Search for diffuse gamma rays from the Galactic anticentre region above 100 GeV with CELESTE

Context. Located in the French Pyrenees, CELESTE was the first ground-based γ-ray telescope with an energy threshold below 100 GeV. It acquired data from 1999 to 2004, and allowed flux measurements of the Crab nebula and the blazars Mrk 421 and Mrk 501. Aims. We search for Galactic diffuse γ-ray emission, which is most significant around the Galactic plane, for b = [−5 ◦ , +5 ◦ ]. Methods. By using the significant data set available for the Crab nebula, we selected Crab OFF-source data at various Galactic latitudes, in order to analyse the diffuse emission. Selection criteria were applied to the sky position, atmospheric conditions, and detector stability. Results. We obtained 108 mn of data in the Galactic anticentre region, providing the first upper limits of around 100 GeV to the diffuse γ-ray emission with atmospheric Cherenkov detectors. These limits are φ UL (E > 140 GeV) = 9.4 × 10 −3 m −2 s −1 sr −1 and φ UL (E > 120 GeV) = 1.2 × 10 −2 m −2 s −1 sr −1 .