NASA's Swift Catches an Anti-glitch from a Neutron Star 

Using observations by NASA's Swift satellite, an international team of astronomers has identified an abrupt slowdown in the rotation of a neutron star. The discovery holds important clues for understanding some of the densest matter in the universe.

While astronomers have witnessed hundreds of events, called glitches, associated with sudden increases in the spin of neutron stars, the sudden spin-down caught them off guard. 

A neutron star is the crushed core of a massive star that ran out of fuel, collapsed under its own weight, and exploded as a supernova. It's the closest thing to a black hole that astronomers can observe directly, compressing half a million times Earth's mass into a ball roughly the size of Manhattan Island. Matter within a neutron star is so dense that a teaspoonful would weigh about a billion tons on Earth. 

Neutron stars possess two other important traits. They spin rapidly, ranging from a few rpm to as many as 43,000, comparable to the blades of a kitchen blender, and they boast magnetic fields a trillion times stronger than Earth's. 

About two dozen neutron stars occasionally produce high-energy explosions that astronomers say require magnetic fields thousands of times stronger than expected. These exceptional objects, called magnetars, are routinely monitored by a McGill team led by Kaspi using Swift's X-Ray Telescope.

From July 2011 to mid-April 2012, the team observed regular X-ray pulses from a magnetar designated 1E 2259+586. The object spins once every seven seconds, or about eight rpm, and observations indicated that it was slowing down at a gradual and stable rate. 

But data acquired at the next scheduled observation, on April 28, showed the star's spin had abruptly slowed by 2.2 millionths of a second, a change of about one-third of a part per million. In addition, the magnetar was found to be spinning down at a much faster rate.

A report on the findings appears in the May 30 edition of the journal Nature.

Astronomers dub this event an anti-glitch because it affected the magnetar in exactly the opposite manner of every other clearly identified glitch seen in neutron stars. 

On April 21, just a week before Swift observed the anti-glitch, the star produced a brief but intense X-ray burst detected by the Gamma-ray Burst Monitor aboard NASA's Fermi Gamma-ray Space Telescope. The scientists think this 36-millisecond eruption of high-energy light likely signaled the changes that drove the magnetar's slowdown. 

The discovery has important implications for understanding the extreme physical conditions present within neutron stars, where matter becomes squeezed to densities several times greater than an atomic nucleus. No laboratory on Earth can reach these conditions. 

While the internal structure of neutron stars is a long-standing puzzle, current theories envision a crust of electrons and ions above an interior containing, among other oddities, a neutron superfluid, which is a bizarre, friction-free state of matter. 

Because the surface of a neutron star accelerates streams of high-energy particles through its intense magnetic field, the crust is always losing energy and slowing down. The fluid interior of the neutron star resists being slowed, so the crust may fracture under the strain, producing an X-ray outburst while also receiving a speedup kick from the faster-spinning interior, producing a conventional glitch.   

But processes that suddenly slow the star's rotation constitute a new theoretical challenge. 

The event's most remarkable aspect is the combination of the abrupt slowdown, the X-ray outburst, and the fact that astronomers now observe the star spinning down at a faster rate than before.

Some properties of conventional glitches have proven puzzling, the researchers note, and they express the hope that the discovery of the anti-glitch phenomenon will initiate renewed progress in understanding neutron star interiors.

This goal is also the main focus of a new NASA Explorer mission recently selected for launch in 2017. The Neutron-star Interior Composition Explorer, or NICER, will be mounted on the International Space Station to search for X-ray outbursts, glitches, anti-glitches and other phenomena that will help theorists produce better models of the exotic interiors of neutron stars. Kaspi is a member of the NICER science team.  

Discovered in 1981 by NASA's Einstein satellite, 1E 2259+586 lies about 10,000 light-years away toward the constellation Cassiopeia. The Swift monitoring study continues a similar program performed by NASA's Rossi X-ray Timing Explorer, which kept an eye on the magnetar for 16 years before being decommissioned in early 2012.