A magnetar is a type of neutron star with an extremely powerful magnetic field, the decay of which powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.[1] 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 was detected on March 5, 1979.[2] During the following decade, the magnetar hypothesis has become widely accepted as a likely explanation for soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs).
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Little is known about the physical structure of magnetars because none are sufficiently close to Earth to facilitate study. Like other neutron stars, magnetars are around 20 kilometres (12 mi) in diameter but have a greater mass than the Sun. The density of a magnetar is such that a thimbleful of its substance, sometimes referred to as neutronium, would have a mass of over 100 million tons.[1] 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.[3] 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.[3]
Starquakes triggered on the surface of the magnetar cause great volatility in the star and 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.[4]
Magnetars are primarily characterized by their extremely powerful magnetic field, which can often reach the order of ten gigateslas. These magnetic fields are hundreds of millions of times stronger than any man-made magnet,[5] and quadrillions of times more powerful than the field surrounding Earth.[6] Earth has a geomagnetic field of 30–60 microteslas, and a neodymium-based, rare-earth magnet has a field of about 1 tesla, with a magnetic energy density of 4.0×105 J/m3. A 10 gigatesla 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, tearing tissues due to the diamagnetism of water. At a distance halfway to the moon, a magnetar could strip information from the magnetic stripes of all credit cards on Earth.[7] As of 2010[update], they are the most magnetic objects ever detected in the universe.[4][8]
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."[2] 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.[2] Currently, the mechanism which generates such strong magnetic fields, if they exist, is unknown.
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 (or 1015 gauss). The result is a magnetar.[9] It is estimated that about one in ten supernova explosions results in a magnetar rather than a more standard neutron star or pulsar.[10]
On March 5, 1979, a few months after the successful dropping of satellites into the atmosphere of Venus, the two Soviet spacecraft that were then drifting through the solar system were hit by a blast of gamma ray 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.[2]
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 ray 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.[11] The direction of the source corresponded with the remnants of a star that had gone supernova around 3000 B.C.[4]
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.[12] On September 24, 2008, ESO announced what it believed was the first optically active magnetar-candidate yet discovered, using ESO's Very Large Telescope. The newly discovered object was designated SWIFT J195509+261406.[13]
As of April 2011[update], twenty one magnetars are known, with five more candidates awaiting confirmation. Examples of known magnetars include:
A full listing is given in the McGill SGR/AXP Online Catalog.[17]
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