<P> The magnetic field strength on the surface of neutron stars ranges from c. 10 to 10 tesla . These are orders of magnitude higher than in any other object: for comparison, a continuous 16 T field has been achieved in the laboratory and is sufficient to levitate a living frog due to diamagnetic levitation . Variations in magnetic field strengths are most likely the main factor that allows different types of neutron stars to be distinguished by their spectra, and explains the periodicity of pulsars . </P> <P> The neutron stars known as magnetars have the strongest magnetic fields, in the range of 10 to 10 tesla, and have become the widely accepted hypothesis for neutron star types soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs). The magnetic energy density of a 10 T field is extreme, exceeding the mass - energy density of ordinary matter . Fields of this strength are able to polarize the vacuum to the point that the vacuum becomes birefringent . Photons can merge or split in two, and virtual particle - antiparticle pairs are produced . The field changes electron energy levels and atoms are forced into thin cylinders . Unlike in an ordinary pulsar, magnetar spin - down can be directly powered by its magnetic field, and the magnetic field is strong enough to stress the crust to the point of fracture . Fractures of the crust cause starquakes, observed as extremely luminous millisecond hard gamma ray bursts . The fireball is trapped by the magnetic field, and comes in and out of view when the star rotates, which is observed as a periodic soft gamma repeater (SGR) emission with a period of 5--8 seconds and which lasts for a few minutes . </P> <P> The origins of the strong magnetic field are as yet unclear . One hypothesis is that of "flux freezing", or conservation of the original magnetic flux during the formation of the neutron star . If an object has a certain magnetic flux over its surface area, and that area shrinks to a smaller area, but the magnetic flux is conserved, then the magnetic field would correspondingly increase . Likewise, a collapsing star begins with a much larger surface area than the resulting neutron star, and conservation of magnetic flux would result in a far stronger magnetic field . However, this simple explanation does not fully explain magnetic field strengths of neutron stars . </P> <P> The gravitational field at a neutron star's surface is about 7011200000000000000 ♠ 2 × 10 times stronger than on Earth, at around 7012200000000000000 ♠ 2.0 × 10 m / s . Such a strong gravitational field acts as a gravitational lens and bends the radiation emitted by the neutron star such that parts of the normally invisible rear surface become visible . If the radius of the neutron star is 3GM / c or less, then the photons may be trapped in an orbit, thus making the whole surface of that neutron star visible from a single vantage point, along with destabilizing photon orbits at or below the 1 radius distance of the star . </P>

Why do neutron stars have strong magnetic fields
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