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The BL Lac object (blazar) Mrk 421 was observed during its outburst in April2004 with the Whipple 10 m telescope for a total of about 24.5 hours. Themeasured gamma-ray rate varied substantially over the range from 4 to 10gamma's/min and eventually exceeded the steady gamma-ray rate of the CrabNebula (standard candle) by a factor of 3. The overall significance of thegamma-ray signal exceeded 70 sigma and the total number of excess events wasmore than 10,000. The signal light curve does not show any particularvariability pattern. This unique Mrk 421 outburst enabled the measurement of ahigh quality spectrum of very high-energy gamma rays in a high state ofemission. This spectrum is a power-law and it extends beyond 10 TeV.

The TeV Energy Spectrum of Mrk 421 Measured in A High Flaring State