The radio structure source of X-ray-selected BL Lacertae objects

Abstract

Arcsecond-size radio structure for a complete sample of 14 X-ray-selected BL Lac objects (XBLs) from the Einstein Extended Medium-Sensitivity Survey is presented. Most objects possess radio morphologies which are similar to those of nearby Fanaroff-Riley type 1 (FR 1) radio galaxies once the effects of surface brightness dimming and beam dilution as a function of redshift are taken into account. In order to test the beamed FR 1 hypothesis for BL Lac objects, the core and extended power levels are determined for these sources and compared with samples of radio-selected BL Lac objects (RBLs) and FR 1 from galaxies from the literature. RBLs and XBLs are found to possess extended (and thus unbeamed) power levels and the largest angular sizes similar to those of the FR 1 galaxies both supporting the beaming hypothesis and a common parent population for XBLs and RBLs.