Chandra Discovers a Cosmic Cannonball
10.28.2007
"This isn't the first time astronomers have found million-mph stars. So-called "hypervelocity stars" have been previously discovered shooting out of the Milky Way with speeds around one million miles per hour. One key difference between RX J0822-4300 and these other reported galactic escapees is the source of their speed. Hypervelocity stars are thought to have been ejected by interactions with a supermassive black hole in the Galaxy's center, which can act as a sort of "gravitational slingshot." This neutron star, by contrast, was flung into motion by a supernova. Data suggest the explosion was lop-sided, kicking the neutron star in one direction and the debris from the explosion in the other.
"The breakneck speed of the Puppis A neutron star is not easily explained, however, by even the most sophisticated supernova explosion models. "The puzzle about this cosmic cannonball is how nature can make such a powerful cannon," says Winkler. "The velocity might be explained by an unusually energetic explosion," but researchers remain unsure."
http://science.nasa.gov/headlines/y2007/28nov_cosmiccannonball.htm?list885344

I have proposed an alternative explanation: planetary impacts after the star goes nova.