Astronomy experiment opens new window on high-energy gamma rays

An innovative experiment that uses the Earth’s atmosphere as a gamma-ray telescope has successfully detected its first celestial object through a window in the spectrum previously closed to astronomers,University of Chicago physicists reported today at the American Astronomical Society’s centennial meeting in Chicago.

The detection demonstrates the ability of the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) to view the gamma-ray spectrum of the sky that ranges from approximately 10 billion electron volts to 300 billion electron volts, said Chicago physicists Corbin Covault and René Ong. Neither the orbiting Compton Gamma-Ray Observatory nor ground-based experiments have previosly been able to detect gamma rays in this energy range. Gamma rays contain vastly more energy than visible light rays, which have the energy of one electron volt.

"We have been running long enough to have a preliminary result on the detection of an important source, the Crab Nebula, which we observed several times since the fall," Covault said. The STACEE team will announce more detailed results later this year following further analysis.

"STACEE is a new experiment that does something completely different from anything that’s ever been done before, doing high-energy astronomy using gamma rays," Ong said. The project contributes to the growing field of high-energy astronomy, which applies techniques used in particle physics experiments to the study of celestial phenomena.

Other objects that STACEE will view in the future may derive their power from supermassive black holes, neutron stars or other objects that generate high-energy particles. "When an object emits these very high-energy particles, they’re doing something special," Ong said. "We hope that it represents some type of new astrophysics or even perhaps under extreme cases new physics that we couldn’t generate here on Earth."

STACEE detected the Crab Nebula, a remnant of an exploding star, using half the apparatus that it will eventually have. Once construction is complete in early 2000, the experiment will be five to 10 times more sensitive to celestial objects than it is now.

"It will allow us to look further out into the universe," Ong said. "The Crab Nebula is in our galaxy. It’s about 6,000 light years away. The other objects that we’re interested in are quasar-like objects, believed to be driven by supermassive black holes, which might be millions or billions of light years away. By getting better sensitivity, we’ll be able to see more of those as well."

STACEE operates at the National Solar Thermal Test Facility at Sandia National Laboratories in Albuquerque, N.M. By day, the facility’s array of 220 large mirrors, called heliostats, are used to track and focus sunlight for solar energy research. On dark, moonless nights, the STACEE collaboration uses some of those mirrors to collect tiny flashes of blue light known as Cherenkov radiation, which gamma rays produce when they enter Earth’s atmosphere.

The heliostats, each measuring 20 feet by 20 feet, collect the radiation and focus it on a 200-foot central tower. Secondary mirrors mounted on the tower image the radiation onto sensitive light detectors.

"We don’t have to send anything to space. We don’t simply look at light coming down through the atmosphere. We use the whole atmosphere as our Cherenkov detector," Ong said.

The Crab Nebula detection was achieved using only 32 heliostats, 32 photodetectors and their associated electronics. Another 32 heliostats and improved electronics will be added by early 2000.

"The part of the experiment that we construct are the optics that sit on the tower, the associated photodetectors and all the associated electronics," Covault said. "For each heliostat that you wish to collect light from you have to build the appropriate additional materials."

Covault and Ong built STACEE in collaboration with a team of 20 scientists at Chicago and at McGill University in Canada; the University of California, Santa Cruz; the University of California, Riverside; and Barnard College and Columbia University.

The $1 million STACEE project is funded by the National Science Foundation and makes use of the National Solar Thermal Test Facility through a cooperative agreement with the U.S. Department of Energy. STACEE’s collaborators at McGill University received additional funding from the National Sciences and Engineering Research Council of Canada.