Very high-energy observations of the Crab nebula with the GRAAL experiment

Abstract

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 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 gamma-ray emission. Evidence for gamma-ray flux from the Crab pulsar with an integral flux of 2.2 +- 0.4 (stat) $^{+1.9}_{-1.5}$ (syst) x 10$^{-9}$ cm$^{-2}$ s$^{-1}$ above threshold and a significance of 4.5 sigma in a total (usable) observing time of 7 hours and 10 minutes on source was found. No evidence for emission from the other sources was seen. The effect of field-of-view restricted to the central part of a detected airshower on the lateral distribution and timing properties of Cherenkov light and their effect on an efficient gamma-hadron separation are discussed.