A Ubiquitous Absorption Feature in the X-Ray Spectra of BL Lacertae Objects

We present the broad-band (0.5-20 keV) X-ray spectra of five X-ray bright BL Lac objects observed with the Einstein Observatory Solid State Spectrometer (SSS) and Monitor Proportional Counter (MPC) detectors. The combination of moderate energy resolution and broad spectral coverage allows us to confirm the presence of an absorption feature at an energy of ~ 650 eV in the BL Lac object PKS 2155 - 304, originally reported by Canizares & Kruper based on higher resolution Einstein Objective Grating Spectrometer (OGS) data. Furthermore, the SSS/MPC data indicate that such a feature, presumably due to O VIII Lyα resonant absorption, is present in all BL Lac objects in our sample. This suggests that such features are ubiquitous in at least X- ray-bright BL Lac objects.