Editing Neutron star (section)

===Spin down===
[[Image:PPdot2.png|thumb|''P''–''P''-dot diagram for known [[rotation-powered pulsar]]s (red), anomalous X-ray pulsars (green), high-energy emission pulsars (blue) and [[binary pulsar]]s (pink)]]
Over time, neutron stars slow, as their rotating magnetic fields in effect radiate energy associated with the rotation; older neutron stars may take several seconds for each revolution. This is called ''spin down''. The rate at which a neutron star slows its rotation is usually constant and very small.

The [[periodic time]] (''P'') is the [[Rotation period|rotational period]], the time for one rotation of a neutron star. The spin-down rate, the rate of slowing of rotation, is then given the symbol <math>\dot{P}</math> (''P''-dot), the [[derivative]] of ''P'' with respect to time. It is defined as periodic time increase per unit time; it is a [[dimensionless quantity]], but can be given the units of s⋅s<sup>−1</sup> (seconds per second).<ref name="nrao">{{cite web |url=https://www.cv.nrao.edu/~sransom/web/Ch6.html |title=Pulsar Properties (Essential radio Astronomy) |publisher=National Radio Astronomy Observatory |access-date=24 March 2016 |first1=J. J. |last1=Condon |first2=S. M. |last2=Ransom |name-list-style=amp |archive-date=10 April 2016 |archive-url=https://web.archive.org/web/20160410113528/http://www.cv.nrao.edu/~sransom/web/Ch6.html |url-status=live }}</ref>

The spin-down rate (''P''-dot) of neutron stars usually falls within the range of {{val|e=−22}} to {{val|e=−9|u=s⋅s<sup>−1</sup>}}, with the shorter period (or faster rotating) observable neutron stars usually having smaller ''P''-dot. As a neutron star ages, its rotation slows (as ''P'' increases); eventually, the rate of rotation will become too slow to power the radio-emission mechanism, so radio emission from the neutron star no longer can be detected.<ref name="nrao" />

''P'' and ''P''-dot allow minimum magnetic fields of neutron stars to be estimated.<ref name="nrao" /> ''P'' and ''P''-dot can be also used to calculate the ''characteristic age'' of a pulsar, but gives an estimate which is somewhat larger than the true age when it is applied to young pulsars.<ref name="nrao" />

''P'' and ''P''-dot can also be combined with neutron star's [[moment of inertia]] to estimate a quantity called ''spin-down [[luminosity]]'', which is given the symbol <math>\dot{E}</math> (''E''-dot). It is not the measured luminosity, but rather the calculated loss rate of rotational energy that would manifest itself as radiation. For neutron stars where the spin-down luminosity is comparable to the actual [[luminosity]], the neutron stars are said to be "[[rotation powered pulsar|rotation powered]]".<ref name="nrao" /><ref name="pavlov" /> The observed luminosity of the [[Crab Pulsar]] is comparable to the spin-down luminosity, supporting the model that rotational kinetic energy powers the radiation from it.<ref name="nrao" /> With neutron stars such as magnetars, where the actual luminosity exceeds the spin-down luminosity by about a factor of one hundred, it is assumed that the luminosity is powered by magnetic dissipation, rather than being rotation powered.<ref name="ufrgs">{{cite web |url=http://www.if.ufrgs.br/hadrons/zhang.pdf |title=Spin-Down Power of Magnetars |publisher=Universidade Federal do Rio Grande do Sul |access-date=24 March 2016 |first=B. |last=Zhang |archive-date=6 February 2021 |archive-url=https://web.archive.org/web/20210206223627/http://www.if.ufrgs.br/hadrons/zhang.pdf |url-status=live }}</ref>

''P'' and ''P''-dot can also be plotted for neutron stars to create a ''P''–''P''-dot diagram. It encodes a tremendous amount of information about the pulsar population and its properties, and has been likened to the [[Hertzsprung–Russell diagram]] in its importance for neutron stars.<ref name="nrao" />