X‐rays and relativistic beaming in radio‐selected BL Lacertae objects

We have used X‐ray and VLBI observations of twenty three BL Lac objects from the 1 Jy sample of radio bright sources to test models for the orgin of X‐ray emission in BL Lacs. We find a correlation between the X‐ray luminosity and the parsec scale radio emission, consistent with the idea that the X‐rays are relativistically beamed. A simple approximation for the synchrotron self‐Compton process has been used to estimate the minimum Doppler factor for bulk relativistic motion. Relativistic motion is implied for ∼75% of the sources, although there are no clear trends between the minimum Doppler factor and other traits. These results suggest that the soft X‐ray spectra do not contain a significant inverse‐Compton contribution, but this needs to be tested with more sophisticated models.