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

The "Gamma Ray Astronomy at ALmer´ıa" (GRAAL) experiment uses 63 heliostat-mirrors with a total mirror area of ≈ 2500 m2 from the CESA-1 field to collect Cherenkov light from airshowers. 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 . Data sets taken in the period September 1999 September 2000 in the direction of the Crab pulsar were analysed for high energy γ-ray emission. In an analysis searching for an excess of events from the direction of the source evidence for γ-ray flux from the Crab pulsar with an integral flux of 2.2 ± 0.4 (stat) +1.9 -1.5 (syst) × 10-9 cm-2 s-1 above threshold and a significance of 4.5 σ in a total (usable) observing time of 7 hours and 10 minutes on source was found. However, no evidence for an excess in the total counting rate was found. No evidence for emission from the other sources was seen.