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 proportional to E(-2.14 +/-0.03 stat)e(-E/E0) m(-2) s(-1) TeV-1 with E0=4.3 +/-0.3(stat) TeV. Mrk 421 is the second gamma -ray blazar that unambiguously exhibits an absorption- like feature in its spectral energy distribution at 3-6 TeV.