BY DR EMILY BALDWIN
Posted: 12 February, 2009
Astronomers using NASA's Swift satellite and the Fermi Gamma-ray Space Telescope have observed high-energy flares emanating from a neutron star 30,000 light years away.
One theory for the formation of gamma ray flares suggests that they arise when the magnetar’s surface suddenly cracks, releasgin energy stored within its powerful magnetic field. Image: NASA/Goddard Space Flight Center Conceptual Image Lab.
Neutron stars are the dense, dead remnants of massive stars, which cram more mass than the Sun into a core just a few tens of kilometres across. They spin rapidly, unleashing X-ray and gamma ray flares in an unpredictable fashion.
SGR J1550-5418 has long been known as an X-ray source, and during the past two years, astronomers have picked up pulsating radio and X-ray signals from the object. In October 2008 it produced a series of modest eruptions then settled down before roaring back to life on 22 January 2009 with an intense episode.
"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."
SGR J1550-5418 is thus classified as a rare type of neutron star known as a soft gamma ray repeater, which unpredictably sends out a series of X-ray and gamma ray flares. It lies in the southern constellation Norma and is only the sixth soft gamma ray repeater known.
Swift 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 intervening dust clouds. Image: NASA/Swift/Jules Halpem (Columbia University).
Neutron stars have intense magnetic fields and can be further sub-divided into a group known as magnetars, which posses fields 1,000 times stronger than ‘normal’ neutron stars. SGR J1550-5418 falls into this category, and rotates once every 2.07 seconds, therefore giving it the title as the fastest-spinning magnetar currently known.
"The ability of Fermi's gamma-ray burst monitor to resolve the fine structure within these events will help us better understand how magnetars unleash their energy," says Chryssa Kouveliotou of NASA's Marshall Space Flight Center. The object has triggered the gamma ray burst monitor more than 95 times since 22 January, and astronomers believe that these flares are powered by the magnetar tapping into the tremendous energy of their magnetic fields.
The Swift's X-ray telescope has also shed new light on the object, capturing the first ‘light echoes’ ever seen from a soft-gamma-ray repeater. Images acquired when the latest flaring episode began show what appear to be expanding halos around the source. Multiple rings form as X-rays interact with dust clouds at different distances, with closer clouds producing larger rings. But both the rings and their apparent expansion are an illusion caused by the finite speed of light and the longer path the scattered light must travel.
"X-rays from the brightest bursts scatter off of dust clouds between us and the star," says Jules Halpern of Columbia University and who captured the light echo images. "As a result, we don't really know the distance to this object as well as we would like. These images will help us make a more precise measurement and also determine the distance to the dust clouds."
NASA's Wind satellite, the joint NASA-Japan Suzaku mission, and the European Space Agency's INTEGRAL satellite have also detected flares from SGR J1550-5418.
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