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 o bjects in the Einstein Medium Sensitivity Survey (EMSS). The complete "M91" sample first presented in Morris et al. is updated to include 26 BL Lac ob jects in total, and we define a new, virtually complete sample consisting o f 41 EMSS BL Lac objects (the "D40" sample). New high signal-to-noise ratio , arcsecond-resolution VLA observations are also presented for II EMSS BL L ac objects, completing VLA observations of the M91 sample. The addition of four new objects, as well as updated X-ray flux and redshift information, h as 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 conjunctio n with other studies of X-ray-selected BL Lac (XBL) samples, these results solidify negative evolution for XBLs, especially for more extreme high-ener gy-peaked BL Lac objects, for which we find (V-e/V-a) = 0.271 +/- 0.077. Th e 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 d ominated to be consistent with a beamed population of FR-l's seen at interm ediate angles, as suggested by the unified model, if XBLs have moderate out flow velocities (gamma similar to 1).