SWIFT observations of TeV BL Lacertae objects

Context. We present the results of a set of observations of nine TeV detected BL Lac objects performed by the XRT and UVOT detectors on board the Swift satellite between March and December 2005. Aims. Our main goal is the accurate measurement of the spectral shape of TeV detected BL Lacs. Particular attention was given to the presence of intrinsic spectral curvature in the X-ray band. Methods. To perform our X-ray spectral analysis we have assumed either a log-parabolic or a simple power-law model. Results. The X-ray data of many objects in our sample clearly show highly significant spectral curvature. However, in sources with spectral energy distribution peaked at energies lower than ∼0.1 keV the X-ray spectrum is steep and generally consistent with a power law. In most of these cases poor statistics did not allow us to obtain tight constraints on the spectral curvature. We have used UVOT observations to verify if X-ray spectra can be extrapolated to lower frequencies and to search for multiple emission components. Conclusions. The results of our analysis are useful for the study of possible signatures of statistical acceleration processes predicting intrinsically curved spectra and for modelling the SED of BL Lacertae objects up to TeV energies where a corresponding curvature is likely to be present.