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

1. Jan 3, 2016

### 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?

2. Jan 4, 2016

### 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!

3. Jan 4, 2016

### Bernie G

That makes sense and fusion reactions at the poles might cause feeble jets. Accreting hydrogen and helium on a neutron star ..... lets 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.

4. Jan 4, 2016

5. Jan 4, 2016

### Bernie G

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.

6. Jan 4, 2016

7. Jan 4, 2016

### Jonathan Scott

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.

8. Jan 4, 2016

### Bernie G

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.

9. Jan 4, 2016

### 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?

10. Jan 5, 2016

### Jonathan Scott

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

11. Jan 5, 2016

### Chronos

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

12. Jan 5, 2016

### Jonathan Scott

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.

13. Jan 5, 2016

### Chronos

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

14. Jan 6, 2016

### Bernie G

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?

15. Jan 6, 2016

### Jonathan Scott

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).

16. Jan 6, 2016

### Bernie G

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.

17. Jan 6, 2016

### Jonathan Scott

18. Jan 6, 2016

### Jonathan Scott

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.

19. Jan 6, 2016

### Bernie G

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

20. Jan 6, 2016

### Jonathan Scott

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.