SIMBAD references

The synchrotron peak of the X-ray bright High Energy Peaked Blazar PKS 2155-304 occurs in the UV-EUV region and hence its X-ray emission (0.6-10 keV) lies mostly in the falling part of the synchrotron hump. We aim to study the X-ray emission of PKS 2155-304 during different intensity states in 2009-2014 using the XMM-Newton satellite. We studied the spectral curvature of all of the observations to provide crucial information on the energy distribution of the nonthermal particles. Most of the observations show curvature or deviation from a single power law and can be well modeled by a log parabola model. In some of the observations, we find spectral flattening after 6 keV. In order to find the possible origin of the X-ray excess, we built the Multiband Spectral Energy distribution. We find that the X-ray excess in PKS 2155-304 is difficult to fit in the one zone model but, could be easily reconciled in the spine/layer jet structure. The hard X-ray excess can be explained by the inverse Comptonization of the synchrotron photons (from the layer) by the spine electrons.