X-ray spectral evolution of TeVBL Lacertae objects:: eleven years of observations with BeppoSAX, XMM-Newton and Swift satellites

Context. Many of the extragalactic sources detected in gamma rays at TeV energies are BL Lac objects. In particular, they belong to the subclass of high frequency peaked BL Lacs (HBLs), as their spectral energy distributions exhibit a first peak in the X-ray band. At a closer look, their X-ray spectra appear to be generally curved into a log-parabolic shape. In a previous investigation of Mrk 421, two correlations were found between the spectral parameters. One involves the height S-p increasing with the position E-p of the first peak; this was interpreted as a signature of synchrotron emission from relativistic electrons. The other involves the curvature parameter b decreasing as E-p increases; this points toward statistical/stochastic acceleration processes for the emitting electrons. Aims. We analyse X-ray spectra of several TeV HBLs to pinpoint their behaviours in the E-p - S-p and E-p - b planes and to compare them with Mrk 421. Methods. We perfom X-ray spectral analyses of a sample of 15 BL Lacs. We report the whole set of observations obtained with the BeppoSAX, XMM-Newton and Swift satellites between 29/06/96 and 07/04/07. We focus on five sources (PKS 0548-322, 1H 1426+418, Mrk 501, 1ES 1959+650, PKS 2155-304) whose X-ray observations warrant detailed searching of correlations or trends. Results. Within our database, we find that four out of five sources, namely PKS 0548-322, 1H 1426+418, Mrk 501 and 1ES 1959+650, follow similar trends as Mrk 421 in the Ep - S p plane, while PKS 2155-304 differs. As for the Ep - b plane, all TeV HBLs follow a similar behaviour. Conclusions. The trends exhibited by Mrk 421 appear to be shared by several TeV HBLs, such as to warrant discussing predictions from the X-ray spectral evolution to that of TeV emissions.