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Most of the extragalactic sources detected at TeV energies are BL Lacobjects. They belong to the subclass of "high frequency peaked BL Lacs" (HBLs)exhibiting spectral energy distributions with a lower energy peak in the X-rayband; this is widely interpreted as synchrotron emission from relativisticelectrons. The X-ray spectra are generally curved, and well described in termsof a log-parabolic shape. In a previous investigation of TeV HBLs (TBLs) wefound two correlations between their spectral parameters. (1) The synchrotronpeak luminosity L_p increases with its peak energy E_p; (2) the curvatureparameter b decreases as E_p increases. The first is consistent with thesynchrotron scenario, while the second is expected from statistical/stochasticacceleration mechanisms for the emitting electrons. Here we present anextensive X-ray analysis of a sample of HBLs observed with XMM-Newton and SWIFTbut undetected at TeV energies (UBLs), to compare their spectral behavior withthat of TBLs. Investigating the distributions of their spectral parameters andcomparing the TBL X-ray spectra with that of UBLs, we develop a criterion toselect the best HBLs candidates for future TeV observations.

X-ray spectral curvature of High Frequency Peaked BL Lacs: a predictor for the TeV flux