(Originally published by Stanford Linear Acclerator Center)
June 27, 2008
Roughly 6,000 years ago, a 954-year-old neutron star about 10 miles in diameter spinning on its axis 30 times per second used its strong magnetic fields—in a sort of souped up LINAC—to create a gamma-ray photon. This photon escaped from a dense crowd of X-rays, electrons and positrons, all eager to make its acquaintance, into the vast reaches of interstellar space. It is now just outside of our solar system, hurtling towards Earth at the speed of light and sometime next week it will keep an appointment with a three-ton orbiting satellite: the Gamma-ray Large Area Space Telescope (GLAST).
However, just as the gamma ray meets its demise inside the GLAST detector, it will give birth to an electron and a positron, which will continue along similar trajectories before spawning several generations of descendents whose genealogy will be reconstructed using a criss-cross pattern of nearly a million silicon strips and who will hold a family reunion in a block of cesium iodide at the end of the detector where each particle will create a flash of light dependent upon the energy of the original gamma-ray photon. All of this information about the arrival and fate of this gamma ray will be converted into a brief radio message, which will be transmitted to Earth via another satellite, before being logged in SLAC's Large Area Telescope (LAT) Instrument Science Operations Center (ISOC), located in Building 84.
The information transmitted to the ISOC will, for the first time, draw back a curtain of mystery that currently shrouds gamma-ray sources. The source of this gamma ray detailed above is the famous pulsar at the center the Crab Nebula in the constellation Taurus, one of several known sources of gamma rays scattered over the sky. The majority of these sources—up to 10,000 of which should be observed by GLAST—are called blazars. These comprise powerful jets of electrons and positrons created by billion-solar-mass black holes. The jets themselves move at over 99 percent of the speed of light and are probably the source of the most energetic cosmic rays that are measured. Equally impressive are the gamma ray bursts, which probably celebrate the birth of roughly 10-solar-mass black holes each with a brilliant flash of gamma rays, lasting no more than a few seconds that can be seen as far back in time as the epoch of the first stars and galaxies in the universe. More speculatively, the famous dark matter, that comprises five-sixths of the matter in the universe, is thought to comprise supersymmetric particles. These particles occasionally meet and create gamma rays which GLAST may detect. (Similar particles may also be found soon at the Large Hadron Collider or detected directly in an underground laboratory.)
GLAST was launched by NASA on June 11 from Cape Canaveral—a thrilling and nerve-wracking event, web-broadcast to collaborators all around the world including a large crowd in SLAC's Kavli and Panofsky auditoriums. As of this writing, the spacecraft is orbiting, slewing and transmitting just as planned. All 16 of the tracker towers in the LAT have been switched on and are working. Provided all continues to go well—and there is still plenty to keep us awake at night—we should make our appointment with the Crab pulsar gamma ray photon next week. Thereafter, we are all looking forward to answering fundamental and longstanding puzzles concerning pulsars, gamma ray bursts and so on by scanning the whole sky every three hours and analyzing roughly a billion photons over the next decade. It is wonderful to see the adrenaline flowing through the GLAST collaboration and to experience, vicariously, the pleasure of my colleagues when a subsystem on which they have collaborated works as designed.
I am often asked why a lab that specializes in accelerators should be working on GLAST. Two answers have been true since the start of the project. First, high-energy physics grew out of cosmic-ray physics and so it is not a surprise that the science of high-energy cosmic sources still overlaps that of particle physics. In addition to possibly identifying dark matter, we are all intensely curious to learn how Nature manages to construct "Zevatrons." Indeed, there has been a strong give and take between the study of astrophysical particle acceleration and the design of advanced laboratory accelerators. Second, GLAST is a natural collaboration between scientists with different backgrounds. While NASA engineers know how to build, launch and operate spacecraft and astronomers can contribute telescopes operating throughout the electromagnetic spectrum that are needed to make sense of the gamma ray data, it took particle physicists supported by the U.S. Department of Energy and many other agencies around the world to design, fabricate and integrate the LAT—which is far superior to what would have otherwise been flown using pre-existing technology. In addition, there is now a third answer. SLAC has transitioned from a single purpose particle physics facility to a multipurpose laboratory. At a time when there will be a long interval between the termination of BaBar and the start of the next major particle physics construction project, experiments like GLAST and its successors—which we in KIPAC are working hard to develop—will help maintain continuity in the Particle Physics and Astrophysics science program and its core capabilities.
So, let us salute Elliott Bloom and LAT Principle Investigator Peter Michelson for initiating GLAST; Bill Atwood for his excellent early design of the instrument; Burt Richter, Jonathan Dorfan and Persis Drell for their enthusiastic support and for keeping GLAST on track at SLAC; our colleagues in the Department of Energy and the Office of Science, most notably Kathy Turner, for their ongoing support of the project; NASA, especially Mission Scientist Steve Ritz and Mission Manager Kevin Grady, for its leadership and for a perfect launch; as well as ISOC Manager Rob Cameron and the ISOC team, LAT Project Manager Ken Fouts, and past Project Manager Lowell Klaisner. We recognize the contribution of hundreds of close colleagues from all around the world for all that they have achieved: U.C. Santa Cruz, under the leadership of Robert Johnson, for the overall design of the silicon tracker; the Italians, under the leadership of Ronaldo Bellazzini, for building the silicon tracker; the Japanese, under the leadership of Tune Kamae and Takashi Ohsugi, for their design and production of silicon detectors; The Naval Research Laboratory, under the leadership of Neil Johnson, for the CsI calorimeter; the Swedes, under the leadership of Per Carlson, for their contribution of cesium iodide; the French, under the leadership of Isabelle Grenier and David Smith, for their many contributions to the calorimeter; Goddard Spaceflight Center, under the leadership of David Thompson, for the contribution of the Anti-Coincidence detector; and, finally, the SLAC electronics department, under leadership of Gunther Haller, the SLAC mechanical design department, and the SLAC integration and test department for the data acquisition system and flight software, the overall mechanical design and construction of key elements of the instrument, and the assembly and testing of the instrument respectively. Of course we also thank the many more collaborators at SLAC, the Kavli Institute for Particle Astrophysics and Cosmology, Stanford University and all over the world who have been contributing to this grand effort. Wish us well over the coming months as we work to transform individual photon events into fundamental understanding about the workings of the universe around us.