Magnetar Wednesday, June 24, 2009

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

A magnetar is a type of 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.^[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 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).

Description

Little is known about the physical structure of a magnetar because none are close to Earth. Magnetars are around 20 kilometres (12 mi) in diameter but substantially more massive than our Sun. A magnetar is so compressed that a thimbleful of its material would weigh over 100 million tons.^[1] Most known magnetars 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 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.^[4]

Quakes 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.^[5]

Magnetic field

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 thousands of times stronger than any man-made magnet, ^[6] and quadrillions of times more powerful than the field surrounding Earth.^[7] As of 2003^[update], they are the most magnetic objects ever detected in the universe.^[5][8]

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 4.0×10⁵ 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. It has even been said that at a distance halfway to the moon, a magnetar could strip information from a credit card on Earth.^[9]

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 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]

Formation

Magnetar SGR 1900+14 is in the exact center of the image, which shows a surrounding ring of gas seven light-years across in infrared light, as seen by the Spitzer Space Telescope. The magnetar itself is not visible at this wavelength, but it has been seen in X-ray light.

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). The result is a magnetar.^[10]

The supernova might lose 10% of its mass in the explosion. In order for such large stars (10 to 30 solar masses) not to collapse straight into a black hole, they have to shed a larger proportion of their mass— perhaps 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.^[11]

On February 21, 2008 it was announced that NASA and McGill University researchers had discovered a neutron star that temporarily changed from a pulsar to a magnetar. This indicates that magnetars are not merely a rare type of pulsar but may be a (possibly reversible) phase in the lives of at least some pulsars.^[12]

On September 24, 2008, ESO announced what it believes is the first optically active magnetar-candidate yet discovered, using ESO's Very Large Telescope. The newly discovered object is known as SWIFT J195509+261406 ^[13].

History

1979 discovery

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.^[5]

Known magnetars

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

As of May 2007, twelve magnetars are known, with three more candidates awaiting confirmation. 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 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. ^[14]
• 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.

A full listing is given in the magnetar catalog.^[15]

• As of Sep 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 (GRB 070610) ^[13]