Magnetar

A magnetar is a neutron star with an extremely strong 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.

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 under certain circumstances grow even larger, to more than 1011 teslas. 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) to not 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. 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 only form in local regions.

Short lifetime

In the outer layers of a magnetar, which consist of a plasma of heavy elements (mostly iron), tensions can arise that leads 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 life of a magnetar as a soft gamma repeater is short: Starquakes cause large ejections of energy, and matter. The matter is held in the strong magnetic field, and evaporates in minutes. Radial ejection of matter carries away angular momentum which slows the rotation. Magnetars lose rotational speed at a higher rate than other neutron stars, attributed to their high magnetic field. Slowdown weakens the magnetic field, and after only about 10,000 years the starquakes cease. After this, the star still radiates X-rays, and astronomers conjecture it forms an anomalous X-ray pulsar. After another 10,000 years, it becomes completely quiet. Starquakes are blockbuster detonations and some have been directly recorded, such as that at SGR 1806-20 on December 27, 2004, and more are expected to be recorded as telescopes increase in fidelity.

Known Magnetars

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.

As of December 2004, 4 soft gamma repeaters and 5 anomalous X-ray pulsars are known, with a further four candidates in need of confirmation.

Effects of superstrong magnetic fields

A magnetic field above 10 gigateslas is strong enough to wipe a credit card from half the distance of the Moon from the Earth. A small neodymium based rare earth magnet has a field of about a tesla, Earth has a geomagnetic field of 30-60 microteslas, and most media used for data storage can be erased with a millitesla field.

The magnetic field of a magnetar would be lethal at a distance of up to 1000 km, tearing tissues due to the diamagnetism of water.

References

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