Very high-energy gamma-ray observations of the Crab nebula and other potential sources with the GRAAL experiment

The Gamma Ray Astronomy at ALmeria'' (GRAAL) experiment uses 63 heliostat-mirrors with a total mirror area of ~ 2500 m2 from the CESA-1 field at the Plataforma Solar de Almeria’’ (PSA) to collect Cherenkov light from air showers. The detector is located in a central solar tower and detects photon-induced showers with an energy threshold of 250 ± 110 GeV and an asymptotic effective detection area of about 15000 m2. A comparison between the results of detailed Monte-Carlo simulations and data is presented. Data sets taken in the period September 1999 - September 2000 in the direction of the Crab pulsar, the active galaxy 3C 454.3, the unidentified gamma-ray source 3EG 1835+35 and a ``pseudo source’’ were analyzed for high energy gamma-ray emission.Evidence for a gamma-ray flux from the Crab pulsar with an integral flux of 2.2 ± 0.4 (stat) ^+1.7_-1.3 (syst) x 10^-9 cm^-2 sec^-1 above threshold and a significance of 4.5 sigma in a total measuring time of 7 hours and 10 minutes on source was found. No evidence for emission from the other sources was found. Some difficulties with the use of heliostat fields for gamma-ray astronomy are pointed out. In particular the effect of 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. Upon restriction the spread of the timing front of proton induced showers sharply decreases and the reconstructed direction becomes biased towards the pointing direction. This is shown to make efficient gamma-hadron separation difficult.