Ground-Based Astrophysics with the STACEE and Pierre Auger Observatories

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This proposal requests funds to support the Case Western Reserve University (CWRU) group to work on two astrophysics experiments. STACEE is a new detector for ground-based gamma-ray astronomy in the energy range 50 to 1000 GeV, corresponding to an energy regime that has been inaccessible to previous ground-based experiments. STACEE is located at the National Solar Thermal Test Facility (NSTTF) in Albuquerque, NM at Sandia National Laboratories and uses large heliostat mirrors at the NSTTF to collect Cherenkov light from gamma-ray air showers. Construction of the STACEE experiment is now complete, but during construction two major observing campaigns were conducted. In 1998, STACEE detected the Crab nebula with a significance in excess of six standard deviations. During the summer of 2001, STACEE detected the extra-galactic source Markarian 421 during an outburst. Over the next three years, STACEE will observe active galactic nuclei (AGN), gamma-ray bursters (GRB) and several other galactic and extra-galactic objects. With a low energy threshold, many more extra-galactic sources will be visible to STACEE that are unobservable at higher energies due to intergalactic gamma-gamma absorption. STACEE is perhaps uniquely capable of making extended observations of sources in the energy range from 50 to 250 GeV. Collaborators from CWRU have developed improved reconstruction techniques and are responsible for maintaining the STACEE optical system. The group at CWRU is also participating in a limited, but important, role in the Pierre Auger Observatory, a major international experiment to detect the highest energy cosmic rays. AUGER is a hybrid detector made of 1600 water-Cherenkov ground stations and five associated fluorescence detectors. A critical component of each ground station is the GPS clock used to time-tag the arrival of shower particles on the ground. The CWRU group is responsible for calibrating each of the 1600 GPS units that will be used in AUGER to timestamp events to within 20 ns absolute time. This work has a potential broader impact on several areas outside of particle astrophysics. These include (1) the use of heliostats to support remote sensing by earth-viewing satellites, (2) the design and application of new types of non-imaging optical systems, (3) the development of mirror measurement techniques for solar energy applications, and (4) the development of software for high-speed data acquisition.