Editing Neutron star (section)

===Magnetic field===
The magnetic field strength on the surface of neutron stars ranges from {{circa|{{val|e=4}}}} to {{val|e=11}}&nbsp;[[Tesla (unit)|tesla]] (T).<ref name="reisenegger">{{cite arXiv |first=A. |last=Reisenegger |year=2003 |title=Origin and Evolution of Neutron Star Magnetic Fields |eprint=astro-ph/0307133 }}</ref> These are orders of magnitude higher than in any other object: for comparison, a continuous 16&nbsp;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.<ref name="reisenegger"/>

The neutron stars known as [[magnetar]]s have the strongest magnetic fields, in the range of {{val|e=8}} to {{val|e=11|u=T}},<ref name="mcgill">{{cite web |title=McGill SGR/AXP Online Catalog |url=http://www.physics.mcgill.ca/~pulsar/magnetar/main.html |access-date=2 Jan 2014 |archive-date=23 July 2020 |archive-url=https://web.archive.org/web/20200723080137/http://www.physics.mcgill.ca/~pulsar/magnetar/main.html |url-status=live }}</ref> and have become the widely accepted hypothesis for neutron star types [[soft gamma repeater]]s (SGRs)<ref name="sa">{{cite journal |first1=Chryssa |last1=Kouveliotou |first2=Robert C. |last2=Duncan |first3=Christopher |last3=Thompson |date=February 2003 |title=Magnetars |journal=Scientific American |volume=288 |issue=2 |pages=34–41 |doi=10.1038/scientificamerican0203-34 |pmid=12561456 |bibcode=2003SciAm.288b..34K }}</ref> and [[anomalous X-ray pulsar]]s (AXPs).<ref>{{cite journal |first1=V.M. |last1=Kaspi |first2=F.P. |last2=Gavriil |year=2004 |title=(Anomalous) X-ray pulsars |journal=Nuclear Physics B |series=Proceedings Supplements |volume=132 |pages=456–465 |doi=10.1016/j.nuclphysbps.2004.04.080 |arxiv=astro-ph/0402176 |bibcode=2004NuPhS.132..456K|s2cid=15906305 }}</ref> The magnetic [[energy density]] of a {{val|e=8|u=T}} field is extreme, greatly exceeding the [[theoretical total mass-energy|mass-energy]] density of ordinary matter.{{efn|Magnetic [[energy density]] for a [[magnetic field|field B]] is {{nowrap| U {{=}} {{frac|[[Vacuum permeability|μ<sub>0</sub>]] B<sup>2</sup>|2}} .}}<ref>{{cite web |url=http://scienceworld.wolfram.com/physics/MagneticFieldEnergyDensity.html |title=Eric Weisstein's World of Physics |website=scienceworld.wolfram.com |archive-url=https://web.archive.org/web/20190423232524/http://scienceworld.wolfram.com/physics/MagneticFieldEnergyDensity.html |archive-date=2019-04-23}}</ref> Substituting {{nowrap| B {{=}} {{val|e=8|u=T}} ,}} get {{nowrap|U {{=}} {{val|4|e=21|u=J|up=m3}} .}} Dividing by c<sup>2</sup> one obtains the equivalent mass density of {{val|44500|u=kg|up=m3}}, which exceeds the [[standard temperature and pressure]] density of all known materials. Compare with {{val|22590|u=kg|up=m3}} for [[osmium]], the densest stable element.}} Fields of this strength are able to [[Vacuum polarization|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 [[Starquake (astrophysics)#Starquake|starquake]]s, 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&nbsp;seconds and which lasts for a few minutes.<ref>{{cite web |url=http://solomon.as.utexas.edu/magnetar.html |title='Magnetars', soft gamma repeaters & very strong magnetic fields |first=Robert C. |last=Duncan |date=March 2003 |access-date=2018-04-17 |archive-date=2020-01-19 |archive-url=https://web.archive.org/web/20200119142438/http://solomon.as.utexas.edu/magnetar.html |url-status=live }}</ref>

The origins of the strong magnetic field are as yet unclear.<ref name="reisenegger"/> One hypothesis is that of "flux freezing", or conservation of the original [[magnetic flux]] during the formation of the neutron star.<ref name="reisenegger"/> 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.<ref name="reisenegger"/>