Context :

Aims : 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 .

Methods : The image analysis is performed with a combination of a semi-analytical shower model and the statistical moment-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 .

Results : With a 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 .
The power law spectrum with an exponential cut-off is characterized by a photon index of 2.10 \pm 0.04 _ { \mathrm { stat } } \pm 0.10 _ { \mathrm { syst } } and a cut-off energy at 15.7 \pm 3.4 _ { \mathrm { stat } } \pm 2.5 _ { \mathrm { syst } } TeV .
The broken power law spectrum exhibits spectral indices of 2.02 \pm 0.08 _ { \mathrm { stat } } \pm 0.10 _ { \mathrm { syst } } and 2.63 \pm 0.14 _ { \mathrm { stat } } \pm 0.10 _ { \mathrm { syst } } with a break energy at 2.57 \pm 0.19 _ { \mathrm { stat } } \pm 0.44 _ { \mathrm { syst } } TeV .
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 with 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 .

Conclusions :