Radio Morphology and Parent Popultion of X-Ray Selected BL Lacertae Objects

The radio structure of 15 x-ray selected BL Lacertae objects (XBLs) from the HEAO-1 Large Area Sky Survey has been studied with the Very Large 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 detections. Despite the many morphological similarities between Fanaroff- Riley (FR) class I radio galaxies and the HEAO-1 XBL population, we find 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 emission or kiloparsec-scale relativistic motion. A comparison of HEAO-1 XBLs with EINSTEIN XBLs, and radio selected BL Lacertae objects (RBLs) shows the XBLs have similar extended powers compared to FR I radio galaxies, but the RBLs have 5-10 times the extended radio power of typical FR I radio galaxies or XBLs, suggesting RBLs are intrinsically stronger radio sources. We find the ratios of the radio core power to the lobe power and the projected linear sizes of XBLs are intermediate between those of the FR I radio galaxies and RBLs. We also note that previous studies indicate HEAO-1 XBLs reside in poor clusters with approximately the same frequency as FR I radio galaxies. In general, these results are consistent with an FR I radio galaxy-BL Lacertae object unified scheme, where FR I radio galaxies are oriented near the plane of the sky, RBLs are oriented near the line of sight, and XBLs constitute an intermediate population.