Strange ring found circling dead star

The paper entitled An infrared ring around the magnetar SGR 1900+14 relates to the stellar corpse, called SGR 1900+14, which belongs to a class of objects known as magnetars. These are extremely dense collapsed cores of massive stars that blew up in supernova explosions, but unlike other dead stars, they slowly pulsate with X-rays and have tremendously strong magnetic fields.

The stellar corpse, called SGR 1900+14, belongs to a class of objects known as magnetars. These are the cores of massive stars that blew up in supernova explosions, but unlike other dead stars, they slowly pulsate with X-rays and have tremendously strong magnetic fields.

“The universe is a big place and weird things can happen,” said Stefanie Wachter of NASA's Spitzer Science Center at the California Institute of Technology, Pasadena, who found the ring serendipitously. “I was flipping through archived Spitzer data of the object, and that's when I noticed it was surrounded by a ring we'd never seen before.” Wachter is lead author of a paper about the findings in this week’s issue of Nature. You can see the ring at http://www.nasa.gov/mission_pages/spitzer/multimedia/20080528.html .

Wachter and her colleagues think that the ring, which is unlike anything ever seen before, formed in 1998 when the magnetar erupted in a giant flare. They believe the crusty surface of the magnetar cracked, sending out a flare, or blast of energy, that excavated a nearby cloud of dust, leaving an outer, dusty ring. This ring is oblong, with dimensions of about seven by three light-years. It appears to be flat, or two-dimensional, but the scientists said they can't rule out the possibility of a three-dimensional shell.

The discovery could help scientists figure out if a star's mass influences whether it becomes a magnetar when it dies. Though scientists know that stars above a certain mass will “go supernova,” they do not know if mass plays a role in determining whether the star becomes a magnetar or a run-of-the-mill dead star. According to the science team, the ring demonstrates that SGR 1900+14 belongs to a nearby cluster of young, massive stars. By studying the masses of these nearby stars, the scientists might learn the approximate mass of the original star that exploded and became SGR 1900+14.

Dr Jonathan Granot of the University of Hertfordshire’s Centre for Astrophysics Research (http://star.herts.ac.uk/) said: “The shape and size of the dust-free cavity surrounding the magnetar provide unique and valuable information on the activity history of its giant-flares.These are rare events, where only three such events have ever been recorded from all known SGRs, and only one of them from SGR1900+14. The fact that the ring is elongated (with an axis ratio of about 2:1) implies that the giant flare was anisotropic – brighter in some directions relative to others (by a factor of several).”