Slow-spinning radio neutron star breaks all the rules | Hacker News Reader

Slow-spinning radio neutron star breaks all the rules

sydney.edu.au

91 pointsdoener40 comments

I'm a layman here. This is a genuine question because I don't know as much about this as I'd like:

Why do we expect neutron stars to spin rapidly? I understand the "ice skater pulling their arms in" analogy, but why should the pre-collapse star have been appreciably spinning in the first place? To my lay lack-of-understanding, if the neutron star is spinning slowly, then that just implies the earlier version didn't have a whole lot of angular momentum in the first place. What's wrong with that?

You're right, lots of things could happen to neutron stars to make them slow or even stop with the right exchanges. The commonly understood model, though, was that these slow spinning objects would not be pulsars. The radio pulsar mechanism was thought to 'shut off' below certain rotational speeds

Ohhhh, I see! That was the part I wasn’t getting. Thanks!

Was that based on theoretical or empirical analysis?

If empirical, is the data biased? The article mentioned ASKAP reads a lot of the sky at the time. If there are objects emitting every 15 hours, would that even be detected or just look like noise?

The pre-collapse star is spinning for the same reason: any nonzero angular momentum in the very extended gas/dust cloud from which it formed gets concentrated as it collapses into the star. In fact, there's typically so much angular momentum it can't go into the star, but instead either goes into more than one star, or into a pre-planetary disk around the star.

The skater is a small human sized object. They pull their arms in, don't honestly shrink much, but they still spin appreciably faster. When you go from something the size of a pre-collapse star and compare it to the size of a neutron star, the speed up factor is way higher. It doesn't matter how slowly you're spinning when big, you will be spinning fast when you get small as all that angular momentum must be conserved.

I understand that bit, but it's proportional to the shrinkage. If star A is spinning at X turns per unit time, and identical star B is spinning at 4X turns per the same unit time, and they both collapse to the same size neutron star, tiny A' should be spinning at 4X the turns per unit time as little B'.

If the observations in the article are correct, then this neutron star is spinning around 1/10,000th as fast as some others. Neutron star weirdness aside - and that's a whole awful lot of weirdness to set aside - I think that'd mean its earlier version was spinning about 1/10,000th as fast as others. What I don't know, again, as a layman, is whether that's especially unusual or unexpected.

Could it be that instead we're looking at its pole, and the pole has an axial precession of an hour while the little fellow's equator is spinning around quickly?

Or that it has slowed down. Maybe two objects with opposite rotations merged, then collapsed into a neutron star?

I thought that the flashes we see from pulsars were entirely due to the axial precession.

I think you're onto something as earth has precessions of 25,000 years...

Everything in space is spinning. Put three or more objects in space, let them fall towards each other under gravity, and the result will be some amount of rotation. So every star forms from spinning material.

Not an expert. How slow would it have to spin for it to be appreciably “slow” after it speeds up as a neutron star, and how many celestial bodies are spinning that slowly in general?

So, why do bodies accrete in a non-uniform way as to inherit a significant spin far more often than not? Is that bias found in the matter being accreted? Similar to why things seem to rotate the same way: one direction eventually prevails?

Super basic estimate (I'm not an astronomer, this could be wrong): The core of a star has a radius of 100000 km, and after collapse the radius becomes 10 km. Angular momentum is proportional to fr^2, where f is the rotation frequency. Since the radius decreases by 4 orders of magnitude, to keep the same angular momentum the frequency must increase by 8 orders of magnitude. This means that if the new rotation frequency is 1/hour, the original must have been around 10000 years.

Statistically some stars like that probably exist somewhere, but it's unlikely that one of the few thousand neutron stars we know of started that way.

Plus the survivorship bias of those very few. We don’t observe the ones that aren’t spinning much.

It's not a silly question. The USS Enterprise used to stop by pulsars and neutron stars all the time to observe them

It occurred to me that the neutron star may be wobble spinning and we only catch it flashing by now and again when it gets round to pointing in our direction. Does anyone here know if this even a possibility for such an object or would it’s internal uniformity (in composition) prohibit such behaviour?

It should be impossible.

The rotational inertia of these things is absolutely insane. They’re the mass of a star! Nothing short of a very close very massive black hole could alter their spin axis quickly.

They’re also extremely symmetrical objects because of the enormous surface gravity. A “mountain” on a neutron star is about a millimeter high.

It could be a periodic spin change caused by a large, unseen, object orbiting it. Alternatively, the angle of the cone of radiation coming from the poles might be tightening and enlarging on a cyclical basis and we may see the glancing blows as a large sector sweeps by our viewpoint.

Periodic spin changes: https://sciencex.com/news/2021-09-pulse-neutron-star-decades...

I would kill for a video of the radio star.

A pun on https://en.wikipedia.org/wiki/Video_Killed_the_Radio_Star I guess...

Marconi plays the mamba.

My first thought was that maybe it’s falling into a black hole and thus heavily time dilated, but the article doesn’t mention that as a possibility.

So… why isn’t this a serious possibility?

I'm going to take a guess not backed up by any mathematics - to be that close to a black hole, it would have to be well within the Roche limit and get torn apart.

Plus, I'd wager they'd be able to tell if it was eclipsed by the black hole at all.

Plus a very visible Doppler effect. We know many stars orbiting neutron stars or blackholes because the Doppler effect, indicates that it's moving fast around something that we can't see.

> maybe it’s falling into a black hole and thus heavily time dilated

If it were doing that it would have redshifted into undetectability and disappeared from view on a time scale much shorter than 8 months, which is how long it has been observed.

You’re making Sparks here!

The rules are more like guidelines, you see.

The universe has had a lot of opportunities to come up with wacky stuff.

From all I've read for this, it should not be possible for such a slow star to emit anything, ie against known physics of natural objects.

Why are they so quick to discard a technological origin?

> Why are they so quick to discard a technological origin?

A sufficiently advanced society could cause any possible phenomena we ever observe, Occam's razor says you should look for other solutions first.

Occam's razer also says we should not have different laws of physics in different places.

Something very interesting is going on here.

> Occam's razer also says we should not have different laws of physics in different places.

You're starting from an odd place if you're assuming that's the case. There's absolutely no reason to believe the laws of physics are different in this star's case compared to any other we've seen. It's an absurd leap to believe aliens are at work here rather than this being yet another case of our knowledge of physics being incomplete.

> Something very interesting is going on here.

Agreed.

If we interpret Occam's razor as to "choose the simplest solution with the greatest explaining power," then the "solution" may be to admit that we have an inadequate understanding of how these things physically work.

This doesn't rule out the possibility of a technological origin, but I think Ben's point is that we should explore possiblities that generate more answers than questions first.

No, there’s nothing except the fact that the "laws of physics", ie. our models of nature, are eternally incomplete simplifications. All that’s going on is that our models have a prediction error and thus tell us we’ve failed to take something into account. Which is an utterly standard part of scientific progress. Papers will be written, hypotheses proposed, more evidence gathered, and after some number of years we’ll have a new, more complete theory of neutron star evolution and/or emission processes.

> Why are they so quick to discard a technological origin?

For the same reason that they haven't presented God as a serious option. It's not falsifiable, it being true would require a truly enormous amount of preconditions that are utterly unknown, and it's much more likely that something about our current understanding isn't correct.

If you say "aliens!" every time you see something that doesn't make sense, you'll be saying it a lot, but you will find it will actually be normal physics hiding behind a curtain.

That path is riskily close to "shrug God did it! shrug", which is kind of a conversation stopper in science.

Because we’ve run into plenty of “huh we didn’t think those existed” in astronomy so far. We’ll try to understand more mundane possibilities first.

Because as of yet, there's no evidence of ETs, but there's plenty of evidence for natural phenomena that were puzzling for a while.

Ok let's assume aliens at the first sight of unexplained phenomenon. Now what?

Obviously, "now what?" is that they fly here and leave detection traces only certain people can see.

Because "space" can contain almost anything at this point... and I doubt we have discovered 1% of natural entities or combinations of natural entities.

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