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Millisecond pulsars are supposed to be old neutron stars. However, they are spinning even more rapidly than newly formed pulsars. Since pulsars slow down as they age, something must have caused these older pulsars to "spin up" and be rotating as fast as they are. What is the mechanism for doing so?

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  • $\begingroup$ please provide a reference because I found little more than 30 in such circunstances. Even if it is only one a correct physical explanation must exist. $\endgroup$ – Helder Velez Apr 13 '18 at 0:07
  • $\begingroup$ Frankly, @Helder this is a term you can look up in the Wikipedia where you will learn that we've been observing them for more than thirty-five years. You'll also find more than a dozen scholarly references. $\endgroup$ – dmckee Apr 21 '18 at 23:32
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They accrete gas from a disk, fed by either a wind or Roche lobe overflow from their companion. Almost all known millisecond pulsars are in binary systems, but I think some in globular clusters may have been disrupted by three-body encounters, so appear to be isolated.

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  • $\begingroup$ please provide a reference, because in such scenario the pulsar will have to share its own momentum with the accreted material that was not rotating faster than the pulsar. Then, imo, the pulsar will have to slow down inevitably. $\endgroup$ – Helder Velez Apr 13 '18 at 0:04
  • $\begingroup$ The answer can not be correct, imo. Before the reunion the matter that is being accreted is rotating much slower than the pulsar and has to be accelerated upon reunion. Thus the momentum of the pulsar has to be shared and the pulsar have to slow down. The ice skater analogy is no good because the arms are already rotating at the same speed. Think as if you are the motor of a rotating carrousel and 100 people jumped inside at once and their velocity had to go from 0 to... Dont you think that they will slow you down? $\endgroup$ – Helder Velez Apr 13 '18 at 0:23
  • $\begingroup$ Identical oppinon as mine can be found "Faster inner annuli get a spin down torque from slower outer ones, so they lose angular momentum in favour of the angular of momentum of the outer annuli." in PSE $\endgroup$ – Helder Velez Apr 13 '18 at 0:57
  • $\begingroup$ Not only can this answer be correct, but it is consistent with the theorized origins of largely uniform direction of rotation and roughly planar nature of stellar systems. Keep in mind that very slow speed motions at large distance can still contribute considerable angular moment to a system and if some interaction brings that mass much closer to the center the result is a non-trivial angular acceleration. The usual terrestrial model is rapid increase in angular velocity of a spinning ice skater as the arms are pulled in. $\endgroup$ – dmckee Apr 21 '18 at 23:27
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There are a couple of pieces of observational evidence that support the explanation Jeremy provided. Many millisecond pulsars have been found via x-ray or gamma ray observations, and is interpreted as accretion in a disk on to the surfaces of the pulsars. The falling material speeds up the rotation of the pulsar due to conservation of angular momentum. Pulsars which are in the process of "consuming" the mass of a companion star are often called "black widow pulsars".

Currently, something like 30% of millisecond pulsars are thought to be isolated, with 70% in binary systems. Two systems are known to have planetary mass companions, the most recent having been discovered last year (Transformation of a Star into a Planet in a Millisecond Pulsar Binary). While three body encounters may account for some of the solitary millisecond pulsars known, some millisecond pulsars may completely consume their donor stars. Some authors have argued that one of the millisecond pulsars with planets likely formed as a millisecond pulsar, and formed with a very low magnetic field (Implications of the PSR 1257+12 Planetary System for Isolated Millisecond Pulsars). Since the rate of "spin-down" of a pulsar, that is how quickly its period gets longer, depends on the strength of its magnetic field, a pulsar with a weak magnetic field will stay at its rotation speed for a much longer time.

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