UV imaging of extensive air showers at TeV energies.

Abstract

This work represents the first successful detection of a flux of cosmic gamma rays at TeV energies by an air-Cherenkov imaging telescope from observations made during periods of bright moon light. The detection is based on two years of observations made on the Crab Nebula, a known source of TeV gamma rays, during periods of bright moon light using the Whipple Observatory's 10-meter imaging air Cherenkov telescope outfitted with a camera with spectral sensitivity restricted to UV wavelengths (below 300nm) in place of the conventional visible light sensitive camera. A UV-sensitive camera is obtained by using solar-blind photomultiplier tubes and a UV filter in place of the visible light sensitive photomultiplier tubes in the Whipple camera. The observations result in a detection of a gamma-ray flux from the Crab Nebula at a statistical significance of 5.4 sigma. The performance of this UV-sensitive camera is discussed with particular emphasis on the effects of operating in a bright moonlit environment. The observed signal is shown to behave in a manner which is consistent with what is expected from a gamma-ray flux. Using Monte Carlo simulations, the energy threshold of the camera is estimated to be 0.9TeV and the integral flux derived from this detection is found to be in good agreement with other measurements. Using the experience gained from working with the UV images a second approach to observing in the presence of the moon is tested utilizing the visible sensitive photomultiplier tubes from the conventional Whipple camera with a UV filter to attenuate, but not eliminate, light above 300nm. With this hybrid camera a flux of gamma rays from the Crab Nebula is detected at the 5 sigma level at an estimated energy threshold of 0.7TeV. These two detection's of the Crab Nebula establish the feasibility of utilizing the air-Cherenkov imaging technique during periods of bright moon light, a new capability that would increase the duty cycle of observation.