Published in:

Astrophysical Journal Letters 679,1 (2008) L13-L15;


The BL Lac object (blazar) Mrk 421 was observed during its outburst in 2004 April with the Whipple 10 m telescope for a total of about 24.5 hr. The measured gamma-ray rate varied substantially over the range from 4 to 10 minute(-1) and eventually exceeded the steady gamma-ray rate of the Crab Nebula (standard candle) by a factor of 3. The overall significance of the gamma-ray signal exceeded 70 sigma and the total number of excess events was more than 10,000. This unique Mrk 421 outburst enabled the measurement of a high-quality spectrum of very high energy gamma-rays in a high state of emission. This spectrum is a power law and it extends beyond 10 TeV.


BL Lacertae objects : individual (Markarian 421);; galaxies : active;; Astronomy & Astrophysics

Date of this Version

January 2008



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