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 White, Giommi & Angelini (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 find that BL Lacs have energy power-law spectral indices between 0 and 3 with a mean value alpha(x) similar to 1.4. Significant differences, however, are present between high-energy cut-off BL Lacs (HBLs), normally selected in the X-ray band, and low-energy cut-off BL Lacs (LBLs), generally found in radio surveys. HBLs have steeper X-ray spectral slopes (alpha(x) similar to 1.5) well correlated with alpha(ox), and anti-correlated with the X-ray-to-radio flux and cut-off frequency, with convex overall broad-band spectra. LBLs, on the other hand, have flatter X-ray spectra (alpha(x) similar to 1.1) and concave optical-X-ray continuum. We interpret these results in terms of different mechanisms being responsible for the X-ray emission in the two classes, namely, synchrotron and inverse Compton for HBLs and LBLs, respectively. The observed differences are consistent with the hypothesis that HBLs and LBLs are powered by essentially the same non-thermal engines differing mainly in their synchrotron cut-off energy.