مَگنِتاَختَر یا مگنتار (به انگلیسی: Magnetar) یا ستاره مغناطیسی (مغنااختر) نوعی ستاره نوترونی است که میدان مغناطیسی بسیار نیرومندی دارد.
نظریهٔ مربوط به مگنتارهاتوسط رابرت دانکن و کریستوفر تامسون در سال 1992 ارائه شدهاست،ولی اولین انفجار اشعهءگاما مربوط به یک مگنتار در 5 مارس 1979 کشف و ثبت شد.در طول یک دههٔ بعد از آن فرضیه ءمگنتار به صورت گسترده به عنوان تکرارکنندهٔ اشعهٔ گاما و اشعه ء ایکس غیرعادی اختر تپنده پذیرفته شد.
مگنتار اسجیآر ۱۸۰۶–۲۰ یک ستاره نوترونی مگنتار است که در صورت فلکی کمان قرار دارد. فاصله این مگنتار از زمین حدود 50 هزار سال نوری است.
یک انفجار به صورت ستاره لرزه در سطح این مگنتار اتفاق افتاد که در 27 دسامبر سال 2004 تشعشعات آن به زمین رسید . این انفجار در 0.1 ثانیه به اندازه ای انرژی آزاد کرد که خورشید ما در طی 100 هزار سال آزاد میکند . این درخشانترین رویدادی است که در روی زمین با منشا خارج از منظومه شمسی دیده شدهاست. این انفجار اگر در 10 سال نوری زمین اتفاق می افتاد تمام لایه ازون زمین را نابود میکرد و اثری مشابه یک انفجار هسته ای با قدرت 12 هزار تن TNT در فاصله 7.5 کیلومتری ایجاد میکرد.
تصویری هنری از انفجار اشعه گاما در ستاره مگنتار SGR 1806-20
Like other neutron stars, magnetars are around 20 kilometres (12 mi) in diameter and have a mass about 1.4 solar masses. They are formed by the collapse of a star with a mass 10–25 times that of the Sun. The density of the interior of a magnetar is such that a tablespoon of its substance would have a mass of over 100 million tons. Magnetars are differentiated from other neutron stars by having even stronger magnetic fields, and by rotating more slowly in comparison. Most magnetars rotate once every two to ten seconds, whereas typical neutron stars rotate one to ten times per second. A magnetar's magnetic field gives rise to very strong and characteristic bursts of X-rays and gamma rays. The active life of a magnetar is short. Their strong magnetic fields decay after about 10,000 years, after which activity and strong X-ray emission cease. Given the number of magnetars observable today, one estimate puts the number of inactive magnetars in the Milky Way at 30 million or more.
Starquakes triggered on the surface of the magnetar disturb the magnetic field which encompasses it, often leading to extremely powerful gamma-ray flare emissions which have been recorded on Earth in 1979, 1998 and 2004.
Neutron Star Types (24 June 2020)
Magnetars are characterized by their extremely powerful magnetic fields of ∼109 to 1011T. These magnetic fields are a hundred million times stronger than any man-made magnet, and about a trillion times more powerful than the field surrounding Earth. Earth has a geomagnetic field of 30–60 microteslas, and a neodymium-based, rare-earth magnet has a field of about 1.25 tesla, with a magnetic energy density of 4.0 × 105 J/m3. A magnetar's 1010 tesla field, by contrast, has an energy density of 4.0 × 1025 J/m3, with an E/c2 mass density more than 10,000 times that of lead. The magnetic field of a magnetar would be lethal even at a distance of 1,000 km due to the strong magnetic field distorting the electron clouds of the subject's constituent atoms, rendering the chemistry of life impossible. At a distance of halfway from Earth to the moon, an average distance between the Earth and the Moon being 384,400 km (238,900 miles), a magnetar could strip information from the magnetic stripes of all credit cards on Earth. As of 2010[update], they are the most powerful magnetic objects detected throughout the universe.
As described in the February 2003 Scientific American cover story, remarkable things happen within a magnetic field of magnetar strength. "X-rayphotons readily split in two or merge. 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." In a field of about 105 teslas atomic orbitals deform into rod shapes. At 1010 teslas, a hydrogen atom, 1.06 × 10−10m, becomes a spindle 200 times narrower than its normal diameter.
Origins of magnetic fields
The dominant theory of the strong fields of magnetars is that it results 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. But another theory is that they simply result from the collapse of stars with unusually high magnetic fields.
Magnetar SGR 1900+14 (center of image) showing a surrounding ring of gas 7 light-years across in infrared light, as seen by the Spitzer Space Telescope. The magnetar itself is not visible at this wavelength but has been seen in X-ray light.
In a supernova, a star collapses to a neutron star, and its magnetic field increases dramatically in strength through conservation of magnetic flux. 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 108teslas, to more than 1011 teslas (or 1015gauss). The result is a magnetar. It is estimated that about one in ten supernova explosions results in a magnetar rather than a more standard neutron star or pulsar.
On March 5, 1979, a few months after the successful dropping of satellites into the atmosphere of Venus, the two unmanned Soviet spaceprobes, Venera 11 and 12, 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.
Artist's impression of a gamma-ray burst and supernova powered by a magnetar 
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. 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. 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. 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. In 2018, the result of the merger of two neutron stars was determined to be a hypermassive magnetar.
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.
As of July 2021[update], 24 magnetars are known, with six more candidates awaiting confirmation. A full listing is given in the McGill SGR/AXP Online Catalog. Examples of known magnetars include:
SGR 1806−20, located 50,000 light-years from Earth on the far side of the Milky Way 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 Space Telescope discovered a ring of matter around this magnetar. It is thought that this ring formed in the 1998 burst.
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.
As of September 2008[update], 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).
SWIFT J1822.3 Star-1606 discovered on 14 July 2011 by Italian and Spanish researchers of CSIC at Madrid and Catalonia. This magnetar contrary to previsions has a low external magnetic field, and it might be as young as half a million years.
3XMM J185246.6+003317, discovered by international team of astronomers, looking at data from ESA's XMM-Newton X-ray telescope.
SGR 1935+2154, emitted a pair of luminous radio bursts on 28 April 2020. There was speculation that these may be galactic examples of fast radio bursts.
Swift J1818.0-1607 , X-ray burst detected March 2020, is one of five known magnetars that are also radio pulsars. It may be only 240 years old.
Unusually bright supernovae are thought to result from the death of very large stars as pair-instability supernovae (or pulsational pair-instability supernovae). However, recent research by astronomers has postulated that energy released from newly formed magnetars into the surrounding supernova remnants may be responsible for some of the brightest supernovae, such as SN 2005ap and SN 2008es.
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