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Artist's conception of a magnetar, with magnetic field lines

A magnetar is a neutron star with an extremely powerful magnetic field, the decay of which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma-rays, and which can be anywhere around the order of 20 kilometres in diameter.^[1] The theory regarding these objects was formulated by Robert Duncan and Christopher Thompson in 1992, but the first recorded burst of gamma rays that was thought to have been sourced from a magnetar was made on March 5, 1979.^[2] In the course of the decade that followed, the magnetar hypothesis has become widely accepted as a likely physical explanation for observable objects known as soft gamma repeaters and anomalous X-ray pulsars.

Little is known about the physical nomenclature of a magnetar, due to none being especially proximate to Earth. Magnetars are somewhere around 20 kilometres in total diameter. Despite this, they are many times more massive than our Sun; so compressed is the magnetar, that a thimbleful of the composite matter is estimated to weigh over 100 million tons.^[1] Most of the magnetars recorded rotate very rapidly, at least several times per second.^[3] The active life of a magnetar is short. Their strong magnetic fields decay after about 10,000 years, after which point activity and strong X-ray emission cease. Given the number of magnetars observable today, one estimate puts the number of "dead" magnetars in the Milky Way at 30 million or more.^[4]

Quakes triggered on the surface of the magnetar cause great volatility in the star's composition and the magnetic field which encompasses it, often leading to the extremely powerful gamma ray flares which have been recorded on Earth in 1979, 1998 and 2004.^[5]

Magnetars are primarily characterised by their extremely powerful magnetic field, which can often reach the order of 10 gigateslas. These magnetic field's are billions of trillions of times stronger than any magnet created by man on Earth, and quadrillions of times more powerful than the field surrounding Earth.^[6] As of 2007, they are the most magnetic objects ever detected in the universe.^[5][7]

A magnetic field of 10 gigateslas is enormous. 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 x 10⁵ J/m³. A 10 gigatesla field, by contrast, has an energy density of 4.0 x 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. It has even been said that, at a distance halfway to the moon, a magnetar could strip information from a credit card on Earth.^[8]

As described in a 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. Also, when entering the magnetic field, photons of polarized light change speed (and therefore wavelength). Since the field can prevent electrons from vibrating as they normally do in response to light, light waves can slip past electrons without losing energy. Empty space itself experiences vacuum birefringence, gaining the ability to split light into different polarizations (like an immaterial version of a calcite crystal).^[2]

The field also stretches atoms into long cylinders. In a field of about 10⁵ teslas atomic orbitals deform into cigar shapes. At 10¹⁰ teslas, a hydrogen atom becomes a spindle 200 times narrower than its normal diameter.^[2]

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 the magnetic field of a neutron star, normally an already enormous 10⁸ teslas could, through the dynamo mechanism, grow even larger, to more than 10¹¹ teslas (or 10¹⁵ gauss). Such a highly magnetic neutron star is called a magnetar.^[9]

The supernova might lose 10% of its mass in the explosion. In order for such large stars (10–30 solar masses) not to collapse straight into a black hole, they have to shed a larger proportion of their mass—maybe another 80%.

It is estimated that about 1 in 10 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 aircraft 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 rise 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, at the time, 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 was constituted the strongest wave of extra-solar gamma rays ever detected; it was over 100 times more intense than any other previous extra-solar bursts.^[5]

On 27 December, 2004, SGR 1806-20 (artist's conception shown) released a burst of gamma rays so powerful that it had effects on Earth's atmosphere, at a range of over 50,000 light years.

Examples of known magnetars include

• SGR 1806-20, located 50,000 light-years from Earth on the far side of our Milky Way galaxy 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 (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 to shut-down to prevent damage and to saturate instruments on BeppoSAX, WIND and RXTE.
• 1E 1048.1-5937, located 9,000 light-years away in the constellation Carina. The original star, out of which the magnetar formed, had a mass 30 to 40 times that of the Sun.

A full listing is given in the magnetar catalog.^[11]

As of May 2007, 12 magnetars are known, with a further three candidates in need of confirmation.

• Neutron star
• Pulsar
• Black hole
• Supernova
• Gamma ray burst
• Gamma radiation

Specific

Books and literature

• Peter Douglas Ward, Donald Brownlee Rare Earth: Why Complex Life Is Uncommon in the Universe. Springer, 2000. ISBN 0387987010.
• Chryssa Kouveliotou The Neutron Star-Black Hole Connection. Springer, 2001. ISBN 140200205X.

General

• "Origin of magnetars", CNN, 2 February 2005. 
• "The Brightest Blast", Sky and Telescope, 18 February 2005. 

• Recording (and animation) of XTE J1810-197.
• Creation of magnetars solved Formed when the biggest stars explode
• NASA: "Magnetar" discovery solves 19-year-old mystery Citat: "...suggested a magnetic field strength of about 800 trillion [g]auss...").
• Robert C. Duncan, University of Texas at Austin: 'Magnetars', Soft Gamma Repeaters & Very Strong Magnetic Fields
• NASA Astrophysics Data System (ADS): Duncan & Thompson, Ap.J. 392, L9) 1992
• NASA Astrophysics Data System (ADS): Katz, J. I., Ap.J. 260, 371 (1982)
• NASA ADS, 1999: Discovery of a Magnetar Associated with the Soft Gamma Repeater SGR 1900+14
• Chryssa Kouveliotou, Robert Duncan, and Christopher Thompson, "Magnetars," Scientific American, Feb. 2003, pp. 34-41 (PDF)
• Robert C. Duncan and Christopher Thompson (June 10, 1992). "Formation of Very Strongly Magnetized Neutron Stars: Implications for Gamma-Ray Bursts". Astronomical Journal 392 (1): pp. L9–L13. 
• Strange Pulsing Star Puzzles Astronomers - A magnetar found to emit radio waves, contrary to previous theories.
• 04/04/07: X-ray Satellites Catch Magnetar in Gigantic Stellar 'Hiccup'