Searching for gamma rays under dark desert skies

On the outskirts of Albuquerque, in the darkened desert when the moon is new, a team of University of Chicago astrophysicists brings a giant array of solar collectors to life. For 14 nights every month, the National Solar Thermal Test Facility at Sandia National Laboratory turns into a nighttime observatory, scanning the skies for evidence of high-energy gamma-ray radiation from distant galaxies.

This gamma-ray "telescope," called the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE), concentrates the faint blue light that is the signature of high-energy gamma rays -- known as Cherenkov radiation -- that hits the top of our atmosphere. "The idea for using a solar facility for this type of astronomy was first floated about 15 years ago," said Rene Ong, the Grainger Assistant Professor in Physics, "but it wasn't until 1994 that we really began to take it seriously and built our first test equipment. And over this past summer we made our first observations."

"We're looking in a region of the electromagnetic spectrum that has never been explored before," explained Corbin Covault, Assistant Professor in Physics, who with Ong and Mark Dragovan, Senior Scientist in Astronomy & Astrophysics, forms the nucleus of the Chicago team. "Every time we've opened up a new window in the spectrum -- for example when astronomers first began to explore the Universe in the infrared, or Xray or radio wavelengths -- we've discovered something new."

The energy in the gamma-ray portion of the electromagnetic spectrum is comparable to that produced in the most powerful particle accelerators here on Earth. Astronomers think they may be produced by neutron stars -- dense, highly magnetized stars that give off gamma ray radiation as they spin -- or such objects as active galaxies that have massive gravity fields.

"Active galaxies are fantastic objects," said Ong. "An entire, massive galaxy is compressed into a volume about the size of our solar system -- something like one billion stars are in a single black hole. The enormous gravitational forces lead to particle acceleration and radiation in the form of gamma rays. Enormous black holes are so fantastic, and they sound so much like science fiction, that many people originally didn't think they could ever exist. Now that gamma rays have actually been seen from some of them we want to find more, and figure out how they work."

One advantage of STACEE is that it utilizes an existing facility in its off hours. During the day, the solar facility's 220 heliostats, which are essentially adjustable mirrors, focus the light of the sun onto a 200-foot central tower, and this intense beam of visible light is used in solar energy research.

At night, STACEE uses 50 of the heliostats to focus the light from the night sky onto the tower, where the collaborators have mounted a secondary mirror and a detector to look for Cherenkov radiation.

When gamma rays from space reach the top of the Earth's atmosphere, they cannot penetrate; instead, they trigger a shower of particles that fly through the atmosphere, faster than the speed that light normally travels through the air. Because they are traveling faster than light, they generate the visual equivalent of a sonic boom, a faint blue glow, called Cherenkov radiation, that actually falls in the visible region of the spectrum.

Cherenkov radiation from a gamma ray is like a headlight beam streaming through the atmosphere -- by the time it reaches the ground, it has spread out to cover an area about 200 meters across. More conventional ground-based gamma-ray telescopes can collect only a small fraction of this beam, and so only "see" the highest-energy (and therefore brightest) of the gamma-ray sources. Because the STACEE collectors are spread out over a large area, they can collect a larger fraction of the "beam" and thus see much fainter sources -- at a fraction of the cost of building a giant gamma-ray telescope from the ground up.

"We have to trick the heliostats into thinking the sun is up at night," said Covault, "but it turns out it's not that difficult -- all I had to do was get into the control room and rewrite some of the software that controls them."

STACEE will be able to detect gamma rays in an energy range of about 10 GeV to 300 GeV. Currently, there are many sources (detected from space) that produce gamma rays with energies of less than 10 GeV, but only a few known sources that have energies higher than 300 GeV. "We don't know what's happening in the middle range," said Covault. "Where's the cutoff? Understanding the maximum energy these things emit is important for understanding how they work."

Gamma rays leave a particularly brief signature, so things like an airplane or a flash of lightning can never be misinterpreted as gamma rays. "The pulse of light we're looking for is only a few nanoseconds (billionths of a second) long. So even though it's very faint, we can easily distinguish it from other things that might pass through our field of view," said Covault. "Nothing else you can think of in the atmosphere is as fast as that."

The timing of the pulse across the heliostat field lets the scientists determine precisely where the gamma rays originated.

"In many ways, the techniques we're using are the same as those used in high-energy particle physics, even though we're really an astrophysics experiment," said Covault. "However, STACEE is a fairly small collaboration, and our plan is to build the experiment and have data within just a few years, so it's a perfect experiment for students to get involved with. Everyone in our group is making major contributions to the experiment." The Chicago group includes Zoa Conner (a Fermi postdoctoral fellow), Mark Chantell, graduate student Scott Oser, and undergraduates Anthony Miceli, Howie Marion and Jaci Conrad.

STACEE also includes collaborators at McGill, UC-Santa Cruz and UC-Riverside, and is funded by the National Science Foundation under a program that encourages young researchers and emphasizes outreach and teaching.

This past summer, the initial installation of the detector was completed and the first test observations made. This winter, a second mirror will be installed. Construction of the experiment is expected to be completed in one to two years. After proving successful with 50 heliostats, it can be scaled up to take advantage of the full 220 at Sandia, and perhaps duplicated on a 2,000-heliostat solar test facility in California.