Explosive aftermath: sluggish neutron star puzzles astronomers.

Even if astronomers don't quite know how stars blow up, they thought that they at least understood what those stellar explosions leave behind. But an X-ray-emitting object at the heart of a young supernova remnant called RCW 103 doesn't fit the textbook view.

Its slow rotation, as well as an outburst that has yet to completely fade after 6 years, make this remnant "absolutely unique among this type of object," says Gordon Garmire of Pennsylvania State University in University Park. Garmire and his colleagues, as well as a team in Italy, report some unexpected properties of the remnant.

When a star heavier than eight times the sun's mass explodes, its core collapses to form a neutron star or a black hole. The star then hurls its outer layers into space, creating a bubble of glowing gas.

Close X-ray scrutiny of the neutron star inside RCW 103 belies that simple picture. Two teams--one led by Garmire using NASA's Chandra X-ray Observatory and the other led by Andrea De Luca of the National Institute of Astrophysics in Milan using the European Space Agency's XMM-Newton satellite--have found that X rays emitted by the star wax and wane every 6.7 hours. De Luca and his colleagues describe their study in an upcoming Science. Observations by Garmire's team, to be reported July 20 at a meeting of the Committee on Space Research in Beijing, indicate that the 6.7-hour period is the rate at which the neutron star spins.

That's a puzzle, says De Luca, because a 2,000-year-old neutron star born in isolation ought to be spinning thousands of times faster.

Just as curiously, earlier observations by Garmire and his colleagues showed that between October 1999 and January 2000, the object became 50 times brighter. Two years later, it was radiating at half that brightness.

The data suggest that the neutron star has a low-mass companion in an elongated orbit, Garmire and his colleagues propose. Whenever the companion star comes close to the neutron star, it feeds a disk of material surrounding the compact body and creates an outburst like the one seen 6 years ago, he asserts. By exerting a drag on the neutron star's magnetic field, the companion also slows the rotation.

One problem with that scenario, both Garmire and De Luca acknowledge, is that a partner often gets such a strong kick from a supernova that it escapes. But as Garmire's team envisions it, the companion closely orbited the massive star before it went supernova--so closely that it grazed the heavy star's atmosphere. This would have triggered shock waves that drove off much of the star's mass before it ever exploded. In that case, the kick from the supernova wouldn't be large enough to split the pair.

It's still possible that a second explanation suggested by both teams could account for the neutron star's properties. In that scenario, the neutron star would have been born solo but highly magnetized. The interaction of this huge magnetic field with a disk of debris from the supernova explosion would have acted as a brake, slowing the neutron star's spin.

But the data seem to favor a companion, says Garmire. That's important, he says, because half the stars in the universe probably have partners.