Astronomers Link Gamma-Ray Bursts, Supernovae
Harvard-Smithsonian Center for Astrophysics — Imagine a cosmic explosion so powerful, it blazes across the entire Universe. Such explosions exist in the form of gamma-ray bursts. Now, in the best tradition of the crime scene investigations (CSI), astronomers have assembled the clues pointing to the source of these gigantic blasts.
“There should no longer be doubt in anybody’s mind that gamma-ray bursts and supernovae are connected,” said Dr. Thomas Matheson of the Harvard-Smithsonian Center for Astrophysics (CfA), a member of the team that made this discovery.
The investigation began on March 29 when NASA’s High-Energy Transient Explorer satellite (HETE) discovered one of the brightest and closest gamma-ray bursts on record.
Located in the constellation Leo, the 30-second burst outshone the entire Universe in gamma rays, and its optical afterglow was still over a trillion times brighter than the Sun two hours later.
Through observations of that afterglow on subsequent nights, astronomers spotted the telltale signs of a supernova. The team cannot yet determine the timing of the burst relative to the supernova (whether one preceded the other or whether both began at the same time), but the same event – a star explosion – was certainly the trigger for both.
Gamma-ray bursts are incredibly bright flashes of high-energy radiation that likely signal the birth of black holes. Bursts occur at random locations scattered across the sky, and few last more than a minute, making them a challenge to study.
A supernova is the explosion of a star at least eight times as massive as the Sun. When such stars deplete their nuclear fuel, they no longer have the energy to support their mass. Their cores implode, forming either a neutron star or (if there is enough mass) a black hole.
Matheson’s colleagues on this find include Dr. Peter Garnavich of Notre Dame and Dr. Krzysztof Stanek of the CfA. The data were obtained by nearly two dozen scientists primarily using the 6.5-meter MMT telescope at Mount Hopkins, Arizona. When the burst was discovered, the astronomers scheduled to use the MMT agreed to observe it as a target of opportunity in collaboration with Garnavich, Matheson, and Stanek.
“For the first time, we were measuring an event no other human beings had seen before, ” said Stanek. “The MMT was our magic time machine that we used to capture this catastrophic cosmic event.”
Matheson and the others have detected direct evidence that the burst afterglow’s light is exhibiting the same patterns as light from a supernova. Namely, the scientists see changes in light absorbed by silicon and iron atoms (forged in the supernova) as the afterglow slowly fades away. The team is continuing to observe and analyze this unique burst.
Previous observations, particularly from NASA’s Chandra X-ray Observatory controlled at the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, have provided convincing indirect evidence of the gamma-ray burst/supernova connection. Chandra detected iron and other heavy elements, which are formed in supernovae, in the vicinity of gamma-ray bursts.
“All gamma-ray bursts may have associated supernovae that are too faint to observe,” Matheson said, “but this burst, named GRB 030329, was one of the closest known. We caught it in the act.” The burst was approximately two billion light-years from Earth, as opposed to other bursts located upwards of 10 billion light-years away. Because the burst was both close and bright, the supernova was detectable.
Garnavich added that material from the blast was ejected at such high speeds that the source of the gamma-ray burst might actually be a ‘hypernova’ – an explosion ten times more powerful than a typical supernova. “We’ve seen such hypernova explosions in nearby galaxies, although without an accompanying powerful gamma-ray burst, so we’ll be watching this distant explosion closely to see if it fits the hypernova profile,” said Garnavich.
The MMT is a joint venture of the Smithsonian Institution and the University of Arizona. The 6.5-meter-diameter optical telescope is located on the summit of Mt. Hopkins, the second-highest peak in the Santa Rita Range of the Coronado National Forest, approximately 30 miles south of Tucson, Arizona.
HETE was built by MIT as a mission of opportunity under the NASA Explorer Program, with collaboration among U.S. universities; Los Alamos National Laboratory; and scientists and organizations in Brazil, France, India, Italy and Japan.
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Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists organized into six research divisions study the origin, evolution, and ultimate fate of the universe.
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