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Researchers anticipate results on rare gamma rays

University Of Chicago : 02 July, 2007  (Technical Article)
Black holes, remnants of exploded stars and other exotic celestial objects emit streams of gamma rays that carry trillions of times more energy across the universe than visible light. Scott Wakely considers gamma rays the juiciest portion of the electromagnetic spectrum.
“They are really rare and unusual things,” said Wakely, a Research Scientist in the Enrico Fermi Institute.

Since 1989, astronomers have identified approximately a dozen sources of high-energy gamma rays in the sky. That number will rise dramatically in years ahead now that construction has begun on VERITAS, or the Very Energetic Radiation Imaging Telescope Array System, on Kitt Peak in southern Arizona.

Chicago scientists assembled much of the prototype for the array at the Enrico Fermi Institute. They continue to play a key role in the $13.1 million project, which is funded by the U.S. Department of Energy and the National Science Foundation.

The international VERITAS collaboration consists of 10 institutions, including the Smithsonian Astrophysical Observatory, Iowa State University, Purdue University, the University of California at Los Angeles, University of Utah and Washington University in St. Louis. Foreign members are McGill University in Canada, the National University of Ireland, University College, Dublin, and the University of Leeds in the United Kingdom.

“We hosted what was probably the first collaboration meeting here in 1997, and at that time it was just a dream,” said Simon Swordy, Professor in Physics and the College. Chicago members of the VERITAS collaboration include Swordy, Wakely and Lucy Fortson, a Senior Research Associate in Astronomy & Astrophysics and Director of Astronomy at the Adler Planetarium and Astronomy Museum.

Wakely noted that gamma-ray astronomy is a young field. “Over the last 15 years or so, we’ve surveyed about 1 percent of the total sky” at gamma-ray wavelengths, he said. Based on promising early returns that were collected by only a handful of instruments, “there’s a lot more to look at,” Wakely added.

Earth’s atmosphere protects organisms from the hazardous effects of gamma rays, but the VERITAS array of four telescopes together will form one of the most sensitive high-energy gamma-ray observatories in the world. “That opens up a really nice window where people haven’t been able to detect gamma rays,” Fortson said. VERITAS can detect flashes of blue light that gamma rays produce when they enter Earth’s atmosphere.

The collaborators hope to eventually obtain funding for three more telescopes that will increase the array’s sensitivity and allow it to observe two objects at a time.

Fortson is especially interested in using VERITAS to study active galactic nuclei, the ultra-bright but compact cores of galaxies that appear to be powered by black holes. Spewing from AGNs are powerful jets of radiation that span from the radio, or low-energy range of the electromagnetic spectrum all the way up to high-energy gamma rays.

“Those jets aren’t stable. They vary their output,” she said. “That implies a lot about the structure of the black hole. But we also want to look at other wavelengths, like the radio and the optical, and see if they are varying at the same time.” Simultaneous optical measurements, which Fortson will coordinate for the collaboration with astronomers at the Apache Point Observatory in New Mexico, are expected to help yield new insights into the inner workings of black holes.

Wakely, meanwhile, hopes to glean new data about the radiation fields that pervade the universe at a variety of wavelengths by how they affect the flow of gamma rays. Such data could help cosmologists better understand the origin and evolution of the universe.

He and Swordy also will look for evidence of gamma-ray emissions from the remnants of exploding stars, called supernovas. Such evidence could one day solve “possibly the oldest mystery in high-energy astrophysics,” Wakely said. That mystery: the sources that periodically bombard Earth with high-energy cosmic rays.

Swordy noted that the origin of cosmic rays “remains an open question even though there’s been about 90 years of research into it.” These rays, which are subatomic scraps of matter that fly through the universe at nearly the speed of light, can come from any direction. If cosmic rays really are produced in supernova remnants, as scientists suspect, then they should see a secondary flow of gamma rays as the cosmic rays interact with protons in the surrounding environment.
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