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

Free access article

Issue		A&A Volume 391, Number 3, September I 2002


Page(s)		L25 - L28
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:20021034

A&A 391, L25-L28 (2002)
DOI: 10.1051/0004-6361:20021034

Letter

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

A. Djannati-Ataï¹, B. Khelifi¹, S. Vorobiov², R. Bazer-Bachi³, L. M. Chounet², G. Debiais⁴, B. Degrange², P. Espigat¹, B. Fabre⁴, G. Fontaine², P. Goret⁵, C. Gouiffes⁵, C. Masterson¹, F. Piron², M. Punch¹, M. Rivoal⁶, L. Rob⁷ and J.-P. Tavernet⁶

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

(Received 18 June 2002 / Accepted 12 July 2002 )

Abstract
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 $\gamma$ -ray signal of 321 events above $250\:{\rm GeV}$ 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 ${\Phi}_{\rm diff}({400\:{\rm GeV}})=6.73\pm1.27^{\rm stat}\pm1.45^{\rm syst}\times10^{-11}\:{\rm cm^{-2}\:s^{-1}\,TeV^{-1}}$ , and a very steep spectrum, with a differential spectral index of $\gamma=-3.60 \pm 0.57$ which can be refined to $\gamma=-3.66 \pm 0.41$ 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 $\gamma$ -rays by the Intergalactic Infrared field.

Key words: galaxies: active -- galaxies: nuclei -- BL Lacertæ objects: individual: 1ES 1426+428 -- gamma-rays: observations

Offprint request: A. Djannati-Ataï, djannati@in2p3.fr

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