Soft X-ray properties of a complete sample of radio-selected BL Lacertae objects

We report the soft X-ray properties of the complete 1 Jy sample of 34 radio-selected BL Lacertae objects (RBLs) as measured with the ROSAT Position Sensitive Proportional Counter. The 0.1-2.0 keV spectra of RBLs are generally well described by a single power-law model modified at low energies by absorption in our Galaxy. In a few brighter cases, either a convex or concave spectrum was detected. The distribution of the X-ray photon indices for the RBL sample is quite broad, 1 < Gamma < 3, with a measurable intrinsic dispersion. Comparing the ROSAT spectral index distributions of RBLs and other blazars, we find that RBLs have soft X-ray spectra similar to the X-ray-selected BL Lacertae objects (XBLs) of the EMSS complete sample, and both are steeper than more luminous (and more distant) strong emission-line blazars. Sorting the sources according to the ratio of their radio to X-ray fluxes into low-frequency peaked BL Lacertae objects (LBLs, which are mostly RBLs) and high-frequency peaked BL Lacertae objects (HBLs, which are mostly XBLs), the soft X-ray spectral index distributions for the two classes. are statistically different, with LBLs being flatter than HBLs. The ROSAT spectra of RBLs are steeper than those obtained previously with the Einstein Observatory IPC at slightly higher energies, suggesting that for these objects a flatter (Compton) component emerges at or above similar to 1 keV. The ROSAT data confirm that RBLs are variable X-ray sources, on timescales as short as weeks or even hours. Spectral variability was detected for three of the six sources having more than two pointed observations. Two of these are flatter in the fainter state, in contrast to the trend seen in previous X-ray studies of BL Lac objects (mostly XBLs).