A magnetar is a type of neutron star
with an extremely powerful magnetic
field. The magnetic field decay powers
the emission of high-energy
electromagnetic radiation, particularly
X-rays and gamma rays. The theory
regarding these objects was proposed by
Robert Duncan and Christopher Thompson
in 1992, but the first recorded burst of
gamma rays thought to have been from a
magnetar had been detected on March 5,
1979. During the following decade, the
magnetar hypothesis has become widely
accepted as a likely explanation for
soft gamma repeaters and anomalous X-ray
pulsars.
Description
Like other neutron stars, magnetars are
around 20 kilometres in diameter and
have a greater mass than the Sun. The
density of the interior of a magnetar is
such that a thimble full 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 rotating comparatively slowly, with
most magnetars completing a rotation
once every one to ten seconds, compared
to less than one second for a typical
neutron star. This 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.
= Magnetic field=
Magnetars are characterized by their
extremely powerful magnetic fields of
108 to 1011 tesla. These magnetic fields
are hundreds of millions of times
stronger than any man-made magnet, and
quadrillions of 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
>104 times that of lead. The magnetic
field of a magnetar would be lethal even
at a distance of 1000 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 halfway to the moon, a magnetar
could strip information from the
magnetic stripes of all credit cards on
Earth. As of 2010, 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-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." 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.
Origins of magnetic fields
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. A similar
magnetohydrodynamic dynamo process
produces even more intense transient
fields during coalescence of pairs of
neutron stars.
= Formation=
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. 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.
= 1979 discovery=
On March 5, 1979, a few months after the
successful dropping of satellites into
the atmosphere of Venus, the two Soviet
spacecraft, Venera 11 and 12, 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.
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. The
direction of the source corresponded
with the remnants of a star that had
gone supernova around 3000 B.C.E. 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 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.
Anti-glitch issue
Often magnetars speed up and many of the
reasons for this behaviour have not been
fully explained by astrophysics.
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.
Known magnetars
As of November 2013, 21 magnetars are
known, with five more candidates
awaiting confirmation. A full listing is
given in the McGill SGR/AXP Online
Catalog. Examples of known magnetars
include:
SGR 0525-66, in the Large Magellanic
Cloud, the first found
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 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.
SGR 0501+4516 was discovered on 22
August 2008.
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, ESO reports
identification of an object which it has
initially identified as a magnetar,
SWIFT J195509+261406, originally
identified by a gamma-ray burst.
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.
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.
Bright supernovae
A recent progress in theory suggests
that the energy deposition from these
magnetars into the expanding supernova
remnant could possibly explain some
observed cases of unusually bright
supernovae. Traditionally such bright
events are thought to come from very
large stars when they become
pair-instability supernova. However, two
papers published in 2010 by
astrophysicists at the University of
California, Berkeley, University of
California, Santa Cruz and University of
California, Santa Barbara provided
semi-analytical and numerical models to
explain some of the brightest events
ever seen, such as SN 2005ap and SN
2008es. A research led by Matt Nicholl,
of the Astrophysics Research Centre at
Queen's School of Mathematics and
Physics of Queen's University Belfast,
the results of which were published on
October 17, 2013 in Nature, has
explained the newly discovered luminous
transient PTF 12dam through the same
mechanism.
See also
Neutron star
Soft gamma repeater
Pulsar
References
Specific
Books and literature
Peter Douglas Ward, Donald Brownlee Rare
Earth: Why Complex Life Is Uncommon in
the Universe. Springer, 2000. ISBN
0-387-98701-0.
Chryssa Kouveliotou The Neutron
Star-Black Hole Connection. Springer,
2001. ISBN 1-4020-0205-X.
Mereghetti, S.. "The strongest cosmic
magnets: soft gamma-ray repeaters and
anomalous X-ray pulsars". Astronomy and
Astrophysics Review 15: 225–287.
arXiv:0804.0250.
Bibcode:2008A&ARv..15..225M.
doi:10.1007/s00159-008-0011-z. 
General
Schirber, Michael. "Origin of
magnetars". CNN. 
Naeye, Robert. "The Brightest Blast".
Sky and Telescope. 
"SWIFT J1822.31606, stella irrequieta".
Italiaglobale.it. 19 July 2012.
