The high-frequency peaked BL Lac object Mrk421 has shown a strong outburst of activity during February 2010 .
This high state , first detected by VERITAS on 16th of February , was followed up by H.E.S.S .
during four subsequent nights at an average zenith angle of 62.4 ^ { \circ } .
In the first night of observations by H.E.S.S. , the average flux level above 1 TeV was 4 times the level of the Crab Nebula flux , decreasing to a third of this value over the subsequent observation nights .
The H.E.S.S .
data shows an energy spectrum compatible with previous measurements , extending up to 20 TeV .
The observed spectral shape can be well described by a powerlaw with a spectral index of \Gamma = 2.05 \pm 0.22 , an exponential cutoff E _ { cut } = ( 3.4 \pm 0.6 ) TeV and a flux normalization \phi _ { 0 } = ( 1.96 \pm 0.32 ) 10 ^ { -11 } cm ^ { -2 } s ^ { -1 } TeV ^ { -1 } at decorrelation energy E _ { 0 } = 2.739 TeV .
The observed integral flux varies on a night-by-night basis between 1.4 and 4.8 times the flux of the Crab Nebula .