RADIO MORPHOLOGY AND PARENT POPULATION OF X-RAY SELECTED BL LACERTAE OBJECTS

The radio structure of 15 x-ray selected BL Lacertae objects (XBLs) fr om the HEAO-1 Large Area Sky Survey has been studied with the Very Lar ge Array at 1.5 and 5 GHz and with resolutions between 0.4'' and 33''. Extended emission is seen in 12 objects, 4 of which are first-time de tections. Despite the many morphological similarities between Fanaroff -Riley (FR) class I radio galaxies and the HEAO-1 XBL population, we f ind the XBLs exhibit fewer ''classical double'' and more ''amorphous'' morphologies. Both samples contain comparable fractions of one-sided morphologies, which could be evidence for intrinsically one-sided emis sion or kiloparsec-scale relativistic motion. A comparison of HEAO-1 X BLs with EINSTEIN XBLs, and radio selected BL Lacertae objects (RBLs) shows the XBLs have similar extended powers compared to FR I radio gal axies, but the RBLs have 5-10 times the extended radio power of typica l FR I radio galaxies or XBLs, suggesting RBLs are intrinsically stron ger radio sources. We find the ratios of the radio core power to the l obe power and the projected linear sizes of XBLs are intermediate betw een those of the FR I radio galaxies and RBLs. We also note that previ ous studies indicate HEAO-1 XBLs reside in poor clusters with approxim ately the same frequency as FR I radio galaxies. In general, these res ults are consistent with an FR I radio galaxy-BL Lacertae object unifi ed scheme, where FR I radio galaxies are oriented near the plane of th e sky, RBLs are oriented near the line of sight, and XBLs constitute a n intermediate population.