The active galactic nucleus Markarian 421 (Mrk 421) is studied through the EAS-TOP extensive air shower array in the γ-ray energy ranges 20-50 TeV and 80-120 TeV. No evidence of d.c. emission is found in both datasets: the 90% flux limits are Φ(>25 TeV) < 1.3 · 10-13 cm-2 s-1 and Φ(>90 TeV) < 2.5 · 10-4 cm-2 s-1. A straight power law extrapolation up to E0 ≈ 1015 eV of the spectrum obtained from the observations by the EGRET and Whipple experiments - above 100 MeV and 0.5 TeV respectively - is excluded with a confidence level >99.99%, as expected from the absorption of VHE γ-rays by the 2.7K cosmic microwave background radiation. An indication (at ≈ 90% c.l.) of a steepening of the spectrum, with respect to such extrapolation, at γ-ray energies lower than expected from the 2.7K cutoff is derived. This could be due either to a break in the source spectrum or to the absorption of VHE γ-rays by intergalactic infrared photons of wavelength ≈ 100 μm.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics
- Nuclear and High Energy Physics