High zenith angle observations of PKS 2155-304 with the MAGIC-I
  telescope
  The high frequency peaked BL Lac PKS 2155-304 with a redshift of z=0.116 was
discovered in 1997 in the very high energy (VHE, E >100GeV) gamma-ray range by
the University of Durham Mark VI gamma-ray Cherenkov telescope in Australia
with a flux corresponding to 20% of the Crab Nebula flux. It was later observed
and detected with high significance by the Southern Cherenkov observatory
H.E.S.S. Detection from the Northern hemisphere is difficult due to challenging
observation conditions under large zenith angles. In July 2006, the H.E.S.S.
collaboration reported an extraordinary outburst of VHE gamma-emission. During
the outburst, the VHE gamma-ray emission was found to be variable on the time
scales of minutes and with a mean flux of ~7 times the flux observed from the
Crab Nebula. Follow-up observations with the MAGIC-I standalone Cherenkov
telescope were triggered by this extraordinary outburst and PKS 2155-304 was
observed between 28 July to 2 August 2006 for 15 hours at large zenith angles.
Here we present our studies on the behavior of the source after its
extraordinary flare and an enhanced analysis method for data taken at high
zenith angles. We developed improved methods for event selection that led to a
better background suppression. The averaged energy spectrum we derived has a
spectral index of -3.5 +/- 0.2 above 400GeV, which is in good agreement with
the spectral shape measured by H.E.S.S. during the major flare on MJD 53944.
Furthermore, we present the spectral energy distribution modeling of PKS
2155-304. With our observations we increased the duty cycle of the source
extending the light curve derived by H.E.S.S. after the outburst. Finally, we
find night-by-night variability with a maximal amplitude of a factor three to
four and an intranight variability in one of the nights (MJD 53945) with a
similar amplitude.
