Pulsars with irregular periodicity radiation

[MathematicalPhysicist]

have such stars been observed by the astrophysicists?
and if they haven't yet, are they predicted by the current model of neutron stars physics and evolution?

 

[Labguy]

have such stars been observed by the astrophysicists?
and if they haven't yet, are they predicted by the current model of neutron stars physics and evolution?

Yes, they have been observed if you allow the normal "spin-up" and slowing by loss of angular momentum in your definition.

A newborn neutron star can rotate several times a second; sometimes, when they orbit a companion star and are able to accrete matter from it, they can increase this to several thousand times per second, distorting into an oblate spheroid shape despite their own immense gravity (an equatorial bulge).

Over time, neutron stars slow down because their rotating magnetic fields radiate energy; older neutron stars may take several seconds or minutes for each revolution.

The rate at which a neutron star slows down its rotation is usually constant and very small: the observed rates are between 10^-12 and 10^-19 second for each rotation. In other words, for a typical slow down rate of 10^-15 seconds per rotation, then a neutron star now rotating in 1 second will rotate in 1.000003 seconds after a century, or 1.03 seconds after 1 million years.1

Sometimes a neutron star will undergo a glitch: a rapid and unexpected increase of its rotation speed (of the same, extremely small scale as the constant slowing down). Glitches are thought to be the effect of internal re-organizations of the matter composing the neutron star, something similar to starquakes. Such a starquake would register as grade 20 or 25 on the Richter scale.

I don't remember the source (web or book) but the fastest I have read about is 810 RPS.

 

[Chronos]

Neutron stars are pretty weird creatures. We don't have a very good grasp of what happens to matter when it is on the verge of crushing itself into oblivion. They have been known to 'hiccup'. As labguy said, this is usually attributed to accretion or star quakes. 'Glitches', which sounds like what you are interested in, have a rather complicated explanation. Basically, the thing appears to simply stops spinning [albeit briefly]. This is theorized to be due to the crust temporarily losing its grip on the still spinning, superfluidic core of the neutron star. The technical term [physicists like to create technical terms to discourage non-physicists from asking dumb questions they can't answer] is vortice unpinning.

Most scientists will mumble about being late for a conference if you start asking too many neutron star questions. Just try this one on your favorite astrophysicist "Why don't free neutrons in neutron stars decay after about 17 minutes like they do on earth?" Expect either a walk off, complete with blank stare, or, with a withering gaze, "The failure to observe the decay iota --> eta pi pi , can be explained by the KK-bar molecule interpretation of the S and delta scalar mesons." After which, of course, you will walk off with the blank stare.

 

[Astronuc]

Neutron stars are pretty weird creatures.

"Why don't free neutrons in neutron stars decay after about 17 minutes like they do on earth?"

That's an interesting question. To which I would add - "are neutrons in a neutron star actually 'free'?"

Perhaps part of the problem is that the conditions within a neutron star cannot be replicated in a laboratory on the earth.

 

[Danger]

Perhaps part of the problem is that the conditions within a neutron star cannot be replicated in a laboratory on the earth.

My Portuguese friend's mother made biscuits that came pretty close.

I know, I know... get back to GD.

 

[MathematicalPhysicist]

labguy, about your signature according to edmund wellington it's a bit different:
http://web.archive.org/web/20020825045628/www.physicsforums.com/topic.asp?ARCHIVE=&TOPIC_ID=4872

courtesy of greg, who reminded me about the archive.org website.

 

[Astronuc]

M. Coleman Miller's wepage on Neutron Stars

Including the section "What the @#$% makes gamma-ray bursts?" and "Soft gamma-ray repeaters and whopping magnetic fields".

I have to think about this.

I would also wonder - are hadronic resonances ($\Lambda, \Xi, \Sigma, \Delta$) possible in a neutron star? If so, how would one prove such a phenomenon?

 

[Nereid]

I would also wonder - are hadronic resonances (LaTeX graphic is being generated. Reload this page in a moment.) possible in a neutron star?

You mean, theoretically? Depends on who you ask

If so, how would one prove such a phenomenon?

The only approach I can think is extremely good modeling ... which results in the prediction of something we might, just possibly, be able to observe from 'here', in the next 100 years (extrapolating the advance in observational capabilities - in EM, gravitational radiation, neutrinos, and cosmic rays) that is different from models which do not predict such resonances.

Chronos, you're too cynical ... surely the astonishing thing is predictions consistent with observation and experiment are possible at all! I mean, IIRC, one of the 'most accurately determined' things in the whole of science is (something like) the location on a distant neutron star of the source region of sub-pulses (scale: 10^n metres, where n ~= 0-1, over a distance of what, a thousand pc? You do the math, that's one part in ?)

 

[Labguy]

labguy, about your signature according to edmund wellington it's a bit different:
http://web.archive.org/web/20020825045628/www.physicsforums.com/topic.asp?ARCHIVE=&TOPIC_ID=4872

courtesy of greg, who reminded me about the archive.org website.

Well, sombody must have said Nuke 'em if they can't take a joke.(?)

For a short while we both used the same computer as he was my uncle. He died about two years ago. In three weeks I'm moving back to Iowa where I have family ties.

 

[Danger]

For a short while we both used the same computer as he was my uncle. He died about two years ago.

I'm sorry to hear of his demise. Although he was far before my time, I think that I would have liked interacting with him.

 

[Astronuc]

You mean, theoretically? Depends on who you ask

The only approach I can think is extremely good modeling ... which results in the prediction of something we might, just possibly, be able to observe from 'here', in the next 100 years (extrapolating the advance in observational capabilities - in EM, gravitational radiation, neutrinos, and cosmic rays) that is different from models which do not predict such resonances.

Yes, I do mean theoretically, since we cannot observe such resonances - only the subsequent products of their existence, e.g. neutrinos and photons (EM). Presumably there would (could) be a unique photon energy spectrum/signature? But then there is whatever EM interaction between the source and observer.

. . . surely the astonishing thing is predictions consistent with observation and experiment are possible at all!

I am always impressed by how consistent observation and experiment can be with predictions and theoretical models.

 

[Nereid]

Yes, I do mean theoretically, since we cannot observe such resonances - only the subsequent products of their existence, e.g. neutrinos and photons (EM). Presumably there would (could) be a unique photon energy spectrum/signature? But then there is whatever EM interaction between the source and observer.

I'm not very familiar with these resonances, nor with neutron star models, but I expect that they wouldn't be surface (of the NS) phenomena. The modelling needed to show what unique EM sig would be left, after the EM from the resonances had found its way to the surface, would likely be mind boggling ... and may be somewhat of an anti-climax.

Neutrinos, of course, have no such difficulty leaving the scene of the crime without further harrassment, so maybe something in a neutrino sig?

Your post has suggested something else to my tired old brain - maybe a 22nd century version of asteroseismology? Combined with exquisite modelling, maybe the precise radial variation in whatever physical parameters an asteroseismogram of an NS could reveal might show a unique sig, attributable to the resonances?

 

[Nereid]

http://www.arxiv.org/PS_cache/astro-ph/pdf/0405/0405262.pdf , covering similar ground to the Miller website (the overlap isn't perfect though); just for fun

 

[Astronuc]

Just a thought - which relates irregular NS behavior and GR.

The local gravity field is a function of mass, and the angular momentum is also related to mass. Now is the mass - rest mass or is it mass-energy? (It's been a while since I have studied this subject).

I am musing about what happens when matter transforms from particles with rest mass to photons/neutrinos which have no or 'neglible' rest mass. If a large amount of matter was transformed to energy - would it affect the local gravity field?

The angular velocity is also tied to mass distribution, so if NS speeds up/slows down, then is it simply the size/diameter decreasing/increasing - ostensibly due to internal pressure variation?

Asteroseismography would be an interesting subject - but I think one needs some very good observational capability - which I would imagine needs to be much better than HST.

Nereid - Thanks for the link to the paper!

 

[Nereid]

I realize that I may have been too pessimistic - magnetar quakes may enable us to probe the internal state of at least one class of NS long before the application of pedestrian asteroseismology to NS would be possible! :tongue2:

 

[MathematicalPhysicist]

by the way, nereid you reminded me of scientific american special edition entitled "The Secret Lives of Stars" which i have and never had the pleasure to read it thouroughly, thanks for enticing my memory of it. (the magnetars quakes did the trick).

 

[Labguy]

The local gravity field is a function of mass, and the angular momentum is also related to mass. Now is the mass - rest mass or is it mass-energy? (It's been a while since I have studied this subject).

I am musing about what happens when matter transforms from particles with rest mass to photons/neutrinos which have no or 'neglible' rest mass. If a large amount of matter was transformed to energy - would it affect the local gravity field?

There is a short blurb on mass differences at: http://www.astro.umd.edu/~miller/nstar.html where it states:

By the way, the "mass" here is the gravitational mass (i.e., what you'd put into Kepler's laws for a satellite orbiting far away). This is distinct from the baryonic mass, which is what you'd get if you took every particle from a neutron star and weighed it on a distant scale. Because the gravitational redshift of a neutron star is so great, the gravitational mass is about 20% lower than the baryonic mass.