Gamma-ray bursts: a distant stretch?Like firecrackers exploding in the night sky, gamma-ray bursts unleash a torrent of high-energy photons before fizzling out hundredths to tens of seconds later. These flashes of radiation rank among the most mysterious phenomena in the universe: No one has found the sources of the bursts, and it's uncertain whether the flashes originate within our galaxy or far beyond. A new analysis of bursts detected by the Compton Gamma Ray Observatory Compton Gamma Ray Observatory Space observatory in service from 1991 to 2000 that was designed to identify the sources of celestial gamma rays. It was named after physicist Arthur Holly Compton. (GRO GRO Guerrero (Estado de México) GRO General Register Office (UK) GRO Greater Research Opportunities GRO Gamma Ray Observatory GRO Growth-Related Oncogene GRO Greensboro, North Carolina ) spacecraft adds to the evidence that the flashes originate billions of light-years beyond the Milky Way. The finding suggests that the bursts serve as probes of the distant cosmos and bear the imprint of the expanding universe, astronomers reported last month at a meeting of the American Astronomical Society The American Astronomical Society (AAS, sometimes pronounced "double-A-S") is a US society of professional astronomers and other interested individuals, headquartered in Washington, DC. in Arlington, Va. GRO findings first made headlines in 1991, when a set of on-board detectors revealed that gamma-ray bursts are distributed uniformly across the sky (SN: 9/28/91, p.196). Sources in a giant, as yet unseen halo surrounding our galaxy might produce the uniform sprinkling. Alternatively, bursts scattered throughout the cosmos could account for the distribution. If the flashes do come from far away, then the most distant ones should last longer than those emitted closer to our galaxy, Bohdan Paczynskl of Princeton University and Tsvi Piran of the Hebrew University of Jerusalem Hebrew University of Jerusalem Independent university in Jerusalem, Israel, founded in 1925. The foremost university in Israel, it attracts many Jewish students from abroad; Arab students also attend. independently predicted in 1992. That stretching effect, known as time dilation, arises as a consequence of the expansion of the universe. Objects near the edge of the observable cosmos recede faster from Earth than objects nearby. Thus, according to Einstein's theory of relativity theory of relativity Einstein’s contribution to the space-time relationship. [Science: NCE, 843–844] See : Turning Point , observers should find that the more distant bursts last longer. These bursts will also be shifted to longer, or redder, wavelengths, a phenomenon called redshift redshift Displacement of the spectrum of an astronomical object toward longer wavelengths (visible light shifts toward the red end of the spectrum). In 1929 Edwin Hubble reported that distant galaxies had redshifts proportionate to their distances (see . In analyzing more than 700 bursts detected by GRO, astronomers now report that 60 relatively dim flashes last about twice as long, on average, as 46 of the brightest. Dim bursts also appear redshifted, says study collaborator Robert J. Nemiroff of George Mason University Named after American revolutionary, patriot and founding father George Mason, the university was founded as a branch of the University of Virginia in 1957 and became an independent institution in 1972. in Fairfax, Va. If one assumes the bursts appear faint only because they lie farther from Earth, then the findings support the notion of an expanding universe and an origin for the dimmest GRO bursts several billion light-years beyond our galaxy, Nemiroff asserts. Team leader Jay P. Norris of NASA's Goddard Space Flight Center The Goddard Space Flight Center (GSFC) is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately 6.5 miles northeast of Washington, D.C. in Greenbelt, Md., cautions that the findings do not prove that bursts lie outside our galaxy, but they show time dilation "does exist and must now be accounted for by any theory." This time dilation is the first found for any celestial radiation source, Paczynski adds. Because bursts vary widely in shape and duration, the researchers couldn't directly compare individual flashes. Instead, they statistically analyzed groups of dim and bright bursts. In contrast, J. Patrick Lestrade of Mississippi State University Mississippi State University, at Mississippi State, near Starkville; land-grant and state supported; coeducational; chartered 1878 as an agricultural and mechanical college, opened 1880. From 1932 to 1958 it was known as Mississippi State College. in Starkville and his cob leagues reported last year that they had seen direct hints of time dilation among a small group of 20 bursts detected by GRANAT, a Soviet-French satellite. Paczynskl says the new results, combined with the bursts' uniform distribution, offer compelling evidence that the flashes are extragalactic ex·tra·ga·lac·tic adj. Located or originating beyond the Milky Way. Adj. 1. extragalactic - outside or beyond a galaxy; "extragalactic nebula" . But Stirling A. Colgate of the Los Alamos (N.M.) National Laboratory asserts that until astronomers observe the bursts at other wavelengths, it's too soon to make such pronouncements. |
|
||||||||||||||||||
Printer friendly
Cite/link
Email
Feedback
Reader Opinion