2002A&A...391L..25D

The BL Lac Object 1ES 1426+428, at a red-shift of z=0.129, 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 250GeV 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 Φ_diff(400GeV)=6.73±1.27^stat±1.45^systx10^–11cm^–2.s^–1.TeV^–1, and a very steep spectrum, with a differential spectral index of γ=-3.60±0.57 which can be refined to γ=-3.66±0.41 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.