H.E.S.S. Observations of strong flaring activity of Mrk421 in February 2010

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 $\Gamma = 2.05,\pm,0.22$,anexponentialcutoff$E_{cut} = (3.4,\pm,0.6)$,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.