Magnetar

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Artist's conception of a magnetar, with magnetic field lines
Artist's conception of a powerful magnetar in a star cluster

A magnetar is a type of neutron star with an extremely powerful magnetic field (~109 to 1011 T, ~1013 to 1015 G).[1] The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.[2]

The existence of magnetars was proposed in 1992 by

soft gamma repeaters (SGRs).[4][5] Over the following decade, the magnetar hypothesis became widely accepted, and was extended to explain anomalous X-ray pulsars (AXPs). As of July 2021, 24 magnetars have been confirmed.[6]

It has been suggested that magnetars are the source of fast radio bursts (FRB), in particular as a result of findings in 2020 by scientists using the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope.[7]

Description

Like other neutron 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 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.[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 radio pulsars, 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 the Milky 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 powerful gamma-ray flare emissions which have been recorded on Earth in 1979, 1998 and 2004.[10]

Neutron Star Types (24 June 2020)

Magnetic field

Magnetar (artist concept)

Magnetars are characterized by their extremely powerful magnetic fields of ~109 to 1011

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 known lifeforms 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 all credit cards on Earth.[14] As of 2020, they are the most powerful magnetic objects detected throughout the universe.[10][15]

As described in the February 2003

de Broglie wavelength of an electron."[4] In a field of about 105 teslas atomic orbitals deform into rod shapes. At 1010 teslas, a hydrogen atom becomes 200 times as narrow as its normal diameter.[4]

Origins of magnetic fields

The dominant model 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.[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 during coalescence of pairs of neutron stars.[17] An alternative model is that they simply result from the collapse of stars with unusually strong magnetic fields.[18]

Formation

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 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, 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.[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]

1979 discovery

On March 5, 1979, a few months after the successful dropping of Landers into the atmosphere of Venus, the two uncrewed Soviet spaceprobes Venera 11 and 12, then in heliocentric 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]

Eleven seconds later,

U.S. Department of Defense Vela satellites, the Soviet Prognoz 7 satellite, and the Einstein Observatory, all orbiting Earth. Before exiting the solar system the radiation was detected by the International Sun–Earth Explorer in halo orbit.[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 the

SGR 0525-66; the event itself was named GRB 790305b
, the first-observed SGR megaflare.

Recent discoveries

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 at

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.[25] 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 temporary result of the merger of two neutron stars was determined to be a hypermassive magnetar, which shortly collapsed into a black hole.[26]

In April 2020, a possible link between fast radio bursts (FRBs) and magnetars was suggested, based on observations of SGR 1935+2154, a likely magnetar located in the Milky Way galaxy.[27][28][29][30][31]

Known magnetars

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, 24 magnetars are known, with six more candidates awaiting confirmation.[6] A full listing is given in the McGill SGR/AXP Online Catalog.[6] Examples of known magnetars include:

Magnetar—
SGR J1745-2900
Magnetar found very close to the supermassive black hole, Sagittarius A*, at the center of the Milky Way galaxy

Bright supernovae

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[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]

See also

References

Specific
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  2. ^ a b Ward; Brownlee, p.286
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  6. ^ a b c d "McGill SGR/AXP Online Catalog". Retrieved 26 Jan 2021.
  7. ^ Starr, Michelle (1 June 2020). "Astronomers Just Narrowed Down The Source of Those Powerful Radio Signals From Space". ScienceAlert.com. Retrieved 2 June 2020.
  8. ^ a b Kaspi, V. M. (April 2010). "Grand unification of neutron stars". Proceedings of the National Academy of Sciences. 107 (16). Proceedings of the National Academy of Sciences of the United States of America: 7147–7152.
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  9. ^ Condon, J. J. & Ransom, S. M. "Pulsar Properties (Essential radio Astronomy)". National Radio Astronomy Observatory. Retrieved 26 Feb 2021.
  10. ^ a b c Kouveliotou, C.; Duncan, R. C.; Thompson, C. (February 2003). "Magnetars Archived 2007-06-11 at the Wayback Machine". Scientific American; Page 36.
  11. ^ "HLD user program, at Dresden High Magnetic Field Laboratory". Retrieved 2009-02-04.
  12. ^ Naeye, Robert (February 18, 2005). "The Brightest Blast". Sky & Telescope. Retrieved 10 November 2020.
  13. ^ Duncan, Robert. "'MAGNETARS', SOFT GAMMA REPEATERS & VERY STRONG MAGNETIC FIELDS". University of Texas.
  14. ^ Wanjek, Christopher (February 18, 2005). "Cosmic Explosion Among the Brightest in Recorded History". NASA. Retrieved 17 December 2007.
  15. ^ Dooling, Dave (May 20, 1998). ""Magnetar" discovery solves 19-year-old mystery". Science@NASA Headline News. Archived from the original on 14 December 2007. Retrieved 17 December 2007.
  16. doi:10.1086/172580
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  17. S2CID 30023248. Archived from the original on 2018-07-17. Retrieved 2012-07-13. Open access icon
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  19. ^ Kouveliotou, p.237
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  23. ^ Shainblum, Mark (21 February 2008). "Jekyll-Hyde neutron star discovered by researchers]". McGill University.
  24. ^ a b "The Hibernating Stellar Magnet: First Optically Active Magnetar-Candidate Discovered". ESO. 23 September 2008.
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