Exploring the Unknown Very High Energy Gamma-Ray Sky

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High-energy gamma rays are used to study the most extreme objects in the Universe, such as the black holes at the centers of distant galaxies and gamma-ray bursts, the most powerful explosions since the Big Bang. Cosmic gamma rays can also provide clues to new physical principles beyond the scope of the current understanding of subatomic physics and cosmology. The very high-energy (VHE) gamma rays studied in this research have about 100 million times as much energy as the X-rays used for medical images. None the less, their detection is difficult because they are absorbed by the atmosphere. At the lower energies, the number of gamma rays from astrophysical sources is sufficient to observe with detectors on board satellites in orbit above the atmosphere. The highest energy gamma rays are much rarer and require detectors with an area the size of a soccer field, Such detectors can only be built on the ground. These observe gamma rays indirectly by detecting the shower of secondary particles produced when a gamma ray collides with an air molecule in the atmosphere. The researchers will use a large, instrumented reservoir of water at high altitude-called "Milagro" -to detect the secondary particles from gamma-ray showers reaching the ground. The man-made reservoir holds about 5 million gallons of water. The particles produce flashes of light in the water, which are recorded by sensitive detectors called photomultiplier tubes. An opaque black cover over the reservoir keeps out sunlight so that the faint flashes can be detected, and the water is filtered so that it is very clear. Milagro is sensitive to gamma rays primarily with energy between about 100 billion electron-volts and 10 trillion electron-volts (visible light has energies of a few electron-volts). Milagro was built specifically for this purpose by the researchers and their collaborators, primarily with support from NSF. This proposal focuses on using Milagro to explore the unknown VHE behavior of the Universe by way of three principal science topics: emission from gamma-ray bursts, surveying the sky, and studying in more depth the known, but unidentified, sources. Gamma-ray bursts are the most powerful explosions known in the Universe, but the details of how they work and the role they play in other high energy phenomena (For example, are they the accelerators of the highest energy cosmic rays?) are still being explored. Learning whether they are capable of creating VHE gamma rays in general, in particular circumstances, or not at all, will constrain gamma-ray burst models and properties of the burst environment. Surveying the sky and studying the unidentified sources go hand in hand. Surveys are the best way to reveal new sources independent of any preconceived notions about VHE emission. Some of these new sources will have clear associations with known objects detected at other wavelengths (for example, radiowaves, optical light, or X-rays) and may or may not produce surprises in terms of their VHE emission. Others, the unidentified sources, hold the greatest potential for the discovery of new objects and phenomena. The work of this proposal will contribute to the scientific literacy of the public, the educational development of students in the sciences, and the nation's technological infrastructure. The public's curiosity about the Universe creates many opportunities to engage their interest and, in the case of students, participation. Postdoctoral, graduate and undergraduate students, including many from underrepresented groups, play an important part in the research effort. The researchers interact regularly with K-12 students, both as visitors to schools and hosts in their university labs as well as presenting general interest lectures to the public. The collaboration with Los Alamos National Laboratory on the Milagro project advances the national security mission of the Lab by helping to attract and retain top-notch talent there.