Observation of gamma rays greater than 10 TeV from Markarian 421

We have observed Markarian 421 in 2001 January and March with the CANGAROO-II imaging Cerenkov telescope during an extraordinarily high state at TeV energies. From 14 hr of observations at very large zenith angles, similar to70degrees, a signal of 298 +/- 52 gamma-ray-like events (5.7 sigma) was detected at E > 10 TeV, where a higher sensitivity is achieved than those of usual observations near the zenith, owing to a greatly increased collecting area. Under the assumption of an intrinsic power-law spectrum, we derived a differential energy spectrum dN/dE p (3.3 +/- 0.9(stat) +/- 0.3(syst)) x 10(-13) (E/10 TeV)(-(4.0+/-0.60.9, stat +/-0.3syst)) photons cm(-2) s(-1) TeV-1, which is steeper than those previously measured around 1 TeV and supports the evidence for a cutoff in the spectrum of Mrk 421. However, the 4 sigma excess at energies greater than 20 TeV in our data favors a cutoff energy of similar to8 TeV, at the upper end of the range previously reported from measurements at TeV energies.