Swift's X-Ray Telescope (XRT) captured an apparent expanding halo around the flaring neutron star SGR J1550-5418. The halo formed as X-rays from the brightest flares scattered off of intervening dust clouds. Credit: NASA/Swift/Jules Halpern (Columbia Univ.)
In October 2008, the remnants of a collapsed star in the southern constellation Norma roared to life with a burst of gamma radiation. After a brief hiatus it reawakened January 22, sending out blasts of gamma rays powerful enough to fuel a lingering X-ray afterglow. The star was recently renamed SGR 1550-5418 for its new status as the sixth known soft gamma-ray repeater, a source of celestial fireworks thought to be powered by the extreme magnetic field of an unusual neutron star.
SGR 1550-5418's gamma flares traveled across 30,000 light years before they were captured by the Gamma Burst Monitor onboard the Fermi Gamma-ray Space Telescope, a joint collaboration of NASA, DOE, and international partners, in orbit above the Earth. The Swift satellite, also a NASA project, captured the X-rays that followed.
"At times, this remarkable object has erupted with more than a hundred flares in as little as 20 minutes," says Loredana Vetere, who is coordinating the Swift observations at Pennsylvania State University. "The most intense flares emitted more total energy than the sun does in 20 years."
Astronomers theorize that such immense energy releases slow the star's rotation from many per second to one turn in 10 seconds, in as little as 10,000 years. With a period of 2.07 seconds, the new SGR has the fastest known rotation of its class.
SGR 1550-5418 is the newest in a class of astronomical objects prone to irregular outbursts of gamma radiation-the most energetic light in the electromagnetic spectrum. Soft gamma-ray repeaters emit unpredictable, single or repeating bursts at the less-powerful, "soft" end of the gamma range. These outbursts pack enough punch to ionize parts of the Earth's upper atmosphere from 50,000 light years away. SGR 1550-5418's latest gamma activity was followed by a longer-lasting emission of X-ray light that shone in halos around its source as it scattered though interstellar dust.
The intense radiation is thought to result from the violent starquakes of a magnetar, a neutron star with a magnetic field so powerful it causes the star's outer crust to buckle and crack, releasing huge storms of energy in the form of gamma rays. That in turn heats the surrounding matter, which releases the pent-up energy in an X-ray afterglow.
"There is a bright flash of gamma rays that lasts typically less than a second," says Columbia University professor Jules Halpern, who used data from Swift to trace the new SGR's X-ray halos. "That will heat the surface and the magnetosphere of the star, which will emit in X-rays for several hours or days-it's a combination of the heat from the surface and probably a fireball of even hotter electrons in the magnetosphere of the neutron star."
SGR 1550-5418 was first spotted in 2007 and cataloged as another class of neutron star-an anomalous X-ray pulsar, or AXP. "There are two classes of neutron stars that have very strong magnetic fields, SGRs and AXPs," Halpern says, "Normally they're distinct; only the SGRs have the most powerful outbursts." SGR 1550-5418 is now elevated to the status of an SGR because of its energetic outbursts. Its change in behavior could theoretically be part of the evolution of the star, he notes, "but the interval between bursts could be decades." It's been only 30 years since the first SGR was discovered, so only time will tell.
