AXPs are "strange" cousins to normal pulsars, which flicker mostly in visible and radio light hundreds or thousands of times per second. Both types of objects are the corpses of stars that have collapsed under gravity to an ultra-dense sphere called a neutron star, only a few miles (kilometers) across. The new finding provides strong confirmation that the AXP is indeed a neutron star and has properties surprisingly similar to its "non-anomalous" cousins.

At a meeting of high-energy astrophysicists at NASAs Goddard Space Flight Center, Dr. Victoria M. Kaspi, assistant professor of physics in the Massachusetts Institute of Technology's Center for Space, told SPACE.com, "The bottom line is that we are surprised to find these strange pulsars have properties which make them common to normal neutron stars. This may eventually challenge conventional theories about the underlying structure of a neutron star."

"These anomalous X-ray stars still remain very elusive. Were really not sure what their nature is," she cautioned.

The starquake was discovered by Kaspi and co-investigators making observations with NASAs Rossi X-ray Timing Explorer (RXTE) satellite, which can measure rapid fluctuations in X-ray-emitting objects. Kaspi's group carefully monitored the spins of several AXPs in the hope of detecting a so-called spin-up event that would result from a starquake. After two years of waiting and watching, one of their targets, called 1RXS J1708-4009, suddenly started spinning faster as its ultra-dense crust shifted.

This "starquake" observed for the first time in an AXP allows for hidden structure to be probed much like the way geologists study Earths interior from earthquakes. Kaspi found that AXPs must be made like normal neutron stars, with a solid crust surrounding a rapidly rotating "fluid" core. The quake happened when the stars whirling core momentarily "decoupled" from the slower rotating outer crust, like disengaging a clutch in a car.

Scientists have long been puzzled as to why AXPs are so bright in X-rays, shining 100 to 1,000 times stronger than normal neutron stars. It has been suspected that the "engine" behind the X-ray beams is an enormous magnetic field generated by the spinning core, which makes the star radiate a torrent of X-rays. If this is true, then if an AXP strayed any closer to Earth than the moon, the force of its magnetic field -- which is a billion times stronger than the sun's -- would erase every cassette tape, ATM card and computer hard drive on our planet.

There are only five known AXPs in our galaxy. Many more of these objects may exist, but are hard to identify because they are isolated and emit only X-ray radiation. RXTE is one of only three or four satellites optimized for AXP hunts.