The Properties of the X-Ray-selected EMSS Sample of BL Lacertae Objects

We present updated and complete radio, optical, and X-ray data for BL Lac objects in the Einstein Medium Sensitivity Survey (EMSS). The complete ``M91'' sample first presented in Morris et al. is updated to include 26 BL Lac objects in total, and we define a new, virtually complete sample consisting of 41 EMSS BL Lac objects (the ``D40'' sample). New high signal-to-noise ratio, arcsecond-resolution VLA observations are also presented for 11 EMSS BL Lac objects, completing VLA observations of the M91 sample. The addition of four new objects, as well as updated X-ray flux and redshift information, has increased the <V_e/V_a> value for the M91 sample to 0.399+/-0.057 and <V_e/V_a>=0.427+/-0.045 for the newly defined D40 sample. In conjunction with other studies of X-ray-selected BL Lac (XBL) samples, these results solidify negative evolution for XBLs, especially for more extreme high-energy-peaked BL Lac objects, for which we find <V_e/V_a>=0.271+/-0.077. The observed <V_e/V_a>, spectral, and radio properties of XBLs are completely consistent with being the beamed population of low-luminosity, FR-1 radio galaxies. However, our VLA observations do confirm that XBLs are too core dominated to be consistent with a beamed population of FR-1's seen at intermediate angles, as suggested by the unified model, if XBLs have moderate outflow velocities (γ~5).