The ROSAT X-ray spectra of BL Lacertae objects

We study the X-ray spectra of 85 BL Lacertae objects using the hardness ratios as given in the WGA catalogue of ROSAT sources. This sample includes all WGA BL Lacs with high-quality data and comprises about 50 per cent of presently known BL Lacs. We nd that BL Lacs have energy power-law spectral indices between 0 and 3 with a mean value x 1:4. Signi cant di erences, however, are present between high-energy cuto BL Lacs (HBLs), normally selected in the X-ray band, and lowenergy cuto BL Lacs (LBLs), generally found in radio surveys. HBLs have steeper X-ray spectral slopes ( x 1:5) well correlated with ox, and anti-correlated with the X-ray-to-radio ux ratio and cuto frequency, with convex overall broad-band spectra. LBLs, on the other hand, have atter X-ray spectra ( x 1:1) and concave opticalX-ray continuum. We interpret these results in terms of di erent mechanisms being responsible for the X-ray emission in the two classes, namely synchrotron and inverse Compton for HBLs and LBLs respectively. The observed di erences are consistent with the hypothesis that HBLs and LBLs are powered by essentially the same non-thermal engines di ering mainly in their synchrotron cuto energy.