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The Question

(Submitted July 16, 1997)

I have read the article on the X-ray emissions from comet Hyakutake. Your hypothesis on the water cloud around the nucleus is interesting but did you analyze the same activities on comet Hale-Bopp? If so what are other hypothesis or conclusions?

The Answer

Last May in Baltimore, Maryland, USA we had a meeting of all the people interested in the cometary X-ray emission problem. the first time all of us had gotten together since the discovery in 1996. We now have detections of X-rays from some 8 comets, and all bright, nearby comets seem to emit X-rays! (Including Hale-Bopp, although it was much fainter than we expected in the X-ray for such an optically bright and productive comet. It has been proposed that the extremely large amount of dust emitted by the comet, as compared to other comets, may be somehow damping the X-ray emission.)

At the meeting, it became apparent that 3 mechanisms are possible causes of the emission, in oder of likeliness: charge exchange between solar wind heavy ions and cometary neutrals, bremsstrahlung emission, and magnetic field recombination. All of these mechanisms involve interactions between the solar wind and the comet's extended atmosphere and ionosphere.

It is clear that we need more observations to figure out exactly what is going on, though! But it does seem that we will be able to use the X-rays to probe the nature of the solar wind and magnetic field throughout the solar system.

Hope this helps!

Casey Lisse
for Ask an Astrophysicist

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