Magnetar
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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. The theory regarding these objects was formulated by Robert Duncan and Christopher Thompson in 1992. 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.
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[edit] 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 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.
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[citation needed]. This happens when the star already has a fast rotation and strong magnetic field before the supernova. It is thought that a magnetar's magnetic field is created as a result of a convection-driven dynamo of hot nuclear matter in the neutron star's interior that operates in the first ten seconds or so of a neutron star's life. If the neutron star is initially rotating as fast as the period of convection, about ten milliseconds, then the convection currents are able to operate globally and transfer a significant amount of their kinetic energy into magnetic field strength. In slower-rotating neutron stars, the convection currents form only in local regions.
[edit] Short lifetime
In the solid crust of a magnetar, tensions can arise that lead to 'starquakes'. These seismic vibrations are extremely energetic, and result in a burst of X-ray and gamma ray radiation. To astronomers, such an object is known as a soft gamma repeater.
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.[1]
[edit] Known magnetars
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.
- 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.[1]
As of May 2007, 12 magnetars are known, with a further three candidates in need of confirmation.
[edit] Effects of superstrong magnetic fields
A magnetic field of 10 gigateslas is enormous. Earth has a geomagnetic field of 30-60 microteslas, while 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.
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).
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]