X-ray bursts might not happen for larger neutron stars?

[Jonathan Scott]

Reasonable numbers for the X-ray energy released from material impacting the surface plus all fusion reactions should be under 0.1%.

What makes you think the typical kinetic energy of falling material impacting the surface should be any less than that in the accretion disk? I would have expected it to be significantly greater because of the difference in gravitational potential. Also, the energy released by hydrogen fusion to helium is only something like 0.7% of the rest mass, but even that is quite a bit more than your "reasonable" number.

 

[Jonathan Scott]

The magnetic field prevents accreting material from impacting the surface with very high energy.

Why should that be? It will certainly deflect charged material towards the magnetic poles, and some of it may end deflected away instead of towards the neutron star. If you have a reference that says it could somehow cushion the energy enough to allow a "soft landing", please provide it. I thought it was already assumed that the energy of the infalling material was sufficient to cause rapid fusion of hydrogen to helium.

 

[Bernie G]

What makes you think the typical kinetic energy of falling material impacting the surface should be any less than that in the accretion disk? I would have expected it to be significantly greater because of the difference in gravitational potential. Also, the energy released by hydrogen fusion to helium is only something like 0.7% of the rest mass, but even that is quite a bit more than your "reasonable" number.

Thank you for making me think about this more. I was incorrectly assuming material impacting the neutron star surface would only have somewhat more kinetic energy than the temperature of the accretion disk, but sensibly material impacting the poles should have extremely high kinetic impact energy. Almost all energy from fusion reactions results in kinetic energy of the particles produced and the X-ray energy directly produced is trivial. The kinetic energy of all fusion products is still less than that needed to escape the neutron star. Would most of this 0.7% fusion energy released heat the accretion disk which then results in X-rays? Even if it was 0.7% its still a small fraction of 5 or 10%.

Would it be possible for material to impact the poles of a neutron star with a kinetic energy representing maybe 1 - 5% of its rest mass? If so, what could be the result of these very high energy collisions?

 

[Bernie G]

Could material impacting the poles of a neutron star with a kinetic energy representing maybe 1 - 10% (or more) of its rest mass result in huge X-ray production? Could this be a major source of X-rays as it is assumed accretion is?

 

[Jonathan Scott]

Could material impacting the poles of a neutron star with a kinetic energy representing maybe 1 - 10% (or more) of its rest mass result in huge X-ray production? Could this be a major source of X-rays as it is assumed accretion is?

From what I've read, I believe something like this is definitely assumed to be true. In particular, after a big "feed", pulsars also show related pulses in X-rays as the magnetic poles rotate into and out of view. However, I'm not an expert, and you can easily do the research yourself. Most of what I've read on this area recently has been found via Google, including Wikipedia entries (which often have very useful references), introductory material for university courses and papers on ArXiv.

 

[Bernie G]

Yes, checked that out. If lots of accreting material falls directly on the magnetic poles ... what about the jets? That seems inconsistent with lots of accreting material somehow also forming jets above the magnetic poles directed away from the star. Let alone some recent observations which apparently say accretion and the jets do not always happen simultaneously.

 

[Ken G]

It sounds to me like you are basically asking, what is the connection between the thermal kinetic energy of the H and He on the surface of a neutron star, and their infall kinetic energy? I think the latter must vastly exceed the former, so the question is, how much kinetic energy is lost during temperature equilibration, that is, what sets the temperature on the surface?

For most stars, there is no connection between the surface temperature and the infall energy, because surface temperature is set by very different physics. I don't know what physics sets the surface temperature of a neutron star, but it sounds like the X-ray flashes are similar to what are called thermal pulses deep in the interiors of more typical kinds of stars. Your question seems to boil down to, is there ever a mass of a neutron star that yields a surface temperature above about 10⁸ K, such that He would fuse even at the surface? We know that would not be possible over the whole surface-- a blackbody with that T would be spectacularly X-ray bright all the time. But could there be tiny hot spots like that just where accretion is occurring? I don't really know, only that the high T would need to remain very concentrated, and heat transport might be an issue.

Put differently, what I mean is, the infall energy of the H is spectacular, so either it just fuses without equilibrating to a temperature, or it heats the H at the surface above 10 million K. I don't know which, all we know is the H does fuse. You're wondering if there is a connection between the resulting T, and that initial infall energy. That would seem to connect to the question, does the He so produced remain hot long enough to fuse again, or does it cool to the prevailing surface temperature, piling up until it gets thick enough to see the T rise up to He fusion levels? I guess we first have to understand how the H fuses-- does it just crash into a nucleus on the way in, at its infall energy, or does it thermalize first in a hot spot of X-ray gas that is constantly maintained whenever there is accretion? If the latter, what sets the T of that gas?

 

[Jonathan Scott]

Yes, checked that out. If lots of accreting material falls directly on the magnetic poles ... what about the jets? That seems inconsistent with lots of accreting material somehow also forming jets above the magnetic poles directed away from the star. Let alone some recent observations which apparently say accretion and the jets do not always happen simultaneously.

What was the point of my starting a separate thread for a separate question when you keep changing the topic back to your own weird ideas?

Jets do not form above the magnetic poles. They form above the spin axis. It is thought that they may be formed or caused by charged particles from the accretion disks getting caught up in magnetic fields but escaping from the rotation poles rather than falling to the surface.

 

[Ken G]

Another point is, the fusion energy released is insignificant compared to the infall energy, so the steady-state heat input that is needed to maintain X-ray emitting gas in T equilibrium is all from infall. So it sounds like the temperature should not just depend on the depth of the potential well, but also the density of infalling gas. My guess is, you could trade one off against the other-- a deeper well would not need as high an infall density to achieve the same X-ray T in the hot spots.

So let's assume the H fusion happens in X-ray hot spots, and not due to the initial kinetic energy of the H as it smacks into a nucleus of some kind. If so, any time the T of the hot spot is more like 10⁸ K instead of 10⁷ K, the He will not build up. I would imagine that factor 10 increase in T could be accomplished by any combination of deeper well and higher accretion density, so having a higher mass neutron star would simply prevent He buildup at a lower accretion density threshhold, but would still allow He buildup for lower density accretion. That might be enough to suppress the occurrence rate of X-ray flashes, although perhaps not eliminate them altogether.

 

[Jonathan Scott]

So let's assume the H fusion happens in X-ray hot spots, and not due to the initial kinetic energy of the H as it smacks into a nucleus of some kind. If so, any time the T of the hot spot is more like 10⁸ K instead of 10⁷ K, the He will not build up. I would imagine that factor 10 increase in T could be accomplished by any combination of deeper well and higher accretion density, so having a higher mass neutron star would simply prevent He buildup at a lower accretion density threshhold, but would still allow He buildup for lower density accretion. That might be enough to suppress the occurrence rate of X-ray flashes, although perhaps not eliminate them altogether.

Thanks very much for your thoughtful posts. This is helping me get more of a grip on the physics.

I think that protons are expected to hit the surface with kinetic energy equal to a significant percentage of their rest mass, so for example something around 10% would give around 100MeV of energy, corresponding to a temperature of around 10¹² K. This would of course dissipate into the existing material, and I don't know how the heat would transfer, but it certainly seems possible to me that there would be enough energy in hot spots to trigger helium fusion (which I guess is likely to work via the triple-alpha process as usual).

I feel that our attempts in this area are all very speculative, and perhaps this is all the answer I can expect to get for now unless I can find anything more specific myself in research papers. Thanks again.

 

[Bernie G]

I think that protons are expected to hit the surface with kinetic energy equal to a significant percentage of their rest mass, so for example something around 10% would give around 100MeV of energy, corresponding to a temperature of around 10¹² K. This would of course dissipate into the existing material, and I don't know how the heat would transfer, but it certainly seems possible to me that there would be enough energy in hot spots to trigger helium fusion (which I guess is likely to work via the triple-alpha process as usual).

If material impacts with that kind of energy its way way more than that required to initiate fusion. With these intense impacts how could there be any hydrogen or helium buildup, let alone the reactions raised by your original question?

Maybe intense impacts would directly produce some very intense X-rays, somewhat like a giant X-ray tube.

Sorry to digress again but all neutron star discussion is fascinating. As KenG says the fusion energy released should be quite small compared to the infall energy. I'm not trying to annoy you with digressions, but don't see how fusion blasts could be a significant amount of the energy emitted from the surface or vicinity neutron stars. Thats probably a subject for other posts as is the puzzle of jets. Thanks for saying "Jets do not form above the magnetic poles. They form above the spin axis." Wiki confirms that statement. Its a great puzzle.

 

[Ken G]

I think that protons are expected to hit the surface with kinetic energy equal to a significant percentage of their rest mass, so for example something around 10% would give around 100MeV of energy, corresponding to a temperature of around 10¹² K.

Yes, that is my understanding as well. So either the fusion is "prompt" in some sense, meaning that there is not time to equilibrate to some kind of surface temperature, or else that huge temperature doesn't last long enough to get fusion, and the T reaches some equilibrium between heating and cooling rates. But if it's prompt, it's way hot enough to fuse anything, so there'd be no reason for He to build up. So I'm thinking the T must drop pretty fast, and the question becomes, what controls where it ends up? I think that must involve both the mass of the neutron star, and also the density of the infalling gas, as both contribute to the heat source per area per time.

This would of course dissipate into the existing material, and I don't know how the heat would transfer, but it certainly seems possible to me that there would be enough energy in hot spots to trigger helium fusion (which I guess is likely to work via the triple-alpha process as usual).

The missing element is how long that energy stays around, such that the He could be maintained at a high enough energy long enough to fuse. It sounds like the standard picture is that the T of these hot spots is often between the fusion T of H and He, but you are wondering under what circumstances can it be above both of those.

I feel that our attempts in this area are all very speculative, and perhaps this is all the answer I can expect to get for now unless I can find anything more specific myself in research papers. Thanks again.

Yes, there needs to be either some observational constraints, or theoretical understanding of the energy balance that includes where that heat goes. Not simple problems, but I would certainly agree that if data suggests high mass neutron stars show less frequent X-ray bursts, that could be taken as evidence that more of the accretion is leading to hot spots that fuse both the H and the He. Of course, that could lead to buildup of C, which has a notorious penchant for runaway fusion! So you might end up trading one source of X-ray burst for another.

 

[Ken G]

I'm not trying to annoy you with digressions, but don't see how fusion blasts could be a significant amount of the energy emitted from the surface or vicinity neutron stars.

It's because the infall X-rays are emitted continuously throughout the accretion process, but you can "save up" fusable material for a long time, if it is building up, and release it all at once in a thermonuclear runaway. So the time average of the latter would never compete with steady X-ray emission, but in isolated events, it can give a big signal.

Thanks for saying "Jets do not form above the magnetic poles. They form above the spin axis." Wiki confirms that statement. Its a great puzzle.

Yes, I have no idea how the jets work, it sounds like they are not directly powered by accretion, but instead use energy that has first been processed quite a bit.

 

[Jonathan Scott]

Of course, that could lead to buildup of C, which has a notorious penchant for runaway fusion! So you might end up trading one source of X-ray burst for another.

Good point! But it seems likely that the energy needed to start that might well be achievable as well, in which case the fusion would continue past carbon (perhaps all the way to iron, or even more directly adding to the neutronium). And if the energy was not easily achievable, then larger masses might trigger black holes rather than carbon fusion.

Of course, there are multiple ways of distinguishing a neutron star from a black hole, and even if this suggestion of possible suppression of thermonuclear bursts turned out to be valid, they could still be spotted if they showed radio or X-ray pulsar properties (unless those too are suppressed, for example if they then turn into a hypothetical quark star, which some people think might kill the magnetic field and make the surface uniform).

 

[Bernie G]

Jets do not form above the magnetic poles. They form above the spin axis.

That seems odd. The jets look like very hot ionized stuff and so should be directed by a magnetic field. Can you suggest a source that says jets form at the spin axis instead of the magnetic poles?

 

[Jonathan Scott]

That seems odd. The jets look like very hot ionized stuff and so should be directed by a magnetic field. Can you suggest a source that says jets form at the spin axis instead of the magnetic poles?

How about the Wikipedia entry for jets? https://en.wikipedia.org/wiki/Astrophysical_jet

The magnetic poles whirl round the neutron star at the pulsar frequency, so their direction is changing extremely rapidly.

 

[Bernie G]

How about the Wikipedia entry for jets? https://en.wikipedia.org/wiki/Astrophysical_jet
The magnetic poles whirl round the neutron star at the pulsar frequency, so their direction is changing extremely rapidly.

The Wiki article does not source this statement so I added 'citation needed' to the article.

 

[Jonathan Scott]

The Wiki article does not source this statement so I added 'citation needed' to the article.

It seems obvious to me that you couldn't have a "jet" whirling round, unlike the electromagnetic beam which behaves like a lighthouse. But it is of course possible that material is being deflected upwards near the magnetic poles and by symmetry only the material which happens to end up aligned with the spin axis escapes as a coherent jet.

 

[Bernie G]

It seems obvious to me that you couldn't have a "jet" whirling round, unlike the electromagnetic beam which behaves like a lighthouse. But it is of course possible that material is being deflected upwards near the magnetic poles and by symmetry only the material which happens to end up aligned with the spin axis escapes as a coherent jet.

Or the whirling magnetic field from the magnetic poles becomes aligned with the spin axis at some distance from the surface.

 

[Jonathan Scott]

Or the whirling magnetic field from the magnetic poles becomes aligned with the spin axis at some distance from the surface.

You appear to be trying to change the definitions to match your ideas. The facts (as currently understood) are that the magnetic pole is known to rotate about the spin axis and any jets appear along the spin axis.

To be technically accurate, there isn't even such a thing as a moving magnetic field, let alone a whirling one. If the source of a magnetic field is moving or changing, that induces electric fields, in the same way that a moving or changing electrostatic source induces magnetic fields. However, one can use a field line model as an approximate illustration anyway.

 

[Ken G]

The field should trace a helical structure with an axis around the rotation axis. What's not clear is if the field would fan out like a cone, making a conical jet, or if something corrals the field along the rotational axis, as would seem to be needed to get a narrow jet. How does that collimation occur? The field of a tilted rotating bar magnet, or a tilted rotating electric dipole, wouldn't collimate, it would be conical. Unless that changes for relativistic rotation?

 

[Jonathan Scott]

The field should trace a helical structure with an axis around the rotation axis. What's not clear is if the field would fan out like a cone, making a conical jet, or if something corrals the field along the rotational axis, as would seem to be needed to get a narrow jet. How does that collimation occur? The field of a tilted rotating bar magnet, or a tilted rotating electric dipole, wouldn't collimate, it would be conical. Unless that changes for relativistic rotation?

I previously found various papers via Google which discuss this in more detail. They usually mention the usual buzzwords plus "Magnetohydrodynamics" or similar.

However, that isn't the subject of this thread. I keep telling Bernie G that if he wants to discuss a different topic he should start a new thread!

 

[Ken G]

Understood.

 

[Bernie G]

It occurs to me that this type of burst might therefore not be possible if the neutron star were sufficiently massive that the falling hydrogen was already sufficiently energetic to fuse beyond helium at a rate sufficient to prevent any build-up.

See this link!:
http://news.mit.edu/2012/model-bursting-star-0302

 

[Jonathan Scott]

See this link!:
http://news.mit.edu/2012/model-bursting-star-0302

Thanks - that is very interesting, especially the paper referenced by the news article. It's not quite the same as I was suggesting, but it's very closely related.

The interesting point is that rather than faster mass accretion driving somewhat faster pulses of the same amplitude, it's actually driving more frequent pulses of a smaller amplitude. This makes sense because the heat has less time to disperse, so the temperature rises and less additional energy is needed to trigger fusion, which creates a similar scenario to the case I suggested of a somewhat more massive star.

The paper also calls attention to the important distinction that this was seen in a relatively slowly-rotating neutron star, but faster-rotating neutron stars seem to behave a bit differently. This reminds me that the surface of the fastest pulsars is rotating at a significant fraction of the speed of light which probably has an important influence on how material impacts on the surface, even near the rotation axis, but I must admit there are so many factors involved that I can't immediately guess what difference that makes overall.