The existence of magnetars was proposed in 1992 by
Robert Duncan and
Christopher Thompson.[3] Their proposal sought to explain the properties of transient sources of gamma rays, now known as
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[update], 24 magnetars have been confirmed.[6]
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]
Magnetic field
Magnetars are characterized by their extremely powerful magnetic fields of ~109 to 1011T.[6] These magnetic fields are a hundred million times as strong as any man-made magnet,[11] and about a trillion times as powerful as the
field surrounding Earth.[12] 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 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[update], they are the most powerful magnetic objects detected throughout the universe.[10][15]
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."[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
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 108teslas, to more than 1011 teslas (or 1015gauss). 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]
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
speed of light and its detection by several widely dispersed spacecraft, the source of the
gamma radiation could be triangulated to within an accuracy of approximately 2
arcseconds.[21] The direction of the source corresponded with the remnants of a star that had
gone supernova around 3000 BCE.[10] It was in the
Large Magellanic Cloud and the source was named
SGR 0525-66; the event itself was named
GRB 790305b, the first-observed SGR megaflare.
Recent discoveries
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.[23] 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.[24] 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.[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]
As of July 2021[update], 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:
SGR 1806−20, located 50,000 light-years from Earth on the far side of the Milky Way in the constellation of
Sagittarius and the most magnetized object known.
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.[32]
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).[24]
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.[37]
3XMM J185246.6+003317, discovered by international team of astronomers, looking at data from ESA's XMM-Newton
X-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 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. 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 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]
^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.
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