Mrk 421, Mrk 501, and 1ES 1426+428 at 100 GeV with the CELESTE Cherenkov telescope

¹ Centre d'études nucléaires de Bordeaux Gradignan, CENBG UMR 5797 CNRS/IN2P3, Université Bordeaux 1, Chemin du Solarium, BP 120, 33175 Gradignan Cedex, France e-mail: smith@cenbg.in2p3.fr
² Laboratoire de physique théorique et astroparticules (UMR 5207 CNRS/IN2P3), Université Montpellier 2, 34095, France
³ Laboratoire Leprince-Ringuet (UMR 7638 CNRS/IN2P3), École Polytechnique, 91128 Palaiseau, France
⁴ Laboratoire de l'accélérateur linéaire (UMR 8607 CNRS/IN2P3), Université Paris 11, 91898 Orsay, France
⁵ Laboratoire d'astroparticule et cosmologie (UMR 7164 CNRS/IN2P3), Collège de France, 75231 Paris, France
⁶ Astrophysical Sciences Division, Bhabha Atomic Research Centre, Mumbai-400 085, India

Received: 22 June 2006
Accepted: 11 August 2006

Abstract

We have measured the gamma-ray fluxes of the blazars Mrk 421 and Mrk 501 in the energy range between 50 and 350 GeV (1.2 to Hz). The detector, called CELESTE, used first 40, then 53 heliostats of the former solar facility “Thémis” in the French Pyrénées to collect Cherenkov light generated in atmospheric particle cascades. The signal from Mrk 421 is often strong. We compare its flux with previously published multi-wavelength studies and infer that we are straddling the high energy peak of the spectral energy distribution. The signal from Mrk 501 in 2000 was weak (). We obtain an upper limit on the flux from 1ES 1426+428 of less than half that of the Crab flux near 100 GeV. The data analysis and understanding of systematic biases have improved compared to previous work, increasing the detector's sensitivity.