Astrophysics with Very High Energy Gamma Rays

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High-energy gamma rays are used to study the most extreme objects in the Universe, such as pulsars, black holes at the center of active galaxies, and gamma-ray bursts. Cosmic gamma rays can also be signatures for new physics beyond the standard models of particle physics and cosmology. At the low end of the gamma-ray spectrum, the flux of gamma rays is sufficient to detect them with satellites in orbit above the atmosphere. Studies with very-high-energy (VHE) gamma rays, above about 100 GeV, require detectors with thousands of square meters of collection area, which can only be built on the ground. These ground-based devices detect gamma rays indirectly, by detecting the shower of secondary particles produced when a gamma ray interacts in the atmosphere. This award will provide funds for the UCSC group to use the VERITAS (Very Energetic Radiation Imaging Telescope Array System) imaging Cherenkov telescope array in southern Arizona to study VHE gamma rays. They plan to explore the extreme behavior of the two most powerful known classes of gamma-ray source: gamma-ray bursts (GRB) and active galactic nuclei (AGN). The details of how GRBs 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 GRB models and properties of the burst environment. Similarly, measurements of the VHE gamma-ray emission from AGN probe properties of the AGN jet, providing crucial information about the particles being accelerated and how the acceleration works. This work has wide-ranging broader impacts, particularly in the areas of education and technological infrastructure. Graduate and undergraduate students, including students from underrepresented groups, play an important part in the research effort. The researchers interact with the California State University to foster undergraduate interest in research and graduate education. They will work with their VERITAS collaborators to develop exhibits on gamma-ray astrophysics for use at science museums and observatory visitors' centers. There is an important interplay between the VERITAS data and data from other wavelengths, particularly X-ray and lower energy gamma-ray data from satellite missions such as Swift and Fermi. More can be learned from both types of data used together than from either alone.