Parsec-scale radio structure of four X-ray-selected BL Lacertae objects

We have made first-epoch, polarization-sensitive 5 GHz VLBI observations of four X-ray-selected BL Lacertae objects (XBLs) from the HEA0 1 catalog, as well as one emission-line active galactic nucleus (AGN) originally classified as an HEA0 1 XBL. All sources show a core-jet morphology similar to that observed in radio-selected BL Lacertae objects (RBLs). We detect polarized features in the jets of two XBLs with some indications that the parsec-scale magnetic field orientation in XBLs differs from that usually seen in RBLs. It is commonly believed that BL Lacertae objects are intrinsically low-luminosity radio galaxies with relativistic jets oriented close to the line of sight. However, the ratio of the parsec-scale core and kiloparsec-scale extended radio fluxes in XBLs and RBLs differ in detail from the expectations of this unified scheme and simple beaming models. Our VLBI images show that the jets in XBLs fade more quickly than in RBLs, which also suggests the importance of factors other than orientation in distinguishing XBLs and RBLs.