New observations have helped astronomers identify the source of a 2,000-year-old supernova explosion. But this blast, and the neutron star that created it, seem to have a curious history that defies explanation.

Scientists discovered remnants of the supernova dubbed E0102 in the 1980s, located in the Small Magellanic Cloud more than 200,000 light years from Earth. Now, data from the Chandra X-ray Observatory and the European Southern Observatory's Very Large Telescope (VLT) in Chile have enabled astronomers to confirm the presence of a neutron star inside its own visible-light-emitting wreath of gas.

This neutron star doesn’t have a stellar companion. Even more intriguingly, the crushed stellar remnant doesn’t give off radio pulsations or high-energy X-rays, which means it likely has a weak magnetic field, making it the only neutron star with these characteristics to be discovered outside the Milky Way so far.

The neutron star is off-center with respect to the giant X-ray wave that surrounds it, sent propagating outward through space by the supernova explosion. Astronomers have long understood that supernova blasts can be asymmetric. “In such cases, the neutron star can get ‘kicked’ in the opposite direction,” says Frédéric Vogt (European Southern Observatory, Chile). In E0102, such a kick could have displaced the neutron star from its central position.

“It’s a bit like fireworks,” Vogt says. “If you take one of those big exploding ones, and place it on the floor, when the black powder explodes, the surrounding cardboard flies in the opposite direction from the blow out.”

But when Vogt and colleagues took a closer look at the remnant in visible light using the VLT, they saw an off-center ring of visible-light-emitting gas. That prompted another look at numerous X-ray observations of E0102, totaling almost four days’ worth of exposure time, which had previously revealed an X-ray source at the center of the red ring. Vogt and colleagues argue in the April 2nd online issue of Nature Astronomy (read the article preprint here) that this source is in fact the neutron star that was created in the supernova explosion.

If the neutron star had been kicked out of its central position in the supernova remnant, then where did the red ring of gas come from? The neutron star’s gravitational field cannot account for its bull’s-eye location — its gravity is too weak to hold on to the gaseous debris, which has been propagating outward and is already 3 light-years away.

Another possible explanation is that the ring has been moving along with the neutron star since the supernova. However, such a scenario is extremely unlikely, as it would require the ring and the neutron star to be moving at the same speed and in the same direction over the course of millennia.

“It’s hard to explain the location and nature of this ring of gas if the neutron star is moving through space at several hundred kilometer per seconds after being kicked during the supernova explosion,” says Vogt.

A more likely scenario is that the neutron star is now where the explosion happened; VLT data show that neither it nor the red ring has moved sideways significantly; the gas inside the red ring of emission is simply expanding radially over time. This explanation, however, would not account for the neutron star’s off-center location in the X-ray ring.

The problem can be resolved by figuring out where the explosion actually happened. In order to do this, scientists would have to play the explosion backwards. Another team is doing so, using archived images from the Hubble Space Telescope to track the motion of the debris over time. Researchers are also collecting new data that will enable them to figure out the exact location of the supernova. If it turns out that the supernova took place at the neutron star’s current location, the scientists will have the key to solving the puzzle.