Advancing Basic Science for Humanity
On the GLAST Track
There’s far more to the universe than meets the eye. Astronomers have long known this, and much of their big-budget work for the past several decades, from radio telescopes to orbiting observatories, has the goal of “seeing” the cosmos on wavelengths that are inaccessible to human sight.
One of the newest of those projects, now occupying an army of scientists from around the globe, focuses on the end of the electromagnetic spectrum where waves are ultra-short and energy levels are ultra-high – the gamma-ray realm. With the Gamma Ray Large Area Space Telescope (GLAST), due for launch in 2008, astronomers will get an unprecedented view of black holes, pulsars and possibly dark matter. They hope to find answers to some key questions of astrophysics: What causes the massive jets of energy emerging from suspected black holes? Why do pulsars have such strong magnetic fields? Is there a dark-matter particle, as some theorize, that could be detected by the gamma rays emitted when it meets its anti-particle and self-annihilates?
Then there’s the question that arises with any new leap in telescope technology: What’s out there that we haven’t yet seen?
A Worldwide Effort
Artist’s rendering of the GLAST spacecraft in flight. The gold box is the thermal blanket over the top of the LAT instrument. The large wings are solar panels that provide power. (Illustration courtesy of NASA).
GLAST is a project of worldwide scope. The Large Area Telescope (LAT) instrument is funded by NASA and the U.S. Department of Energy and by government agencies in France, Italy, Japan, and Sweden. Its scientific team includes more than 300 researchers, engineers, technical experts and graduate students, representing 34 academic and government institutions in the U.S., Europe and Japan. At the hub of all this activity is the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University (KIPAC).
KIPAC member Peter Michelson, a Stanford professor of physics, leads the GLAST Large-Area Telescope Collaboration, as the global scientific effort surrounding the GLAST mission is officially known. He and other physicists based at Stanford and around the world built the components of the LAT instrument at the heart of GLAST which uses silicon-strip detectors to track gamma ray particles in three dimensions. Michelson says this solid-state technology “has resulted in an enormous gain in capability” over the earlier spark-chamber detectors used in the in the last space-based gamma-ray telescope mission, the Compton Gamma Ray Observatory (CGRO) that operated in the 1990s. It also exemplifies KIPAC’s and Stanford’s melding of particle physics and cosmology. As KIPAC Director Roger Blandford notes, silicon-strip technology “came straight out of particle physics” – specifically, at Stanford. It was first devised some 20 years ago at the Stanford Linear Accelerator Center, a facility operated by the university for the U.S. Department of Energy.
Black Holes and Radiation
GLAST is in part a follow-up to CGRO, which found high-energy radiation from distant galaxies believed to contain supermassive black holes – up to a billion times more massive than the sun. Michelson, who was involved in CGRO, said GLAST will train its more precise instrument on these gamma-ray sources to test theories of how black holes affect their surroundings and generate high-energy radiation.
A top view of the Large Area Telescope (LAT) without its thermal blanket covering. Prominently featured is the anticoincidence detector (ACD), which detects charged particles. Charged cosmic rays and trapped particles are much more abundant than the gamma rays the LAT is designed to observe. They must be identified so they don't confuse the measurement. (Photo courtesy of Stanford University).
“We observed prodigious amounts of energy being emitted from the regions of suspected black holes,” he said, but why and how this happens is not clear. Black holes, by definition, exert a gravitational pull so great that not even light can escape them. But they also seem to throw off huge volumes of energized particles, some of which find their way to telescopes such as CGRO and LAT. Blandford and Roman Znajek explained 30 years ago how this energy might come from the rotation of the black hole within a magnetic field. Michelson says GLAST could confirm key aspects of this process with its detailed new data.
Another mysteriously energetic object is the pulsar, which beams radio waves to earth in repeated short bursts. Astronomers believe that these are spinning neutron stars (extremely dense objects formed from the collapse of massive stars) with strong magnetic fields that emit radio signals in one direction. Because of this so-called “lighthouse effect,” these signals are only detected when the wave-emitting part of the star faces the earth. What would cause these magnetic fields is a mystery. Ordinary stars have magnetic fields generated internally by moving gases acting as dynamos, but neutron stars are packed far too tightly for that. But pulsars emit gamma rays, and scientists such as KIPAC’s Roger Romani can study these to measure their rotation and understand how high-energy radiation is generated in their intense magnetic fields. CGRO detected this radiation, but Michelson says the more sensitive GLAST should provide a much more detailed look (he also expects it to discover more pulsars in our galaxy). Among other things, GLAST data will test theoretical work done by a KIPAC post-doctoral fellow, Anatoly Spitkovsky (now at Princeton), who developed numerical methods for simulate the structure of pulsars’ magnetic fields.
A Smoking Gun for Dark Matter
Detecting dark matter is one of the great challenges in today’s astronomy. This substance doesn’t interact with ordinary matter, so it passes through our bodies – and telescopes – unnoticed. It’s known indirectly through its gravitational effects (based on these, scientists think dark matter outweighs ordinary matter by a factor of at least five). Also, according to some models of its composition, dark matter might be detected through cosmic rays produced when its particles meet its anti-particles and destroy each other, converting their mass into energy through the familiar formula E=mc2. GLAST will be looking for gamma rays at the frequency predicted in such self-annihilation events. A narrow band of them in the high-energy spectrum would be a “smoking gun” for the presence of dark matter, says Michelson, but he thinks it’s more likely that the dark matter emission would be mixed in with gamma rays from other energy sources in the universe and that scientists such as GLAST pioneer Elliott Bloom will have to work very hard to untangle the GLAST data to find it. Either way, GLAST will take the search for dark matter a step forward.
More than a dozen KIPAC scientists are members of the GLAST collaboration. KIPAC’s role in the mission goes beyond the participation of its individual members. It is a “host for the scientific work” of the GLAST LAT Collaboration, Michelson says, serving as a meeting place for GLAST collaborators and attracting visitors from all over the world “who interact with our collaboration and get involved in our science.” In this way, he says, the Institute is both a “facilitator” for GLAST and a model for carrying out truly international projects in an era of globalized science.