Do Non-Rotating Neutron Stars Exist?

[cosmic_tears]

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 process takes billions of years (like every process 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?

Thanks!
Tomer.

 

[mgb_phys]

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!

 

[cosmic_tears]

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?

Thanks!

 

[mgb_phys]

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.

 

[cosmic_tears]

Thank you very much! This helped a lot :)

 

[Orion1]

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.

 

[mgb_phys]

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

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!

 

[nhjansen]

This is all, of course, assuming we know enough about the universe to understand the problem fully. :D Just bringing it back into perspective.

 

[Orion1]

(conservation of angular momentum)

The same phenomenon results in extremely fast spin of compact stars (like white dwarfs, neutron stars and black holes) when they are formed out of much larger and slower rotating stars (indeed, decreasing the size of object 10^4 times results in increase of its angular velocity by the factor 10^8).

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.

Reference:
http://en.wikipedia.org/wiki/Angular_momentum#Conservation_of_angular_momentum

 

[cosmic_tears]

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.

 

[mgb_phys]

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.

 

[cosmic_tears]

1000/s :-\
Makes you want to...
See it.

Thanks. :)

 

[Orion1]

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 a lot 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.

A group of scientists has announced that recent observations of one particularly tempestuous neutron star seems to bolster the theory that irregular gamma and x-ray bursts are caused by starquakes.

Last year, the gamma repeater known as SGR 1900+14 suddenly flashed to life after 20 years of relative dormancy.

The neutron star was identified as a source of gamma ray bursts in 1979, but it was only seen to flash seven times between then and 1998. Last May, though, the star flared up, and it has done so more than 200 times since. Each burst releases as much energy as the Sun does in a year.

"These flashes are really the starquakes," said Robert Duncan, a research astronomer at the University of Texas

Neutron stars are the only stars (along with some white dwarfs) that have a solid surface. They are thought to be the collapsed cores of stars, the remnants of immense explosions called supernovae. About 12 miles (20 kilometers) in diameter, neutron stars contain the mass of the Sun and have a superfluid interior covered by a metallic crust. That crust is thought to be slightly more than a half mile (about 1 kilometer) thick.

A starquake can be thought of as a displacement or rupture of the crust, similar to the tear between two of Earth's tectonic plates along a fault. A displacement as small as a few millimeters could create a typical gamma ray burst, Duncan said.

But the two-hundred-plus typical bursts were impotent flickers compared to the explosion of August 27. On that day the star erupted with a flash more than 1,000 times as powerful as any yet observed. The gamma ray flash lasted almost six minutes. It exploded as much energy during the first second as the sun releases in 1,000 years.

What most amazed Woods, though, was a dramatic deceleration in the star's spin rate -- what looked like the sudden breaking of the whirling magnetic star.

Scientists classify SGRs as magnetars, a 10-member family of neutron stars that has the peculiar characteristic of slowing in rotation very rapidly. The rate of this deceleration -- a few milliseconds per year -- is called the spindown rate. Theorists believe that a strong magnetic field is responsible for applying the breaks.

"During the summer of 1998, we observed a rapid change in the spindown, and it happened in a period when the burst source was very active," Woods said. After that 80-day period, it (the spindown) was about 2 times faster than it was previously."

In May 1998, the rate was measured to be about 2 milliseconds per year. It then increased to about 4.5 milliseconds per year, and today is back to 2 milliseconds per year, Woods said. A rotation now takes almost one two-hundredth of a second longer to complete than it did last year. This is an important difference in a star that spins once every 5.16 seconds.

It exploded as much energy during the first second as the sun releases in 1,000 years.

Reference:
http://www.space.com/scienceastronomy/astronomy/starquake.html

 

[SpaceTiger]

The fastest spinning NSs are probably the youngest...

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.

 

[Orion1]

Of the roughly 1600 pulsars discovered to date, only 30 have periods longer than 3 seconds, with the slowest spinning pulsar having a period of 8.5 seconds. (0.117 Hz)

The scientists discovered the pulsar, named PSR J1748-2446ad, in a globular cluster of stars called Terzan 5, located some 28,000 light-years from Earth in the constellation Sagittarius. The newly-discovered pulsar is spinning 716 times per second, or at 716 Hertz (Hz), readily beating the previous record of 642 Hz from a pulsar discovered in 1982. For reference, the fastest speeds of common kitchen blenders are 250-500 Hz.

why assume a binary system?

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'.

Dense globular clusters of stars are excellent places to find fast-rotating millisecond pulsars. Giant stars explode as supernovae and leave rotating pulsars which gradually slow down. However, if a pulsar has a companion star from which it can draw material, that incoming material imparts its spin, or angular momentum, to the pulsar. As a result, the pulsar spins faster. "In a dense cluster, interactions between the stars will create more binary pairs that can yield more fast-rotating pulsars

1600 pulsars discovered to date, only 30 have periods longer than 3 seconds

the fastest speeds of common kitchen blenders are 250-500 Hz.

Reference:
http://www.nrao.edu/pr/2006/mspulsar/
http://www.jb.man.ac.uk/~pulsar/msc04/
http://xray.sai.msu.ru/~mystery/articles/review/node46.html

 

[SpaceTiger]

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?)

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.

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

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

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?

They can exist for a very long time:

http://www.math.ucr.edu/home/baez/end.html

Can induced torque destroy a NS?

Not by accretion, no.

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

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.