Stanford University News Service
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June 12, 2008
Dan Stober, Stanford News Service: (650) 721-6965, firstname.lastname@example.org
The Gamma-ray Large Area Space Telescope is in orbit 350 miles above Earth, lifted there by a rocket launched from NASA's Kennedy Space Center. The blast-off was a milestone for scientists on the Stanford campus and at the Stanford Linear Accelerator Center, who have been working on the project for the last 15 years.
"It was absolutely beautiful," said Peter Michelson, a Stanford astrophysicist who is a principal investigator on the project. He and two dozen other members of a Stanford contingent watched from a beach Wednesday as the fiery rocket rose through the Florida clouds.
With launch of GLAST, the human view of the violent gamma ray sky is expected to improve dramatically, providing fresh clues to the inside story of how the universe operates now and how it evolved. GLAST may even find definitive signals of dark matter, that mysterious, unseen substance that gravitationally holds the universe together. Such a finding—far from guaranteed—might help rewrite the laws of physics and conceivably push GLAST into Nobel territory.
As it circles Earth, the GLAST observatory, roughly the size of an automobile, contains two main instruments. The first is SLAC's Large Area Telescope. Its wide-angle stare will scan the entire sky every three hours, in essence making a movie of the sky with the most sensitive gamma ray detector ever flown. The second instrument, the GLAST Burst Monitor, watches the sky for powerful bursts of gamma rays and alerts scientists when it spots one, so that they may turn their telescopes in that direction.
The Large Area Telescope—whose design and assembly was managed by SLAC—will not turn on for another 10 days, but there is already a bit of good news, Michelson said. The craft's solar panels, which provide the electricity necessary for all its operations, unfolded and faced the sun.
Over the five- to 10-year lifetime of the spacecraft, GLAST's gusher of data will be sifted and analyzed around the clock at the project science center at SLAC and then shared with scientists around the world. Researchers who photograph the sky in the visible range, or radio waves or X-rays, can combine their images with those produced by GLAST. "They can just grab the data and overplot our results on their results," said SLAC physicist Richard Dubois.
The GLAST telescope will focus its attention on a smorgasbord of celestial delights, including black holes, neutron stars, cosmic rays, blazars, gamma ray bursts and the gamma ray background that glows in the distance like a child's nightlight.
To understand the revealing nature of gamma rays, take a look at the night sky with all its twinkling stars, as humans have for millions of years, and then realize that you are only seeing part of the show. Across the sky, exotic cosmic oddities are lurking, shining brightly with invisible gamma rays as they swirl, wink seductively at astronomers and send beams of high-energy particles across the universe like so many searchlights. You are missing this exhibition, but it is there, hiding in plain sight.
Gamma rays are actually weightless photons, like visible light but much more energetic, beyond the high end of the visible spectrum, beyond violet, ultraviolet and X-rays. They are invisible to the human eye but exist in abundance across the universe. If humans could see gamma rays, you would enjoy a whole new nighttime display.
"You would see a very bright, glowing band across the sky that would correspond to the plane of the Milky Way galaxy," said Michelson, who led the development of the Large Area Telescope. "And then you would see stars, but they wouldn't correspond to the stars you know about. These stars, rather than twinkling, would just be absolutely brilliant, the brightest thing in the sky, and then they'd go away and then they'd reappear; they'd be flickering on and off.
"When you look at the night sky with your eyes, its fairly quiescent and peaceful. The gamma ray sky is not. It's a very different view of the universe. We're seeing exotic things like black holes and neutron stars and coalescing binary systems at the end of their life when they collapse into a black hole and there's an explosion."
It is virtually impossible to study these celestial gamma rays here on Earth. The planet's atmosphere essentially absorbs them. Even in space, it is a difficult task, given that gamma ray photons carry an energy punch a billon-times greater than photons of visible light. Gamma rays fly through a glass lens without a second thought, rendering a conventional telescope useless. As a result, the Large Area Telescope has no glass. It is more of a physicist's particle detector than a conventional telescope, a cousin of the detectors at SLAC that sort out the sub-atomic debris of high-energy collisions inside the two-mile-long accelerator.
The story of GLAST begins at SLAC in 1992, with Michelson heading up an effort to design a successor to an earlier NASA orbiting gamma-ray telescope with Stanford involvement named EGRET. Launched in 1991, EGRET "saw a gamma ray sky that nobody had even dreamed of," said Bill Atwood, a former SLAC physicist (now at UC-Santa Cruz) who played a key role in the design of GLAST. "It was a Fourth of July fireworks show. Things were popping off and flaring, and gamma ray bursts were bursting, and pulsars were pulsing."
Eventually, the effort became a $690-million NASA project, a consortium of six countries and 14 U.S. research institutions. At Stanford, project members came from SLAC, a U.S. Energy Department lab; the Physics Department; the Hansen Experimental Physics Laboratory; and the Kavli Institute for Particle Astrophysics and Cosmology. Success came in part from the unusual mix of particle physicists and astrophysicists. One group works at the smallest of sub-atomic scales, while the other observes events across the whole of the universe.
GLAST's bread-and-butter targets, the ones NASA says are guaranteed to produce results, are known as active galactic nuclei. At the heart of an active galaxy is black hole whose tremendous gravitational pull is sucking the entire galaxy, stars and all, into a mad orbit around the dark hole. Eventually everything spirals in and disappears. "When the galaxy is young and the inner portion of the galaxy is still replete with a lot of garbage, dust, stars, whatever, this black hole is in feeding mode and is accreting this matter into it," Atwood explained. "During this accretion process, the stuff starts swirling around at great velocities. These things are beyond your imagination, both in size and ferocity."
Most intriguingly, radio, optical and X-ray telescopes have seen amazing jets of particles escaping in beams along the spin axis of swirling galaxies. The particles travel within a whisper of the speed of light and somehow stay focused across hundreds of thousands of light-years of space, like searchlights. When they briefly point directly at Earth, astronomers call them blazars.
"We don't know what the jets are made of or how they are produced. It is one of the biggest unsolved mysteries of astrophysics," Michelson said. The gamma rays from the most distant of these objects will have traveled 10 billion years at the speed of light before plunking into GLAST's detectors. "That's a sizable fraction of the size of the known universe," he added.
Atwood says that some of these enormous black holes may prove to be too big to have been created after the universe was born. "If that is the case then some of these black holes in fact will be revealed as primordial," created at the very beginning of the universe. "And that's the quest of all this. Where did we all come from? What are the smoking guns? What are the clues, the relics, left behind from the creation of everything?"
Gamma ray bursts are another GLAST target. First discovered serendipitously in the 1960s by U.S. surveillance satellites looking for gamma rays from Soviet nuclear weapons tests, they last from less than a second to several minutes. "When the gamma ray burst is on, it is the brightest, most energetic sources in the sky. In that brief period of time, the gamma ray burst will put out more energy in one second that the rest of the entire universe," Michelson said. The bursts may be generated by the merger of two neutron stars, or perhaps the merger of a black hole and a neutron star.
To many GLAST participants, the detection of dark matter would be the ultimate discovery. One possibility is that the telescope will detect gamma rays from the annihilation that is predicted to occur when two particles of dark matter meet each other. "That would be an extraordinarily difficult measurement," Michelson said. "But it would be a spectacular result."
Atwood is hoping for a spectacular set of findings involving dark matter. GLAST might find the substance in space at the same time it being created in the new Large Hadron Collider at CERN in Switzerland. If his wish comes true, "We're in for something akin to the birth of quantum mechanics," he said. "It just doesn't get any better. I'm speechless."
Scientists designed the detectors inside GLAST's main telescope to answer two basic questions about each gamma ray it captures: What's the energy of this gamma ray photon? And where did it come from?
When a gamma ray slams into a thin sheet of tungsten inside the detector, Einstein's famous equation E=MC2 comes into play. The gamma ray photon is pure energy—it has no mass. But when it strikes the tungsten, its energy is totally converted into mass. The gamma ray disappears, replaced by a pair of subatomic particles, an electron and its antimatter counterpart, a positron.
When that pair of particles plows into the 3,000 pounds of cesium iodide stuffed into the bottom of the detector, a flash of light is produced; its intensity is proportional to the energy of the incoming particles. Since the energy of the pair of particles is equal to the energy of the gamma ray that created them, researchers have one of their answers.
The answer to the second question—where did that gamma ray come from?—is provided by 800,000 silicon strips that record the path of the electron and positron as they pass through. Those paths point backwards to the sky and the origin of the gamma ray.
If the history of astronomy is any guide, GLAST's opening of a new window to the universe will result in the discovery of something previously unknown and unanticipated. "GLAST is going to knock your socks off," Atwood said.
Peter Michelson, Physics, (650) 723-3004, email@example.com
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