GLAST Spacecraft Will Observe Cosmic Sources Of Gamma Rays As It Orbits Above The Earth

GLAST will explore the universe's most extreme environments, searching for answers to long-standing questions about dark matter, black holes, and gamma-ray bursts. Scientists expect the orbiting telescope to detect thousands of hitherto unknown gamma-ray sources. With its extraordinary sensitivity and wide field-of-view, it is the first imaging gamma-ray observatory capable of scanning the entire sky every three hours on a daily basis.

Bill Atwood, an adjunct professor of physics affiliated with the Santa Cruz Institute for Particle Physics (SCIPP) at UCSC, said GLAST has the potential to answer some of science's fundamental questions. It may even discover evidence of particles whose existence has been hypothesized but not yet confirmed by physicists.

"Elementary particles are the universe's building blocks," Atwood said. "If we understand their properties well, it will really constrain our thinking in terms of how the universe was born and how it evolved."

GLAST's two main components are the Large Area Telescope (LAT) and the GLAST Burst Monitor (GBM). The UCSC team was responsible for the LAT's gamma-ray-detecting system (known as the "Tracker"), which uses silicon-strip technology developed at SCIPP. Atwood came up with the LAT's overall design concept in 1992, while working at the Stanford Linear Accelerator Center (SLAC). UCSC physicist Robert Johnson came on board a couple of years later to help turn Atwood's initial design into reality, and became primarily responsible for managing the Tracker.

Partners in Japan and Italy played major roles in the Tracker project, Johnson said. The silicon strip detectors (more than 11,000 altogether) were manufactured and tested in Japan. Assembly and environmental testing of the 18 Tracker modules (including two spares) were done in Italy.

The LAT weighs three tons and contains almost a million channels of electronics, mostly in the Tracker, which also includes 73 square meters of silicon-strip detectors. Despite its size and complexity, Johnson figured out a way for the whole system to operate on just 160 watts of power--a couple of light bulbs worth.

Over the years, approximately a dozen UCSC undergraduates, six graduate students (two currently working with Johnson), and five postdoctoral physicists contributed to this huge undertaking, including its mechanical design. The team came up with a carbon-based support structure to ensure that the Tracker survives what Atwood described as GLAST's "fire and brimstone" launch aboard a Delta II heavy rocket in early June.

Once in orbit, GLAST will deliver a steady stream of data on gamma-ray sources for astrophysicists to analyze. Gamma rays are the most energetic form of radiation in the electromagnetic spectrum, millions to billions of times more energetic than visible light.

Astrophysicists are especially hopeful that GLAST will give them a better grasp of dark matter. This mysterious form of matter is thought to account for the vast majority of the mass in the universe, although its presence has only been inferred from its gravitational effects.

"It would be most exciting to see a signal from dark matter, although I wouldn't bet a lot of money on it," Johnson said.

Not much is known about the nature of dark matter, but a leading theory is that it consists of weakly interacting massive particles (WIMPs). These theoretical particles are capable of annihilating each other during collisions and emitting a particular gamma-ray signature.

"A spot in the sky spitting out gamma rays with one single energy would be a dead giveaway of dark matter," said David Smith, an associate professor of physics at UCSC. "There'd be parties everywhere if we saw something like that."

Among the main sources of gamma rays are gigantic, active black holes in the centers of galaxies. As matter falls into a black hole, it emits energy in the form of gamma rays and other high-energy radiation. Neutron stars, produced when a massive star's core collapses, are another gamma-ray source. The strong magnetic fields swirling around a neutron star create electric fields that accelerate particles to high energies, resulting in gamma rays when these high-energy particles are magnetically deflected.

The death of a massive star occasionally releases energy in a narrow beam of gamma rays, brighter than all other sources combined. This is called a gamma-ray burst, a phenomenon that GLAST's second component, the burst monitor, was specifically built to study.

The gamma-ray sources GLAST detects could also be useful in looking at the "light pool"--all the light present within the universe. Gamma-ray photons (particles of gamma radiation) can interact with other photons to create an electron and its antimatter counterpart, a positron, Smith explained.

"If we have a far-away source and we know what gamma rays it ought to be making, but we see that some are missing, we know they were essentially eaten up by intergalactic starlight," Smith said. "Seeing how much is missing compared to our expectation tells us how much intergalactic starlight there is, and more about the history of how many stars there were, going back in time."

"We'll learn more about galactic evolution," Atwood added. "It's fundamental science and close to the big issues: How did the universe start" Why are we here in the first place""

Johnson and Atwood plan to be in the front row of the audience at Cape Canaveral when GLAST is finally launched. "We wouldn't miss it for the world," Atwood said.

There have been numerous technical problems along the way, and enormous pressure to stay on schedule, Johnson said. "But it's really great how things finally came together," he said.

Atwood noted that the Geneva-based Large Hadron Collider (LHC), designed to help physicists fill the gaps in describing the fundamental nature of matter, is also set to begin operating this year. UCSC physicists, led by SCIPP director Abraham Seiden, helped design one of the LHC's two particle detectors. Meanwhile, GLAST's Guest Investigator Program, which supports basic research relevant to the GLAST mission, recently selected seven proposals from UCSC faculty for funding, including two from Johnson.

"UCSC will be a very exciting place when all of this new information starts coming together over the next year," Atwood said.