ROSAT OBSERVATIONS OF RADIO-SELECTED BL LAC OBJECTS

ROSAT observations of a homogeneous sample of radio-selected BL Lac objects are presented. Results of a detailed spectral analysis in the soft similar to 0.1-2.0 keV energy range are discussed and compared with similar previously published results. The X-ray spectral shape is discussed in relation to the overall energy distribution, with particular emphasis on the high-energy gamma-ray emission recently detected by the Compton Gamma-Ray Observatory (CGRO) EGRET detector for about half of the objects in the sample. Dividing the objects in our sample on the basis of the radio-to-X-ray flux ratio (alpha(rx)), we show that objects with alpha(rx) > 0.75 have flatter X-ray spectra and are likely to be stronger gamma-ray emitters than objects with alpha(rx) < 0.75. Moreover, we note that the value of the peak energy of the synchrotron component, in a nu-nu f(nu), plot, correlates with alpha(rx) and alpha(x).