Cutoff in the TeV Energy Spectrum of Markarian 421 During Strong Flares in 2001

Exceptionally strong and long-lasting flaring activity of the blazar Mrk 421 occurred between 2001 January and March. Based on the excellent signal-to-noise ratio of the data, we derive the energy spectrum between 260 GeV and 17 TeV with unprecedented statistical precision. The spectrum is not well described by a simple power law even with a curvature term. Instead, the data can be described by a power law with exponential cutoff: dN/dE ∝ Ee m-2 s-1 TeV-1 with E0 = 4.3 ± 0.3stat TeV. Mrk 421 is the second γ-ray blazar that unambiguously exhibits an absorption-like feature in its spectral energy distribution at 3-6 TeV.