Particle Physics Detectors in Space Will Record Gamma Rays in Search of Dark Matter, the Evolution of Stars, and Nature’S Most Powerful Particle Accelerators

Particle physics detectors in space will record gamma rays in search of dark matter, the evolution of stars, and nature’s most powerful particle accelerators. By Heather Rock Woods Supermassive rotating black holes can emit particle jets that radiate in gamma rays. Illustrations: Sandbox Studio 10 Roughly once a day, the universe is rocked by are electrically neutral and always travel at the mighty explosions. We don’t fully understand speed of light. what causes the explosions, but we can detect GLAST will take high-resolution pictures of the results: brief, intense bursts of gamma rays, the invisible gamma-ray sky. Together, the Large the most energetic photons in the universe. The Area Telescope (LAT) and the GLAST Burst bursts can show up at any time, in any part of Monitor (GBM) instruments will investigate the the sky. 10 keV to 300 GeV energy range, which includes Soon a powerful new observatory will be some x-rays. The observatory will be launched orbiting the Earth, capturing gamma rays that into space to escape Earth’s atmosphere, which come from bursts and various other sources. blocks intense gamma radiation from reaching The Gamma-ray Large Area Space Telescope Earth’s surface. The observatory will complete an (GLAST) will explore the high-energy frontier orbit every 95 minutes, and can view the entire in space to help solve the mysteries of dark sky in two orbits. 06| february symmetry 03 | volume | issue 01 matter, supermassive black holes, and the evo- “If you put on gamma-ray glasses, you will lution of stars. see bright point sources, and you will also see a Gamma rays are bards, carrying tales at light diffuse glow in our galaxy,” says Rob Cameron, speed from the farthest reaches and earliest manager of a group at Stanford Linear days of our universe. They are the progeny of Accelerator Center that will operate LAT and wild and violent events, of particles that have process its raw data. been accelerated to fantastic energies, and of The glow shows up on maps as a bright stripe dark matter annihilations. across the plane of our galaxy. In our galaxy, “We’re interested in listening to the stories charged cosmic rays frequently crash into inter- gamma rays are telling us,” says Steve Ritz, mis- stellar gas, creating a gamma-ray haze, akin sion project scientist at NASA’s Goddard Space to the misty glow of the Milky Way viewed from Flight Center. Earth on a dark night. The joint Department of Energy and NASA GLAST will work synergistically with obser- project is a true collaboration of particle physics vatories operating at different wavelengths, know-how with particle astrophysics savvy. as well as with ground-based telescopes that Particle physicists can catch gamma rays so detect flashes of light in the upper atmosphere energetic that they defy telescope mirrors and made by the most intense gamma rays, ener- lenses. Particle astrophysicists know where to getic enough to plow into the upper atmosphere go to hear the tales, and how to interpret them. before perishing. And if gamma rays can tell us anything about “You get the most science out by using other the identity of dark matter, and many scientists kinds of telescopes–optical, radio, x-ray, on the think they can, the results will be spectacularly ground, in space–at the same time as GLAST important to both communities. is observing,” says Roger Blandford, director of the Kavli Institute for Particle Astrophysics and The gamma-ray sky Cosmology, based at Stanford University and Humans can’t see gamma rays. Our eyes only the Stanford Linear Accelerator Center. detect photons—that is, particles of light—in GLAST’s LAT is the successor to an in- a narrow range of energy hovering around two strument called EGRET onboard a NASA obser- electronvolts. Gamma rays are photons at the vatory that provided the first real look at the extreme end of the electromagnetic spectrum, gamma ray sky and produced spectacular results 100,000 electronvolts (100 keV) and up. They and discoveries. GLAST Source: NASA Large Area Telescope Hubble Burst Monitor Visible Light 10 -6 10 -3 10 0 10 3 10 6 10 9 10 12 Energy Spitzer Chandra eV Electromagnetic radiation, carried by photons, occurs over a Spitzer Space Telescope searches in infrared energies, the large range of energies, including low-energy radio waves, Hubble Space Telescope observes primarily in visible mid-range visible light, and high-energy gamma rays. NASA light, the Chandra X-ray Observatory works with x-rays, and missions are looking at different parts of the spectrum: the GLAST (with DOE) will look for gamma rays. 11 “GLAST is an enormous leap in capability family that particle physicists call upon to unify over EGRET,” says GLAST principal investigator the electromagnetic, weak, and strong forces. To Peter Michelson of Stanford. “It has a broader learn what makes up a large portion of the uni- energy range, better sensitivity, greater clarity. verse while simultaneously explaining subatomic The discovery reach is enormous, and it has the occurrences of dark matter will unify particle capability to follow up on EGRET measure- physics and astrophysics like never before. ments and questions.” In one or two days, it will cover the entire sky Ancient starlight with the same sensitivity EGRET had after 15 “Astronomy is a time machine,” says Michelson. months, allowing GLAST to make observations “GLAST will see some very old photons that significant to a broad range of science topics. come from when the universe was young.” Some of the gamma rays will have traveled Understanding cosmic for more than 10 billion years, packing with them accelerators information about what it was like when they Only the most powerful accelerators in the uni- departed, like an elderly refugee carrying a pho- verse can accelerate particles to energies where tograph of her hometown, taken when she they radiate gamma rays. GLAST will help was a young child. determine how these different accelerators work. Gamma rays speed virtually unimpeded and GLAST will observe thousands of active gal- unchanged through the universe. However, axies that emit vast amounts of energy, each there is a small probability that they will interact with a supermassive black hole at its center. Some with starlight and other photons of lower energy emit the energy as jets of particles, bursting and be converted into other particles. GLAST forth perpendicular to the plane of the galaxy. will measure this attenuation in gamma rays to The particles radiate gamma rays, which GLAST learn how many stars existed in a particular will see if the jets point directly toward Earth. region when a gamma ray flew through. “By watching gamma rays, we’re watching “GLAST will help us know how long the stars black holes digest and burp matter,” says have been shining,” says Ritz. Michelson. “We will learn a lot about the black holes by watching that.” Puzzling gamma-ray bursts Pulsars are another kind of cosmic accelerator. Gamma-ray bursts have no obvious counterpart Giant stars run out of fuel, get violently crushed in other parts of the electromagnetic spectrum. by gravity, explode brilliantly as supernovae, and “Just for a few seconds, those sources in gamma leave a collapsed core as heavy as our sun yet rays are brighter than the entire universe–all only 10 kilometers across. This extremely dense stars, galaxies, all radiation in that second,” says core–a pulsar–rotates quickly, shaking off elec- Michelson. “Then it goes away and is never seen.” tromagnetic radiation in periodic pulses, like Scientists expect to witness some 200 flashes of light from a lighthouse. bursts a year, using both GBM and LAT to find “We expect to see hundreds of gamma-ray them, track where they came from, and deter- pulsars with LAT and learn details of the emission mine their energy. mechanism,” says Cameron. “The engine that powers these explosions is not yet clear,” says Ronaldo Bellazzini of INFN The search for dark matter Pisa and head of the Italian part of the GLAST The matter we are familiar with, which radiates collaboration. Candidates include the birth of photons of various sorts, accounts for only four black holes from massive star collapses, and the percent of what exists in the universe. The rest mergers of black holes or neutron stars. of the matter–called dark matter–does not Researchers hope the bursts will reveal more emit photons. Yet GLAST hopes to find, confirm, about dark energy, the mysterious force that is or set limits on dark matter by collecting high- speeding up the expansion of the universe. GBM energy photons. How could that be possible? can detect bursts that occurred very long Theories predict that when high-mass dark mat- ago, extending the known history of past expan- ter particles collide, they destroy each other sion rates. but create gamma rays with an energy specific The oldest bursts might also allow scientists to the masses of the colliding particles. With to find signs of gravitons, the postulated particles careful analysis, GLAST researchers expect to that transmit gravity. Graviton effects might slow pick out this dark matter “signature.” down the highest-energy gamma rays ever A leading candidate for dark matter is a so slightly in the course of billions of light-years, theoretical particle called the neutralino. Neutra- making them arrive just after lower-energy linos are part of the hypothesized supersymmetry gammas from a burst. 12 Surviving in space A unique collaboration Almost immediately after EGRET’s results started In addition to the technical and scientific efforts streaming in, a few physicists from SLAC and to build, launch, and do science with the obser- Stanford “talked about what the next technology vatory, the mission has faced two more challenges: for a high-energy gamma ray mission might building a strong collaboration from researchers be,” recalls Elliott Bloom, co-chair of the SLAC/ in different fields, and forging strong working GLAST Physics Department.

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