When in a supernova a star collapses to a neutron star, its magnetic field increases dramatically in strength. Duncan and Thompson calculated that the magnetic field of a neutron star, normally an already enormous 1012 tesla could under certain circumstances grow even larger, to about 1015 tesla. Such a highly magnetic neutron star is called a magnetar.
In the outer layers of a magnetar, which consist of a plasma of heavy elements (mostly iron), this causes tension which 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.
It is estimated that about 1 in 10 supernova explosions results in a magnetar rather than a more standard neutron star or pulsar. It only happens when the star already has a fast rotation and strong magnetic field before the supernova.
The life of a magnetar as a soft gamma repeater is short: The energy of these explosions slows the rotation (causing magnetars to rotate much slower than other neutron stars of a similar age) and lessens the electric field, and after only about 10,000 years the starquakes are over. After this, the star still radiates X-rays, forming an object known to astronomers as an anomalous X-ray pulsar[?] (AXP). After yet another 10,000 years, it is completely quiet. At the moment (2000), 4 soft gamma repeaters and 6 anomalous X-ray pulsars are known.
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