The Properties of the X-Ray-Selected EMSS Sample of BL Lac Objects

Abstract

We present updated and complete radio, optical and X-ray data for BL Lacs in the Einstein Medium Sensitivity Survey (EMSS). The complete "M91" sample first presented in Morris et al. is updated to include 26 BL Lacs in total and we define a new, virtually complete sample consisting of 41 EMSS BL Lacs (the "D40" sample). New high signal-to-noise, arcsecond-resolution VLA observations are also presented for eleven EMSS BL Lacs, 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/v_max value for the M91 sample to 0.399\pm0.057; and v/v_max = 0.427\pm0.045 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 Lacs. The observed v/v_max, 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-1s seen at intermediate angles, as suggested by the unified model, if XBLs have moderate outflow velocities (gamma approx 5).