NASA's Chandra Discovers Exotic Neutron Stars: Magnetars Not so Rare

Magnetars are some of the most extreme objects known in the universe. The dense remains of dead stars that erupt sporadically with bursts of high-energy radiation, magnetars were once thought to be relatively rare. Now, though, NASA's Chandra X-ray Observatory and other satellites have revealed that magnetars may be far more diverse and common than previously believed.

Magnetars form when a massive star runs out of fuel. When this happens, the star's core collapses and forms a neutron star, an ultradense object about 10 to 15 miles wide. The gravitational energy released in this process blows the outer layers away in a supernova explosion and leaves the neutron star behind.

Most of these neutron stars spin rapidly--a few times a second. However, a small fraction of these stars have a very low spin rate of once every few seconds. These slow-moving neutron stars are dubbed "magnetars."

Most magnetars have extremely high magnetic fields on their surface that are ten to a thousand times stronger than the ones found on an average neutron star. Now, new research has revealed that one particular magnetar named SGR 0418+5729 (SGR 0418 for short) has a magnetic field that's more like an average neutron star.

During the course of their observations, astronomers measured how the magnetar's rotation speed changed during an X-ray outburst. These outbursts are likely caused by fractures in the crust of the neutron star precipitated by the buildup of stress in a relatively strong, would-up magnetic field lurking just beneath the surface.

"This low surface magnetic field makes this object an anomaly among anomalies," said co-author GianLuca Israel of the National Institute of Astrophysics in Rome in a news release. "A magnetar is different from typical neutron stars, but SGR 0418 is different from other magnetars as well."

After examining the unusual magnetar, the researchers then modeled the evolution of the cooling of the neutron star and its crust, as well as the gradual decay of its magnetic field. The scientists found that the magnetar is about 550,000 years old, which makes it older than most other magnetars. In fact, it's very possible that its age explains why the surface magnetic field strength has declined.

The findings suggest that there may be many more elderly magnetars with strong magnetic fields hidden under the surface, which means that their birth rate could be as much as five to ten times higher as previously thought. The study also has huge implications for further research on these neutron stars, and may even reveal a bit more about the origin of gamma-ray bursts.

The findings will be published in The Astrophysical Journal.