BeppoSAX observations of 1-Jy BL Lacertae objects -: I

We present new BeppoSAX observations of seven BL Lacertae objects selected from the 1-Jy sample plus one additional source. The collected data cover the energy range 0.1-10 keV (observer's frame), reaching similar to 50 keV for one source (BL Lac). All sources characterized by a peak in their multifrequency spectra at infrared/optical energies (i.e., of the low-energy peaked BL Lac type, LBL) display a relatively flat (alpha (x) similar to 0.9) X-ray spectrum, which we interpret as inverse Compton emission. Four objects (two-thirds of the LBLs) show some evidence for a low-energy steepening, which is probably due to the synchrotron tail merging into the inverse Compton component around similar to1-3 keV. If this were generally the case with LBLs, it would explain why the 0.1-2.4 keV ROSAT spectra of our sources are systematically steeper than the BeppoSAX ones (Delta alpha (X) similar to 0.5). The broad-band spectral energy distributions fully confirm this picture, and a synchrotron inverse Compton model allows us to derive the physical parameters (intrinsic power, magnetic field, etc.) of our sources. Combining our results with those obtained by BeppoSAX on BL Lacs covering a wide range of synchrotron peak frequency, nu (peak), we confirm and clarify the dependence of the X-ray spectral index on nu (peak) originally found in ROSAT data.