A magnetar is a type of neutron star believed to have an extremely powerful magnetic field (∼109 to 1011 T, ∼1013 to 1015 G). The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays. The active life of a magnetar is short.

Like other neutron stars, magnetars are around 20 kilometres (12 mi) in diameter and have a mass about 1.4 solar masses. They are formed by the collapse of a star with a mass 10–25 times that of the Sun. The density of the interior of a magnetar is such that a tablespoon of its substance would have a mass of over 100 million tons. Magnetars are differentiated from other neutron stars by having even stronger magnetic fields, and by rotating more slowly in comparison.

Most magnetars rotate once every two to ten seconds, whereas typical neutron stars rotate once in less than a few seconds. A magnetar’s magnetic field gives rise to very strong and characteristic bursts of X-rays and gamma rays. 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.

The theory regarding these objects was proposed by Robert Duncan and Christopher Thompson in 1992, but the first recorded burst of gamma rays  from a magnetar had been detected on March 5, 1979. During the following decade, the magnetar hypothesis became widely accepted as a likely explanation for soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs).

The dominant theory of the strong fields of magnetars is that it results from a magnetohydrodynamic dynamo process in the turbulent, extremely dense conducting fluid that exists before the neutron star settles into its equilibrium configuration. These fields then persist due to persistent currents in a proton-superconductor phase of matter that exists at an intermediate depth within the neutron star (where neutrons predominate by mass).

When in a supernova, a star collapses to a neutron star, and its magnetic field increases dramatically in strength through conservation of magnetic flux. Halving a linear dimension increases the magnetic field fourfold.

Duncan and Thompson calculated that when the spin, temperature and magnetic field of a newly formed neutron star falls into the right ranges, a dynamo mechanism could act, converting heat and rotational energy into magnetic energy and increasing the magnetic field, normally an already enormous 108 teslas, to more than 1011 teslas (or 1015 gauss). The result is a magnetar. It is estimated that about one in ten supernova explosions results in a magnetar rather than a more standard neutron star or pulsar.

On 1. June 2020, astronomers reported narrowing down the source of fast radio bursts (FRBs), which may now plausibly include “compact-object mergers and magnetars arising from normal core collapse supernovae”.

As of March 2016, 23 magnetars are known, with six more candidates awaiting confirmation. Some of known magnetars are SGR 0525−66  in the Large Magellanic Cloud, SGR 1806−20- located on the far side of the Milky Way in the constellation of Sagittarius, SGR 1900+14- located 20,000 light-years away in the constellation Aquila. 

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