Spectrum and variability of the Galactic Center VHE gamma-ray source
  HESS J1745-290
  A detailed study of the spectrum and variability of the source HESS J1745-290
in the Galactic Center (GC) region using new data from the H.E.S.S. array of
Cherenkov telescopes is presented. Flaring activity and Quasi Periodic
Oscillations (QPO) of HESS J1745-290 are investigated. The image analysis is
performed with a combination of a semi-analytical shower model and the
statistical moments based Hillas technique. The spectrum and lightcurves of
HESS J1745-290 are derived with a likelihood method based on a spectral shape
hypothesis. Rayleigh tests and Fourier analysis of the H.E.S.S. GC signal are
used to study the periodicity of the source. With three-fold increase in
statistics compared to previous work, a deviation from a simple power law
spectrum is detected for the first time. The measured energy spectrum over the
three years 2004, 2005 and 2006 of data taking is compatible with both a power
law spectrum with an exponential cut-off and a broken power law spectrum. The
curvature of the energy spectrum is likely to be intrinsic to the photon source
as opposed to effects of interstellar absorption. No significant flux variation
is found. Increases in the gamma-ray flux of HESS J1745-290 by at least a
factor of two would be required for a 3 sigma detection of a flare of time
scales of an hour. Investigation of possible QPO activity at periods claimed to
be detected in X-rays does not show any periodicities in the H.E.S.S. signal.
