Collecting heat during the day, observing the sky at night : the use of a heliostat field near Almeria to search for cosmic gamma rays
The GRAAL experiment (Gamma Ray Astronomy at ALmeria) is the result of the conversion of a solar power plant near Almeria into a Cherenkov telescope with a total mirror area of 2500 m^2 for very high energy gamma astronomy. The detector is located in a central solar tower and detects photon-induced showers with an energy threshold of 250 GeV and an effective detection area of about 15000 m^2. The aim of the experiment was the search for very high energy gamma sources. This thesis describes the installation of the detector and the results of its operation during more than 2 years. The methods developed for the Monte Carlo simulation and the analysis of the data are discussed. A special emphasis is put on the general problems encountered on the application of this new technique to the observation of gamma-rays common to all heliostat arrays. In particular, the effect of a field of view restricted to the central part of a detected air shower on the lateral distribution and timing properties of Cherenkov light are discussed. Under angular restriction the differences between gamma and hadron induced showers obliterate, making an efficient gamma-hadron separation difficult. More than 250 hours of usable data were taken with the GRAAL detector on 18 different sources, among them the Crab nebula, the blazar Markarian 421 and the gamma-ray burst GRB010222. Evidence for a gamma-ray flux from the direction of the Crab pulsar and Markarian 421 was found, proving the feasibility of solar arrays for the observation of gamma-ray sources.
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