Abstract: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 find that BL Lacs have energy power-law spectral indices between 0 and 3 with a mean value α x ∼ 1.4. Significant differences, however, are present between high-energy cutoff BL Lacs (HBLs), normally selected in the X-ray band, and lowenergy cutoff BL Lacs (LBLs), … Show more
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