Massaro, F.F.MassaroTramacere, A.A.TramacereCavaliere, A.A.CavalierePerri, MatteoMatteoPerriGiommi, PaoloPaoloGiommi2020-09-172020-09-172008-02-01https://hdl.handle.net/20.500.13025/958Astronomy and Astrophysics, Volume 478, Issue 2, February I 2008, pp.395-401Context 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<SUB>p</SUB> increasing with the position E<SUB>p</SUB> 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<SUB>p</SUB> increases; this points toward statistical/stochastic acceleration processes for the emitting electrons. <BR />Aims We analyse X-ray spectra of several TeV HBLs to pinpoint their behaviours in the E_p-S<SUB>p</SUB> and E_p-b planes and to compare them with Mrk 421. <BR />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. <BR />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 E_p-S<SUB>p</SUB> plane, while PKS 2155-304 differs. As for the E_p-b plane, all TeV HBLs follow a similar behaviour. <BR />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. Appendix A is only available in electronic form at http //www.aanda.orggalaxies activegalaxies BL Lacertae objects generalX-rays galaxiesradiation mechanisms non-thermal10.1051/0004-6361 20078639http://adsabs.harvard.edu/abs/2008A&A...478..395M2008A&A...478..395M