Non-rotational neutron stars?

  1. Hey!
    I have a question, whose answer I was unable to get after reading some articles regarding neutron start.
    I've understood that neutron stars lose their rotational energy and emit fragments of it to earth, thus slowing down through the years.
    I know that this proccess takes billions of years (like every proccess out there!!! :), but I wondered if, out of all the neutron stars we have discovered - are there any who are not rotating at all? Or maybe rotating very slowly? Meaning, are there any n.stars known to us that have lost all their rotational energy? What happens then?
    Is it even possible to hunt them if they aren't rotating? Most of the energy they emit comes from the rotational energy - are they invisible to us when they stop rotating?

  2. jcsd
  3. mgb_phys

    mgb_phys 8,952
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    They don't emit rotation to earth as such, they transfer angular momentum to any bits of dust and gas around them, but since they are formed from a supernova which tends to remove most of the matter in their system there isn't much of this left.
    It would be very difficult to detect non rotating NS, it's only from the pulsar jets from rotating ones we get any signal.

    The slow down rate is incredibly low, 10^-10 to 10^-20 /s, or in other words if a N. star had a period of 1s after 1 million years it would rotate at 1.03 seconds!
  4. Thanks for the reply.
    1. Are the pulsar jets connected to the rotational speed? I guess so.... in that case - here's another detection method gone bad when the NS (thanks for the initials :) stops rotating.
    2. Even though the slowing-down rate is incredibly low, there are probably millions of NS out there, so some should be rotating really slowly, no?
    3. What does, theoreticaly, happen to a NS that loses all it's rotational energy?

  5. mgb_phys

    mgb_phys 8,952
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    1, I don't think the strength of the jets are connected to rotation speed but the rotating magentic field is needed for them.
    2, Some neutron stars will have almost stopped rotating, absolutely zero rotation speed is difficult to reach of course.

    3, Apart from being a traffic hazard to UFOs not much! It just sits there essentially for ever occasionally emitting X rays if any matter lands on it.

    I was wrong, you can detect non-pulsar neutron stars wether rotating or not if a companion survived the initial supernova, you can then detect the wobble in the orbit of the other star as it rotates around the invisible neutron star. You could also detect x-rays from any gas being pulled off the companion and dumped onto the surface of neutron star. If it is rotating and has a magnetic field (neutron stars have strong fields) you will also get pulsar jets from this material.
  6. Thank you very much! This helped a lot :)

  7. It is impossible for a stellar nova to NOT induce angular momentum on a core element during its gravitational collapse phase, nor can an Equation of State be demonstrated that can NOT induce angular momentum.

    Non-rotational neutron stars are a violation of conservation of angular momentum, they do not exist.

    Also, a neutron stars companion would induce angular momentum on the neutron star.
    Last edited: Aug 29, 2007
  8. mgb_phys

    mgb_phys 8,952
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    A supernova creating a non rotating neutron star is somewhat unlikely.
    But since a rotating neutron star slows down as it loses angular momentum to any other massive object nearby then an eternally-rotating one would be a violation of conservation of angular momentum!
  9. This is all, of course, assuming we know enough about the universe to understand the problem fully. :D Just bringing it back into perspective.

  10. Reducing the angular momentum of a neutron star from 10^8 to 0 with a binary companion in the lifetime of the Universe is highly improbable. However, anyone is welcome to perform the GR calculation.

    A closed binary system may alternate between spin and orbital momentum, however either probably not absolutely to total or zero in the classical sense, the system would either collapse or fly apart.

    Last edited: Aug 30, 2007
  11. Well, why assume a binary system?

    Is the slowing-down rate of a neutron star decreasing with time? If so, it's probably right - a neutron star will never really get to "zero" angular speed.
    But if the slowing-down rate is pretty much a constant - I still don't see why theoratically a neutron star couldn't slow down to zero.
    Unless, of course, the star itself can't exist (as a whole) with a small ang. velocity - therefore it "dies" (again!) before slowing more.
  12. mgb_phys

    mgb_phys 8,952
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    Most stars are in binary systems - so assuming that the companion survived the SN the NS would be in a binary.
    The slow down rate is constant so it would eventually stop ( for very large values of eventually - as Orion says) unlike say cooling where the rate is proportional to temperature and so something never entirely cools.
    The slowest radio emitting NS have a period of under a day, of course there may be slower ones that we can't detect - the fastest spin at nearly 1000/s !

    There is no theoretical reason why a NS cannot exist with zero rotation, it's just going to take an impossibly long time to get there.
  13. 1000/s :-\
    Makes you want to...
    See it.

    Thanks. :)

  14. The fastest spinning NSs are probably the youngest and have a discus or oblate spheroid geometry, while the slowest spinning NSs are probably the oldest and evolve from discus or oblate spheroid to spheroid then to sphere while experiencing periodic seismic activity to its changing stellar core geometry.

    There also appears to be alot of dynamics involved with its angular momentum and Equation of State, such as its dynamic composition, mass, radius, crust thickness, and magnetic field strength as a few dynamic factors...

    Meaning, a NS is going to really shake and shudder and even explode before its angular momentum shuts down.

    Last edited: Aug 31, 2007
  15. SpaceTiger

    SpaceTiger 2,977
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    The fastest spinning neutron stars are those that have been sped up by close binary interactions (accretion torques). This is the theoretical origin of the "millisecond pulsars", which are not necessarily the youngest neutron stars. The Crab pulsar, for example, is not a millisecond pulsar.

    It is true, though, that neutron stars not experiencing accretion torques will slow down with time.

  16. SpaceTiger, True, I grant that my statement is not an absolute, however you have failed to reconcile my statement regarding NSs that exist singular, without binary companions. The physics of 'millisecond pulsars' are 'induced' as opposed to the physics of singular NSs that exist in an undisturbed natural state from induced external influences in their entire existences. (lifetimes?)

    Why assume a binary system?

    What is the ratio of singular versus binary systems for NSs?

    I calculate a ratio of 600 singular to 21 binary or NS s-b ratio: 28.571

    This also raises the issue of NS existence 'lifetimes', spin or not, induced torque or not, singular or binary, NS stellar quake explosions, can a NS exist forever or not?

    Can induced torque destroy a NS?

    A NS s-b ratio of 28.571 seems to indicate that singular NSs are created more frequently, however it may also be an indication that binary NS systems explode at a faster rate from induced torque.

    Can supernovae also destroy the NSs that they can temporarily create?

    There also appears to be a theoretical GR upper limit to how fast a NS can rotate, even with induced torque, around 716 Hertz, without exploding,

    NSs that fall outside the spectrum rotational range of 0.117 Hz to 716 Hz, probably all explode.

    Calculating the de-acceleration and acceleration rates for NSs to these lower and upper limit rotational frequencies results in the determination of a NSs stellar existence 'lifetime'.

    Last edited: Sep 10, 2007
  17. SpaceTiger

    SpaceTiger 2,977
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    The physics aren't induced, but the rotation rates are. Disregarding millisecond pulsars, then what you said would be true on average -- younger neutron stars would spin more quickly.

    Where is this number coming from? Who's assuming a binary system?

    They can exist for a very long time:

    Not by accretion, no.

    A star in a binary system with a neutron star might become unbound from that neutron star after a supernova, but the neutron star itself would remain intact.
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