Letter to the Editor

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

¹ Laboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, 34095 Montpellier, France e-mail: Richard.Britto@lpta.in2p3.fr
² CENBG, Université Sciences et Techniques Bordeaux 1, CNRS/IN2P3, 33175 Gradignan, France
³ Laboratoire de Physique Appliquée et Automatique, Université de Perpignan “via Domitia”, 66860 Perpignan, France
⁴ AstroParticule et Cosmologie, Université Denis Diderot-Paris 7, CNRS/IN2P3, CEA, Observatoire de Paris, 75205 Paris, France

Received: 5 December 2008
Accepted: 23 February 2009

Abstract

Context. Located in the French Pyrénées, 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 (E > 140 GeV) = 9.410^-3 m^-2 s^-1 sr^-1 and (E > 120 GeV) = 1.210^-2 m^-2 s^-1 sr^-1.