XMM-Newton and Swift observations of High Frequency Peaked BL Lacs in the TeV era

Most of TeV detected extragalactic sources are BL Lac objects. They belong to the subclass of "high frequency peaked BL Lacs" (HBLs) exhibiting their spectral energy distributions with a lower energy peak in the X-ray band,interpreted as synchrotron emission. The X-ray spectra are generally curved, and well described in terms of a log-parabolic shape. In a previous investigation of TeV HBLs (TBLs) we found two correlations between their spectral parameters: (1) The synchrotron peak luminosity L_p increases with its peak energy E_p, (2) the curvature parameter b decreases as E_p increases. The first is consistent with the synchrotron scenario, while the second is expected with statistical/stochastic acceleration mechanisms for the emitting particles. Here, we present an extensive X-ray analysis of a sample of HBLs observed with XMM Newton and SWIFT but undetected at TeV energies (UBLs), to compare their spectral behavior with that of TBLs. We investigate the distributions of their spectral parameters, providing an interpretation of both the E_p and b distributions in terms of systematic and stochastic acceleration processes. We also compare the X-ray spectral behavior of UBLs with that in gamma-rays and propose a criterion to select the best HBLs candidates for future TeV observations.