Feb. 5, 1999 CHAMPAIGN, Ill. -- A team of astronomers studying supernova remnants has found direct evidence linking the pattern of their X-ray emission to the size -- and therefore, age -- of the remnants.
"As supernova remnants grow in size, we discovered that the X-ray emission from the center of the remnant becomes brighter than that from the edge," said Rosa Williams, a graduate student in astronomy at the University of Illinois. "Our results provide new insight into the distribution of gas shocked by the supernova blast wave, and the nature of material between stars."
When a massive star nears the end of its life, the outward pressure of its thermonuclear reactions can no longer counter the inward pull of gravity. The star collapses in on itself and, within a fraction of a second, rebounds in a tremendous explosion called a supernova. Much of the star's material is flung outward, forming an expanding sphere of gas and dust called a supernova remnant. X-rays are produced by supernova remnants when the supersonically expanding shock hits the surrounding material.
"Since the shock front is roughly spherical, we normally expect the X-rays to form a spherical shell structure, like a soap bubble," Williams said. "The shell appears brightest at the edge, where our line of sight intersects the most X-ray-emitting material. While the shocked material in the interior of the shell can also produce X-rays, this gas is usually too diffuse to be detected."
Nonetheless, it has long been known that some supernova remnants show X-ray emission over the entire face of the remnant, sometimes even brighter toward the center, Williams said. "If this emission is indeed coming from inside the remnant, where the shock has already passed, then something must be raising the density of the material in the interior until the hot gas is dense enough, and therefore bright enough at X-ray wavelengths, to be seen by our instruments."
Using X-ray observations from the ROSAT satellite, Williams and her colleagues -- You-Hua Chu and John R. Dickel of the U. of I., Robert Petre of the Goddard Space Flight Center, R. Chris Smith of the Cerro-Tololo Inter-American Observatory, and Maritza Tavarez of the University of Michigan -- studied the structure and distribution of gas in supernova remnants within the Large Magellanic Cloud, a satellite galaxy of our own Milky Way. They classified the objects by their X-ray structures, and determined that these structures vary according to the remnant's size: Smaller remnants have shell-like X-ray emission, while larger remnants appear to have more X-ray emission from inside the shell.
"Because supernova remnants are continually expanding, a remnant becomes larger as it gets older," Williams said. "So it looks like what we're seeing is a change in the remnants as they age, with more and more of the X-ray emission coming from the inside of the remnants."
If this is indeed the case, said Williams, who presented the team's findings at the January meeting of the American Astronomical Society, theories to explain the presence of detectable X-ray emission from the interior of supernova remnants must also account for the age progression.
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