des magnétars effectuent une rotation complète une fois toutes les 10 secondes,[2] comparé à moins d'une seconde pour une étoile à neutrons typique. Ce champ magnétique donne naissance à des rayons gamma et X très forts et d'énergie caractéristique. La vie active d'un magnétar est courte : leur puissant champ magnétique décline après environ 10 000 ans, après quoi, l'activité et la forte émission de rayons X cessent. Étant donné le nombre de magnétars observables aujourd'hui, on estime le nombre de magnétars inactifs dans la Voie Lactée à 30 millions ou plus[2].

Un Tremblement d'étoile se déclenche à la surface du magnétar perturbant le champ magnétique qui l'inclut, souvent menant à de puissants sursaut gamma qui ont été enregistré sur Terre en 1979, 1998 et 2004[3].

Champ magnétique modifier

Les magnétars sont caractérisés par leur champ magnétique extrêmement puissants de 108 à 1011 tesla[4]. Ces champs magnétiques sont des centaines de millions de fois plus forts que les aimants que l'Homme a pu fabriquer,[5] et des quadrillions de fois plus puissants que le champ magnétique terrestre[6]. La Terre possède un champ magnétique de 30-60 microteslas, et un aimant au néodyme, un aimant de terre rare possède un champ d'environ 1,25 tesla, avec une énergie magnétique d'une densité de 4.0×105 J/m3. Un magnétar avec un champ de 1010 tesla, a, en comparaison, une énergie d'une densité de 4.0×1025 J/m3, avec une masse E/c2 supérieur à 104 fois celle du plomb. Le champ magnétique d'un magnétar serait létal même à une distance de 1 000 km à cause de la forte distorsion, due au champ magnétique, des nuages d'électrons des atomes constituant un organisme ; rendant la chimie de la vie impossible[7]. À une distance de 190 000 km (la moitié de la distance Terre-Lune), un magnétar pourrait effacer toutes les informations contenues dans les bandes magnétiques des cartes de crédit sur Terre[8]. À partir de 2010, ils sont les objets les plus magnétiques jamais détectés dans l'univers[3],[9].

As described in the February 2003 Scientific American cover story, remarkable things happen within a magnetic field of magnetar strength. "X-ray photons readily split in two or merge together. The vacuum itself is polarized, becoming strongly birefringent, like a calcite crystal. Atoms are deformed into long cylinders thinner than the quantum-relativistic de Broglie wavelength of an electron."[10] In a field of about 105 teslas atomic orbitals deform into rod shapes. At 1010 teslas, a hydrogen atom becomes a spindle 200 times narrower than its normal diameter.[10]

Origins of magnetic fields modifier

The strong fields of magnetars are understood as resulting from a magnetohydrodynamic dynamo process in the turbulent, extremely dense conducting fluid that exists before the neutron star settles into its equilibrium configuration. These fields then persist due to persistent currents in a proton-superconductor phase of matter that exists at an intermediate depth within the neutron star (where neutrons predominate by mass). A similar magnetohydrodynamic dynamo process produces even more intense transient fields during coalescence of pairs of neutron stars.[11]

Formation modifier

Magnetar SGR 1900+14 is in the exact center of the image, which shows a surrounding ring of gas seven light-years across in infrared light, as seen by the Spitzer Space Telescope. The magnetar itself is not visible at this wavelength, but it has been seen in X-ray light.

When, in a supernova, a star collapses to a neutron star, its magnetic field increases dramatically in strength. Halving a linear dimension increases the magnetic field fourfold. Duncan and Thompson calculated that, when the spin, temperature and magnetic field of a newly formed neutron star falls into the right ranges, a dynamo mechanism could act, converting heat and rotational energy into magnetic energy, and increasing the magnetic field, normally an already enormous 108 teslas, to more than 1011 teslas (or 1015 gauss). The result is a magnetar.[12] It is estimated that about one in ten supernova explosions results in a magnetar rather than a more standard neutron star or pulsar.[13]

1979 discovery modifier

On March 5, 1979, a few months after the successful dropping of satellites into the atmosphere of Venus, the two Soviet spacecraft that were then drifting through the Solar System were hit by a blast of gamma radiation at approximately 10:51 EST. This contact raised the radiation readings on both the probes from a normal 100 counts per second to over 200,000 counts a second, in only a fraction of a millisecond.[10]

This burst of gamma rays quickly continued to spread. Eleven seconds later, Helios 2, a NASA probe, which was in orbit around the Sun, was saturated by the blast of radiation. It soon hit Venus, and the Pioneer Venus Orbiter's detectors were overcome by the wave. Seconds later, Earth received the wave of radiation, where the powerful output of gamma rays inundated the detectors of three U.S. Department of Defense Vela satellites, the Soviet Prognoz 7 satellite, and the Einstein Observatory. Just before the wave exited the Solar System, the blast also hit the International Sun-Earth Explorer. This extremely powerful blast of gamma radiation constituted the strongest wave of extra-solar gamma rays ever detected; it was over 100 times more intense than any known previous extra-solar burst. Because gamma rays travel at the speed of light and the time of the pulse was recorded by several distant spacecraft as well as on Earth, the source of the gamma radiation could be calculated to an accuracy of about 2 arcseconds.[14] The direction of the source corresponded with the remnants of a star that had gone supernova around 3000 B.C.E.[3]

Recent discoveries modifier

On February 21, 2008 it was announced that NASA and researchers at McGill University had discovered a neutron star with the properties of a radio pulsar which emitted some magnetically powered bursts, like a magnetar. This suggests that magnetars are not merely a rare type of pulsar but may be a (possibly reversible) phase in the lives of some pulsars.[15] On September 24, 2008, ESO announced what it ascertained was the first optically active magnetar-candidate yet discovered, using ESO's Very Large Telescope. The newly discovered object was designated SWIFT J195509+261406.[16] On September 1, 2014, ESA released news of a magnetar close to supernova remnant Kesteven 79. Astronomers from Europe and China discovered this magnetar, named 3XMM J185246.6+003317, in 2013 by looking at images that had been taken in 2008 and 2009.[17] In 2013, a magnetar PSR J1745-2900 was discovered, which orbits the black hole in the Sagittarius A* system. This object provides a valuable tool for studying the ionized interstellar medium toward the Galactic Center.

Anti-glitch issue modifier

Often magnetars speed up (and more rarely slow down) and many of the reasons for this behaviour have not been fully explained by astrophysics.[18]

Astronomers have theorized that glitches occur when fluid inside the star rotates faster than the crust and suddenly transfers some extra momentum during a disturbance. They think the spectacular outbursts of x-rays occur in the 20 to 30 percent of glitches where the disturbance is violent enough to crack the crust. Because the strange 2012 outburst was accompanied by a slowdown, it has been called an anti-glitch.[19]

« That kind of behaviour is "not at all what you expect if that whole picture of glitches in these stars is correct. It shouldn't happen that way," Kaspi added. "It's hard to imagine how fluid interior is slower than crust."

Kaspi has since contacted theorists who have helped her come up with a possible explanation — that pockets of fluid rotating slower than the crust could be responsible.[20] »

Known magnetars modifier

On 27 December 2004, a burst of gamma rays from SGR 1806-20 passed through the Solar System (artist's conception shown). The burst was so powerful that it had effects on Earth's atmosphere, at a range of about 50,000 light years.

Modèle:As of, 21 magnetars are known, with five more candidates awaiting confirmation.[4] A full listing is given in the McGill SGR/AXP Online Catalog.[4] Examples of known magnetars include:

  • SGR 1806-20, located 50,000 light-years from Earth on the far side of our Milky Way galaxy in the constellation of Sagittarius.
  • SGR 1900+14, located 20,000 light-years away in the constellation Aquila. After a long period of low emissions (significant bursts only in 1979 and 1993) it became active in May–August 1998, and a burst detected on August 27, 1998 was of sufficient power to force NEAR Shoemaker to shut down to prevent damage and to saturate instruments on BeppoSAX, WIND and RXTE. On May 29, 2008, NASA's Spitzer telescope discovered a ring of matter around this magnetar. It is thought that this ring formed in the 1998 burst.[21]
  • SGR 0501+4516 was discovered on 22 August 2008.[22]
  • 1E 1048.1−5937, located 9,000 light-years away in the constellation Carina. The original star, from which the magnetar formed, had a mass 30 to 40 times that of the Sun.
  • Modèle:As of, ESO reports identification of an object which it has initially identified as a magnetar, SWIFT J195509+261406, originally identified by a gamma-ray burst (GRB 070610).[16]
  • CXO J164710.2-455216, located in the massive galactic cluster Westerlund 1, which formed from a star with a mass in excess of 40 solar masses.[23][24][25]
  • SWIFT J1822.3 Star-1606 discovered on 14 July 2011 by Italian and Spanish researchers of CSIC and Catalonia's space studies institute. This magnetar contrary to previsions has a low external magnetic field.
  • 3XMM J185246.6+003317 Discovered by international team of astronomers, looking at data from ESA's XMM-Newton X-ray telescope.

See also modifier

Modèle:Portal

References modifier

Specific
  1. ↑ Ward ; Brown lee, p.286
  2. ↑ a et b (en) « Magnetars, Soft Gamma Repeaters and Very Strong Magnetic Fields », Robert C. Duncan, University of Texas at Austin, (consulté le )
  3. ↑ a b et c Kouveliotou, C.; Duncan, R. C.; Thompson, C. (February 2003). "Magnetars". Scientific American; Page 36.
  4. ↑ a b et c « McGill SGR/AXP Online Catalog » (consulté le )
  5. ↑ (en) « HLD user program, au Dresden High Magnetic Field Laboratory » (consulté le )
  6. ↑ (en) Robert Naeye, « The Brightest Blast », Sky & Telescope, (consulté le )
  7. ↑ (en) Robert Duncan, « `MAGNETARS', SOFT GAMMA REPEATERS & VERY STRONG MAGNETIC FIELDS », University of Texas (consulté le )
  8. ↑ (en) Christopher Wanjek, « Cosmic Explosion Among the Brightest in Recorded History », NASA, (consulté le )
  9. ↑ (en) Dave Dooling, « "Magnetar" discovery solves 19-year-old mystery », Science@NASA Headline News, (consulté le )
  10. ↑ a b et c Kouveliotou, C.; Duncan, R. C.; Thompson, C. (February 2003). "Magnetars". Scientific American; Page 35.
  11. ↑ Daniel J. Price et Stephan Rosswog, « Producing Ultrastrong Magnetic Fields in Neutron Star Mergers », Science, vol. 312, no 5774,‎ , p. 719–722 (PMID 16574823, DOI 10.1126/science.1125201, Bibcode 2006Sci...312..719P, arXiv astro-ph/0603845, lire en ligne) Accès libre
  12. ↑ Kouveliotou, p.237
  13. ↑ S. B. Popov et M. E. Prokhorov, « Progenitors with enhanced rotation and the origin of magnetars », Monthly Notices of the Royal Astronomical Society, vol. 367, no 2,‎ , p. 732–736 (DOI 10.1111/j.1365-2966.2005.09983.x, Bibcode 2006MNRAS.367..732P, arXiv astro-ph/0505406) Accès libre
  14. ↑ Cline, T. L., Desai, U. D., Teegarden, B. J., Evans, W. D., Klebesadel, R. W., Laros, J. G.,, « Precise source location of the anomalous 1979 March 5 gamma-ray transient », Journal: Astrophysical Journal, vol. 255,‎ , L45–L48 (DOI 10.1086/183766, Bibcode 1982ApJ...255L..45C) Accès libre
  15. ↑ Mark Shainblum, « Jekyll-Hyde neutron star discovered by researchers] », McGill University,
  16. ↑ a et b « The Hibernating Stellar Magnet: First Optically Active Magnetar-Candidate Discovered », ESO,
  17. ↑ « Magnetar discovered close to supernova remnant Kesteven 79] », ESA/XMM-Newton/ Ping Zhou, Nanjing University, China,
  18. ↑ (en) Emily Chung, « 'Extreme' star's sudden slowdown stumps astronomers », CBC News,‎ (lire en ligne)
  19. ↑ R. F. Archibald, V. M. Kaspi, C. -Y. Ng, K. N. Gourgouliatos, D. Tsang, P. Scholz, A. P. Beardmore, N. Gehrels et J. A. Kennea, « An anti-glitch in a magnetar », Nature, vol. 497, no 7451,‎ , p. 591–593 (PMID 23719460, DOI 10.1038/nature12159, Bibcode 2013Natur.497..591A, arXiv 1305.6894) Accès payant
  20. ↑ (en) « Glitches in a Neutron Star - Pt. 4 », CBC News,‎ (lire en ligne)
  21. ↑ « Strange Ring Found Around Dead Star »
  22. ↑ Francis Reddy, European Satellites Probe a New Magnetar (NASA SWIFT site, 06.16.09)
  23. ↑ Westerlund 1: Neutron Star Discovered Where a Black Hole Was Expected
  24. ↑ Magnetar Formation Mystery Solved, eso1415 - Science Release (14 May 2014)
  25. ↑ Wood, Chris. "Very Large Telescope solves magnetar mystery" GizMag, 14 May 2014. Accessed: 18 May 2014.
Books and literature
General
  • (en) Michael Schirber, « Origin of magnetars », CNN,‎ (lire en ligne)
  • (en) Robert Naeye, « The Brightest Blast », Sky and Telescope,‎ (lire en ligne)
  • (en) « SWIFT J1822.31606, stella irrequieta », Italiaglobale.it,‎ (lire en ligne)

Modèle:Commons and category

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