July 21, neutron stars

Comments and questions about the APOD on the main view screen.
Odegard
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July 21, neutron stars

Post by Odegard » Thu Jul 21, 2005 6:11 pm

On todays APOD and in this pressrelease, it is said that the accretion of matter on a neutron star spins it up. Can this be correct?

An accretion disk lights up due to internal friction forces .The inner ring spins faster than the outer ring. The outer ring slowes the inner ring, creating the friction which radiates (often) in X-rays.

My question is, how can this spin the neutron star up? I'd say the star would indeed slow down.

Any comments?
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makc
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Post by makc » Fri Jul 22, 2005 7:24 am

according to Kerr solution, the rotation is sort of "illusion" caused by space/time curvature. Matter around is in its free fall - it's falling down, but due to curvature when we "map" it back to flat space it seems to rotate with a star. Sure, matter can truly rotate around a star, but you wouldn't be able to separate two effects.

Odegard
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Post by Odegard » Fri Jul 22, 2005 7:56 am

Roy Kerr solved Einsteins equations to find that objects influence their surroundings, called frame dragging.

Frame dragging was indeed observed not long ago, however, my question has nothing to do with frame dragging.

There is no doubt that material in an accretion disk spirals inwards. It's the only way to explain the massive radiation observed from objects with accretion disks (mini-quasars, black holes and more).

We can look at it in a stricly mechanical way. A body is rotating. Matter falls into the object. The rotation will slow down. It's a textbook example explaining the conservation of angular momentum (think ice skater pulling his or her arms to the body to increase rotation speed).

Thank you for your reply. I hope we can get to the bottom of this.
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makc
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offtopic:

Post by makc » Fri Jul 22, 2005 9:00 am

just seen that "maintainer of mirror". wondering, how many MBs are all APODs together?

Odegard
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Post by Odegard » Fri Jul 22, 2005 9:05 am

Hehe, I guess it's rather big. I only started two months ago but I enjoy free storage and bandwidth (atleast for the time being).

APOD just celebrated their 10th year online. Images vary from a few kilobytes to several megabytes. Also, the main site stores two versions of the images. Anyway, let's say each APOD weighs in, on average, 300 kilobytes (remember, images from 5-10 years back weren't that big). 10 years is >3500 APODs totalling just over one gigabyte.

My guess is that the whole archive occupies between one and a few gigabytes of diskspace.
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dlw
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Accretion-powered spin?

Post by dlw » Fri Jul 22, 2005 4:57 pm

I too wonder about this assumption. I read the link from the APOD description but it didn't explain the physics of this.

Conservation of momentum would require that accretion -decrease-, as opposed to increase, spin. However, I suppose that if the accretion resulted in a tangentially directed release of nuclear energy (fusion?) then perhaps that might increase the spin.

Is there a reasonable cogent explanation of "accretion-powered spin" somewhere on the web?

Odegard
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Post by Odegard » Fri Jul 22, 2005 6:03 pm

Well, I finally did what I should've done from the beginning - read the paper.

It says in the introduction already that neutron stars not only accrete matter but also angular momentum from the companion star. There is no explanation of this in the paper, but they referred to another paper by D. Bhattacharyaa and E. P. J. van den Heuvelc. The paper is published here. Sadly, I have no affiliation with any institution with access to this paper at the moment, so I can't read it. Perhaps someone else can?
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