The existence of magnetars was proposed in 1992 byRobert Duncan andChristopher Thompson.[3] Their proposal sought to explain the properties of transient sources of gamma rays, now known assoft gamma repeaters (SGRs).[4][5] Over the following decade, the magnetar hypothesis became widely accepted, and was extended to explainanomalous X-ray pulsars (AXPs). As of July 2021[update], 24 magnetars have been confirmed.[6]
Like otherneutron stars, magnetars are around 20 kilometres (12 mi) in diameter, and have a mass of about 1.4 solar masses. They are formed by the collapse of a star with a mass 10–25 times that of theSun. 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.[2] Magnetars are differentiated from other neutron stars by having even stronger magnetic fields, and by rotating more slowly in comparison. Most observed magnetars rotate once every two to ten seconds,[8] whereas typical neutron stars, observed as radiopulsars, rotate one to ten times per second.[9] 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 compared to other celestial bodies. 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 theMilky Way at 30 million or more.[8]
Starquakes triggered on the surface of the magnetar disturb the magnetic field which encompasses it, often leading to extremely powerfulgamma-ray flare emissions which have been recorded on Earth in 1979, 1998 and 2004.[10]
Magnetars are characterized by their extremely powerful magnetic fields of ~109 to 1011T.[6] These magnetic fields are a hundred million times stronger than any man-made magnet,[11] and about a trillion times more powerful than thefield surrounding Earth.[12] Earth has ageomagnetic field of 30–60 microteslas, and aneodymium-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 of4.0×1025 J/m3, with anE/c2 mass density more than 10,000 times that oflead. 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 sustaining life impossible.[13] 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 wipe information from the magnetic stripes of allcredit cards on Earth.[14] As of 2020[update], they are the most powerful magnetic objects detected throughout the universe.[10][15]
As described in the February 2003Scientific American cover story, remarkable things happen within a magnetic field of magnetar strength. "X-rayphotons readily split in two or merge. The vacuum itself ispolarized, becoming stronglybirefringent, like acalcite crystal.Atoms are deformed into long cylinders thinner than the quantum-relativisticde Broglie wavelength of an electron."[4] In a field of about 105 teslasatomic orbitals deform into rod shapes. At 1010 teslas, ahydrogen atom becomes 200 times as narrow as its normal diameter.[4]
The dominant model of the strong fields of magnetars is that it results from amagnetohydrodynamic dynamo process in the turbulent, extremely dense conducting fluid that exists before the neutron star settles into its equilibrium configuration.[16] 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 duringcoalescence of a pair of neutron stars.[17] An alternative model is that they simply result from the collapse of stars with unusually strong magnetic fields.[18]
Magnetar SGR 1900+14 (center of image) showing a surrounding ring of gas 7 light-years across in infrared light, as seen by theSpitzer Space Telescope. The magnetar itself is not visible at this wavelength but has been seen in X-ray light.
In asupernova, a star collapses to a neutron star, and its magnetic field increases dramatically in strength through conservation ofmagnetic flux. Halving a linear dimension increases the magnetic field strength fourfold. Duncan and Thompson calculated that when the spin, temperature and magnetic field of a newly formed neutron star falls into the right ranges, adynamo 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 amagnetar.[19] It is estimated that about one in ten supernova explosions results in a magnetar rather than a more standard neutron star or pulsar.[20]
On March 5, 1979, a few months after the successful dropping oflanders into the atmosphere ofVenus, the two uncrewed Soviet spaceprobesVenera 11 and12, then inheliocentric orbit, 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.[4]
This was the strongest wave of extra-solar gamma rays ever detected at over 100 times as intense as any previously known burst. Given thespeed of light and its detection by several widely dispersed spacecraft, the source of thegamma radiation could be triangulated to within an accuracy of approximately 2arcseconds.[21] The direction of the source corresponded with the remnants of a star that hadgone supernova around 3000 BCE.[10] It was in theLarge Magellanic Cloud and the source was namedSGR 0525-66; the event itself was namedGRB 790305b, the first-observed SGR megaflare.
Artist's impression of a gamma-ray burst and supernova powered by a magnetar[22]
On February 21, 2008, it was announced that NASA and researchers atMcGill 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.[23] On September 24, 2008,ESO announced what it ascertained was the first optically active magnetar-candidate yet discovered, using ESO'sVery Large Telescope. The newly discovered object was designated SWIFT J195509+261406.[24] On September 1, 2014,ESA released news of a magnetar close to supernova remnantKesteven 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.[25] In 2013, a magnetarPSR J1745−2900 was discovered, which orbits theblack hole in theSagittarius A* system. This object provides a valuable tool for studying the ionizedinterstellar medium toward theGalactic Center. In 2018, the temporary result of themerger of two neutron stars was determined to be a hypermassive magnetar, which shortly collapsed into a black hole.[26]
On 27 December 2004, a burst of gamma rays fromSGR 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 50,000light-years.
As of July 2021[update], 24 magnetars are known, with six more candidates awaiting confirmation.[6] A full listing is given in theMcGill SGR/AXP Online Catalog.[6] 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 ofSagittarius and the most magnetized object known.
SGR 1900+14, located 20,000 light-years away in the constellationAquila. 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 forceNEAR Shoemaker to shut down to prevent damage and to saturate instruments onBeppoSAX,WIND andRXTE. On May 29, 2008, NASA'sSpitzer Space Telescope discovered a ring of matter around this magnetar. It is thought that this ring formed in the 1998 burst.[32]
1E 1048.1−5937, located 9,000 light-years away in the constellationCarina. The original star, from which the magnetar formed, had a mass 30 to 40 times that of theSun.
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).[24]
SWIFT J1822.3 Star-1606 discovered on 14 July 2011 by Italian and Spanish researchers ofCSIC 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.[37]
3XMM J185246.6+003317, discovered by international team of astronomers, looking at data from ESA's XMM-NewtonX-ray telescope.[38]
SGR 1935+2154, emitted a pair of luminous radio bursts on 28 April 2020. There was speculation that these may be galactic examples offast radio bursts.
Swift J1818.0-1607, X-ray burst detected March 2020, is one of five known magnetars that are also radio pulsars. By its time of discovery, it may be only 240 years old.[39][40]
Unusually bright supernovae are thought to result from the death of very large stars aspair-instability supernovae (or pulsational pair-instability supernovae). However, recent research by astronomers[41][42] 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.[43][44][45]
^Cline, T. L., Desai, U. D., Teegarden, B. J., Evans, W. D., Klebesadel, R. W., Laros, J. G. (Apr 1982). "Precise source location of the anomalous 1979 March 5 gamma-ray transient".The Astrophysical Journal.255: L45–L48.Bibcode:1982ApJ...255L..45C.doi:10.1086/183766.hdl:2060/19820012236.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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