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

[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.