Mel linked to a video from Harvard (of all places) that contained the 'official' description of this celestial firecracker:
http://chandra.harvard.edu/photo/2009/casa/
(Highlight added by me)This slower velocity is explained by a special type of energy loss by the blast wave. Electrons are accelerated to high energies as they travel backwards and forwards across the shock front produced by the blast wave. As the electrons travel around magnetic fields in the shock they lose energy by producing synchrotron emission and glowing in X-rays. Scientists think heavier particles like protons and ions are accelerated in the same way. The energy lost by these heavier particles can amount to a large fraction of the energy from the supernova explosion, resulting in a slower shock velocity.
Blinded by the science of it all... the last paragraph reads:
"Magnetic fields" is a link in that last paragraph, and (rather hilariously I think) it links you to:The rapid flickering not only gives information about acceleration of particles to high energies, but it also shows that relatively strong magnetic fields have been generated in the shock front.
http://chandra.harvard.edu/resources/glossaryM.html
(Highlight added by me)magnetic field Field which accompanies any electric current or changing electric field, and governs the influence of magnetized objects on one another.
Moral of the Story: Don't send your kids to Harvard!
Mike H.