ROSAT observations of BL Lacertae objects.

We present soft X-ray spectra of 74 BL Lacertae objects observed with the PSPC detector on board of the ROSAT satellite. The sample contains all BL Lac objects detected during the pointed observation phase as a target or serendipitously. We have investigated the soft X-ray and broad band spectral properties and discuss the consequences for the X-ray emission processes. For the first time a clear dependence of the X-ray spectral steepness on the radio to X-ray spectral energy distribution is found: αrx and αx are correlated in the X-ray selected (XBL) subsample and anticorrelated in the radio selected (RBL) subsample. The objects with intermediate αrx values thus do have the steepest soft X-ray spectra. Simulated PSPC spectra based on a set of simple two component multifrequency spectra are in good agreement with the measurements and suggest a broad range of synchrotron cutoff energies. We have calculated synchrotron self-Compton beaming factors for a subsample of radio bright objects and find a correlation of the beaming factors δIC with αrx and αx . The most extreme RBL objects are very similar to flat spectrum radio quasars in all their broad band and X-ray properties.