Growing pains.
This artist's conception shows a neutron star known as a magnetar crackling with extremely powerful magnetic activity.

"When you hear hoofbeats," the old saying goes, "think horse, not zebra." But what if your horse suddenly grows zebra stripes? That's the predicament astronomers faced when a star they were observing--a rapidly spinning remnant of a supernova called a pulsar--started emitting powerful bursts of x-rays considered the hallmark of a much-rarer object called a magnetar. The finding strongly suggests that pulsars, also known as neutron stars, and magnetars are linked and paves the way for a better understanding of stellar evolution.
Pulsars are the dense cores left over after stars of a certain mass explode into supernovae. Weighing as much or more than the sun but only as big as asteroids, they can rotate tens or even hundreds of times a second (versus once a day for Earth). Sky surveys have identified about 1800 pulsars within the Milky Way, most of which emit pulsing radio signals that rise and fall as the pulsars spin.

The stripe-changing pulsar, named PSR J1846-0258, lies about 20,000 light-years away in the constellation Aquila. A team of researchers from NASA and elsewhere was observing it using the Rossi X-ray Timing Explorer (RXTE) spacecraft when the star suddenly erupted in a blast of x-rays. The display, reported online today in Science, made PSR J1846-0258 a candidate for being a magnetar--a type of neutron star with an enormously powerful magnetic field. Magnetars, so rare that only a dozen or so have been discovered, routinely emit high-energy x-rays and even gamma rays. But no one had ever observed a pulsar emitting such bursts.

"The bursts were completely unexpected," says astrophysicist and lead author Fotis Gavriil of NASA's Goddard Space Flight Center in Greenbelt, Maryland. Because PSR J1846-0258 is a very young pulsar (a mere 1000 years old) and because its magnetic field strength is considerably lower than those from bona fide magnetars, Gavriil says, the researchers suspect it is still evolving. He says the discovery raises important questions about the two types of stars: Do pulsars behave like magnetars only periodically and then revert? Did all magnetars originate as pulsars? "We really need to follow this source, and others like it, to answer these questions," he says.

Astrophysicist Duncan Lorimer of West Virginia University in Morgantown calls the discovery "fantastic." A decade ago, he says, very little was known about any connections between pulsars and magnetars. Now, Lorimer says, the evolutionary connections between the two are strengthening, and observations like this one will help "elucidate our understanding of what happens to a young neutron star after its birth in a supernova." And astrophysicist Robert Duncan of the University of Texas, Austin, calls the findings "fascinating and important," because they represent the first time that magnetically generated x-rays have been seen coming from a rotationally driven pulsar. Duncan, who developed the theoretical behavior of magnetars in 1992, says he is not so sure the object will turn out to be a magnetar, but "neutron stars are constantly surprising scientists, so future observations … will certainly be interesting."