2001MNRAS.328..931P

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 ∼50keV 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 (α_X∼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 ∼ 1-3 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 (Δα_X∼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, ν_peak, we confirm and clarify the dependence of the X-ray spectral index on ν_peak originally found in ROSAT data.