Letter to the Editor

Detection of the BL Lac object 1ES 1426+428 in the Very High Energy gamma-ray band by the CAT Telescope from 1998–2000

¹ Physique Corpuscul aire et Cosmologie, IN2P3/CNRS, Collège de France et Université Paris VII, 75231 Paris Cedex 05, France
² Laboratoire Leprince-Ringuet, IN2P3/CNRS, École Polytechnique, 91128 Palaiseau, France
³ Centre d'Études Spatiales des Rayonnements, INSU/CNRS, Université Paul Sabatier, 31028 Toulouse, France
⁴ Groupe de Physique Fondamentale, Université de Perpignan, 66860 Perpignan, France
⁵ Service d'Astrophysique, CEA/DSM/DAPNIA, Centre d'Études de Saclay, 91191 Gif-sur-Yvette, France
⁶ Laboratoire de Physique Nucléaire et de Hautes Énergies, IN2P3/CNRS, Universités Paris VI/VII, 75252 Paris Cedex 05, France
⁷ Institute of Particle and Nuclear Physics, Charles University, 11636 Prague, Czech Republic

Corresponding author: A. Djannati-Ataï, djannati@in2p3.fr

Received: 18 June 2002
Accepted: 12 July 2002

Abstract

The BL Lac Object 1ES 1426+428, at a red-shift of , has been monitored by the Cat telescope from February 1998 to June 2000. The accumulation of 26 h of observations shows a γ-ray signal of 321 events above at 5.2 standard deviations, determined using data analysis cuts adapted to a weak, steep-spectrum source. The source emission has an average flux of , and a very steep spectrum, with a differential spectral index of which can be refined to using a higher flux data subset. If, as expected from its broad-band properties, the Very High Energy emission is hard at the source, these observations support a strong absorption effect of γ-rays by the Intergalactic Infrared field.