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

[Jonathan Scott]

A method of definitely distinguishing a neutron star from a possible stellar black hole is that it produces X-ray bursts, which have a sharp rise time and may last for an extended period. I had previously thought these occurred when hydrogen fell to the surface and was immediately fused to helium, but I've now learned that bursts are thought to occur when an amount of helium has built up and undergoes further fusion in a chain reaction.
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. I don't know the details of the required energy, but it seems that this could mean that neutron stars above some mass threshold might not produce X-ray flashes, making it difficult to tell the difference from a black hole.
Is it known whether this burst suppression might actually occur within the range of masses expected for neutron stars, and if so can anyone point me to any further information on the subject?

 

[Bernie G]

Its an interesting idea. If hydrogen can do it maybe helium could do it. You are suggesting the smaller mass "black holes" might be compact stars? Expect a lot of flak!

 

[Bernie G]

helium has built up and undergoes further fusion in a chain reaction...
... 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.

That makes sense and fusion reactions at the poles might cause feeble jets. Accreting hydrogen and helium on a neutron star ... let's say at the magnetic poles ... should impact with more energy needed to cause fusion. Plus the accreting plasma should have undergone significant fusion on the way down which heats the incoming plasma even more so a mix of light weight + heavier ions and electrons will rain on the magnetic poles. I was inaccurate before. Yes, fusion reactions are puny compared to a neutron star's surface gravity. Most of the fusion products don't even have the energy to escape the surface gravity. But would light weight ions get velocities to escape the star? My guess is yes. Its worth figuring out the what the velocity of a hydrogen ion at fusion temperatures at the poles could be ... my guess is its more than the escape velocity.

 

[Jonathan Scott]

This is not relevant to my question in this thread. Please resist the temptation to promote your own strange ideas about neutron stars in other people's threads!

 

[Bernie G]

A method of definitely distinguishing a neutron star from a possible stellar black hole is that it produces X-ray bursts, which have a sharp rise time and may last for an extended period. ... 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.

What do you mean by "at a rate sufficient to prevent any build-up"? Does that mean to prevent all accretion?

Fusion reactions significantly above the surface and at/near the surface could probably create feeble jets.

 

[Bernie G]

What do you mean by "at a rate sufficient to prevent any build-up"? Does that mean to prevent all accretion?

Or does it mean there is no hydrogen or helium at the surface? That would be something!

 

[Jonathan Scott]

What do you mean by "at a rate sufficient to prevent any build-up"? Does that mean to prevent all accretion?

If the thermonuclear X-ray flash is caused by a build-up of helium undergoing a chain reaction on a large scale, then I would expect that if the energy of the incoming particles is somewhat higher it could be sufficient to trigger such reactions frequently and locally on a small scale (or even continuously), preventing any significant build-up on a large scale, so there would be no major X-ray burst events.

And yes, in that case, the surface would be mostly higher elements, although I'm not an expert on the relevant fusion paths so I couldn't give numbers.

 

[Bernie G]

If the thermonuclear X-ray flash is caused by a build-up of helium undergoing a chain reaction on a large scale, then I would expect that if the energy of the incoming particles is somewhat higher it could be sufficient to trigger such reactions frequently and locally on a small scale (or even continuously), preventing any significant build-up on a large scale, so there would be no major X-ray burst events.

I think that is expressed quite well and a good idea. My nagging probably helped you write it better. I don't know of course but wish you well on resolving this.

 

[Bernie G]

Nothing would please me more than your idea being correct, but at what mass do you expect this to take effect? Presumably above 2 solar masses? Is there any observed compact star or small black hole that could be a candidate?

 

[Jonathan Scott]

Nothing would please me more than your idea being correct, but at what mass do you expect this to take effect? Presumably above 2 solar masses? Is there any observed compact star or small black hole that could be a candidate?

That's exactly the sort of thing I'd like to know myself, which is why I started this thread.

 

[Chronos]

The insinuation here is neutron stars have too much gravity to release xray bursts, I assume this logic also applies to quasars.

 

[Jonathan Scott]

The insinuation here is neutron stars have too much gravity to release xray bursts, I assume this logic also applies to quasars.

The X-ray bursts being referred to here are thought to emanate from the surface of a neutron star. They consist of sudden bright flashes which then die away relatively slowly as from thermal cooling, and I think they also typically show periodic patterns with the same frequency as the associated pulsar. My suggestion is that neutron stars over some threshold mass might not exhibit this effect because the energy of the infalling hydrogen would be enough to cause frequent small amounts of fusion rather than larger bursts. This would mean that the absence of such flashes would not necessarily rule out a neutron star.

Anything larger than that threshold would not be expected to emit thermonuclear X-ray flashes of that type, especially if it was large enough to be a black hole, which would not have a surface.

Of course, both neutron stars and quasars are thought to emit lots of X-rays, primarily from their accretion disks, and those emissions can show various fluctuations too. Current observations do not have sufficient resolution to determine where the X-rays come from, so the only way to tell where they originate depends on matching up frequencies of periodic fluctuations with other information which suggests whether it is likely to be the surface or part of the accretion disk.

 

[Chronos]

Agreed. Jonathan. That point was never in dispute. I only wished to point out you should never ignore the obvious.

 

[Bernie G]

If the thermonuclear X-ray flash is caused by a build-up of helium undergoing a chain reaction on a large scale, then I would expect that if the energy of the incoming particles is somewhat higher it could be sufficient to trigger such reactions frequently and locally on a small scale (or even continuously), preventing any significant build-up on a large scale, so there would be no major X-ray burst events.
...
From your other post: It would certain stir things up a bit if someone could spot an object soon that GR says should be a black hole but which clearly isn't, such as a pulsar around 30 solar masses!).

Even if it turns out not to be correct your first paragraph is so well written its convincing. The idea deserves at least 50 out of 100 points just for the quality of writing!

But what could be the support mechanism to prevent collapse for a solid compact star larger than 5 solar masses? Degeneracy pressure? A 5 solar mass 30-km radius star would have less core pressure and surface gravity than a 2 solar mass 12-km radius star. Could a 5 solar mass 30-km radius star be stable or should it contract?

 

[Jonathan Scott]

But what could be the support mechanism to prevent collapse for a solid compact star larger than 5 solar masses?

You do make it difficult to keep a thread on topic, but I guess we can make a minor diversion here!

The only way a dense object significantly larger than a normal neutron star could fail to be a black hole is if GR is wrong in some way and the surface gravity for a given mass is less than it would be with GR, for example if the effective potential (time-dilation factor) actually varies something like $\exp(-Gm/rc^2)$ rather than the Schwarzschild expression $\sqrt{1-2Gm/rc^2}$ (where to be boringly accurate I should mention that the first is expressed in isotropic coordinates, and satisfies the solar system tests, but the second is the conventional expression in Schwarzschild coordinates, so $r$ doesn't have exactly the same meaning in both cases). This sort of potential would allow objects of unlimited mass with unlimited surface redshift. In the same way as for neutron stars, it is possible that beyond certain thresholds such objects would collapse into different types of matter, but would still have a surface.

I'm not aware of any alternative theory which makes such predictions and is considered sufficiently mainline for discussion in these forums, so although I think it's acceptable to discuss this in the context of what sort of observation evidence would suggest a problem with GR, it's probably not acceptable to speculate here on what any alternative gravity theory might look like (unless you can provide links to appropriate peer-reviewed references).

 

[Bernie G]

You do make it difficult to keep a thread on topic, but I guess we can make a minor diversion here!

It was your broaching a 30 solar mass compact star on another thread that made me consider your idea with a twist ... a type of large compact star with a large enough radius that it would behave differently than a 2 solar mass neutron star. If it had (arbitrarily) about the same mass/radius as a neutron star GR factors shouldn't make it collapse. I will reword this and post it on the recent "Do Black Holes Exist" thread.

 

[Jonathan Scott]

It was your broaching a 30 solar mass compact star on another thread that made me consider your idea with a twist ... a type of large compact star with a large enough radius that it would behave differently than a 2 solar mass neutron star. If it had (arbitrarily) about the same mass/radius as a neutron star GR factors shouldn't make it collapse. I will reword this and post it on the recent "Do Black Holes Exist" thread.

Please don't! That thread has already strayed too far from its purpose.

 

[Jonathan Scott]

It was your broaching a 30 solar mass compact star on another thread that made me consider your idea with a twist ... a type of large compact star with a large enough radius that it would behave differently than a 2 solar mass neutron star. If it had (arbitrarily) about the same mass/radius as a neutron star GR factors shouldn't make it collapse. I will reword this and post it on the recent "Do Black Holes Exist" thread.

The purpose of this thread was to ask if anyone has any further information on a specific scientific possibility based on what I believe to be current mainstream physics, which if true would mean that certain compact stars with masses slightly exceeding current known masses of neutron stars might still be neutron stars even though they do not exhibit X-ray bursts, making it more difficult to establish the threshold for black hole formation.

If you want to discuss a different topic, please start a new thread, but note that Physics Forums is not a place to discuss personal theories or other vague handwaving ideas that are not based on mainstream physics.

 

[Bernie G]

A method of definitely distinguishing a neutron star from a possible stellar black hole is that it produces X-ray bursts, which have a sharp rise time and may last for an extended period. I had previously thought these occurred when hydrogen fell to the surface and was immediately fused to helium

Could X-ray bursts happen in the accreting matter above a neutron star?

 

[Jonathan Scott]

Could X-ray bursts happen in the accreting matter above a neutron star?

I'm certain that the density of matter in accretion flows would be far too low to support a sudden thermonuclear chain reaction of this type. Accreting material still emits X-rays from being heated by friction.

 

[Bernie G]

... the density of matter in accretion flows would be far too low to support a sudden thermonuclear chain reaction of this type...

The buildup of an ocean of hydrogen or helium on the star surface and then igniting makes less sense to me than fusion reactions first occurring above the star. The extremely hot ionized plasma above the surface is contained there for a long time. Why do we build Tokamaks? I think large fusion reactions or explosions would occur prior to or within 3 Schwarzschild radius and no hydrogen or helium would reach the surface. Just my opinion.

Maybe some gamma ray bursts are so large they can't be explained by just fusion reactions. If the energy released by these bursts is more than about 1% of of the mass accretion rate then more than just fusion is going on.

 

[Jonathan Scott]

You're doing it again! Your strange ideas are not the topic of this thread.

The difference in density between the accretion disk and the surface is many orders of magnitude, and material at the surface also has significantly more energy from gravity. Describing either hydrogen or helium on the surface as an "ocean" is not very appropriate as it would be a thin and extremely dense layer because of the gravitational field.

Fusion can occur in high temperature plasma (as it does in normal stars) but the rate is extremely slow compared with the X-ray bursts. Tokamaks need to squeeze plasma to extremely high densities to achieve even a brief flash of continuous fusion.

 

[Hornbein]

A method of definitely distinguishing a neutron star from a possible stellar black hole is that it produces X-ray bursts, which have a sharp rise time and may last for an extended period. I had previously thought these occurred when hydrogen fell to the surface and was immediately fused to helium, but I've now learned that bursts are thought to occur when an amount of helium has built up and undergoes further fusion in a chain reaction.
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. I don't know the details of the required energy, but it seems that this could mean that neutron stars above some mass threshold might not produce X-ray flashes, making it difficult to tell the difference from a black hole.
Is it known whether this burst suppression might actually occur within the range of masses expected for neutron stars, and if so can anyone point me to any further information on the subject?

There are neutron stars that have continuous fusion. But I don't recall where in Arxiv to find this, or anything more about it.

 

[Chronos]

Even black holes can emit xrays, so, it looks rather pointless to debate if a sufficiently massive neutron star might suppress xray emissions.

 

[Jonathan Scott]

Even black holes can emit xrays, so, it looks rather pointless to debate if a sufficiently massive neutron star might suppress xray emissions.

In this thread I'm specifically only interested in the very bright sudden X-ray flashes, followed by decay on a thermal profile, which are thought to occur when fusion occurs suddenly after an accumulation of material (presumed to be helium) has built up on a surface, providing evidence of the existence of such a surface.

 

[Bernie G]

Accreting material still emits X-rays from being heated by friction.

Shouldn't the radiation from the total of all fusion reactions be far smaller than the radiation from accreting material?

 

[Jonathan Scott]

Shouldn't the radiation from the total of all fusion reactions be far smaller than the radiation from accreting material?

Material falling to the surface is expected to emit X-rays from hitting the surface anyway, and continuous or frequent fusion would add a bit to that. I don't know how that compares with emission from the accretion flows, but I would have expected it to be comparable, although perhaps at a somewhat higher energy, and potentially showing variation at the pulsar frequency.

As far as I know, it is not easy to distinguish whether continuous X-rays are coming from the accretion disk or from the surface, but the bright X-ray bursts (thermonuclear flashes) are briefly much stronger and stand out.

 

[Bernie G]

Even black holes can emit xrays, so, it looks rather pointless to debate if a sufficiently massive neutron star might suppress xray emissions.

But his concept is about a particular type of X-ray emission being suppressed. Maybe "suppressed" is not the right word. The disturbing implications of his suggestion are that 5 or 10 solar mass "black holes" might not be black holes, and could a non-black hole compact star more massive than a conventional neutron star exist without collapsing. What could be its support mechanism?

 

[Jonathan Scott]

But his concept is about a particular type of X-ray emission being suppressed. Maybe "suppressed" is not the right word. The disturbing implications of his suggestion are that 5 or 10 solar mass "black holes" might not be black holes, and could a non-black hole compact star more massive than a conventional neutron star exist without collapsing. What could be its support mechanism?

That's an entirely different idea!

According to GR, there should be some threshold where neutron stars or their exotic relations collapse to black holes, probably somewhere in the range of 2 to 3 solar masses. My suggestion is that if that threshold is a bit higher than currently observed neutron star masses, but neutron stars above the currently observed maximum mass might not produce the bright flashes which provide clear evidence of a surface, then the absence of such flashes would not be a reliable way to distinguish such objects from black holes. Other evidence could still be used (for example pulsar emissions, or periodic X-ray fluctuations which appear to emanate from a surface).

 

[Bernie G]

Material falling to the surface is expected to emit X-rays from hitting the surface anyway, and continuous or frequent fusion would add a bit to that. I don't know how that compares with emission from the accretion flows, but I would have expected it to be comparable...

I like your original idea but don't see how X-ray energy from material impacting the surface plus X-ray energy from all fusion reactions could compare to the radiation from accreting material approaching a magnetized neutron star. The magnetic field prevents accreting material from impacting the surface with very high energy. Reasonable numbers for the X-ray energy from accreting material are 5% or 10% of the accreting material rest mass. Reasonable numbers for the X-ray energy released from material impacting the surface plus all fusion reactions should be under 0.1%. If a neutron star had gamma ray bursts comparable to the energy released by the accreting radiation, impacting material and fusion reactions could not explain it.

 

[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.