A New Sample of X-ray--Selected BL Lacertae Objects and Implications for Unified Schemes

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We present a well-defined sample of ~ 30 new BL Lacertae Objects discovered in the Rosat All-Sky Survey (RASS). This sample is the largest sample of X-ray--selected BL Lacertae Objects (XBLs) yet created from the RASS and increases the number of XBLs that were known prior to the Rosat mission by nearly 50%. The objects were discovered among the >2,000 sources with entries in both the RASS and GreenBank 5 GHz radio catalogs (Gregory & Condon 1991). Our followup high resolution VLA observations of these Rosat-GreenBank sources produced a database of subarcsecond positions and core radio fluxes which were correlated with POSS I plates, giving broadband radio, optical and X-ray colors. Following the method of Schachter et al. (1993), we concentrated our followup spectroscopic observations on objects with flat radio--optical and optical--X-ray spectral indices, although other objects were observed to assess biases in this selection criterion. All together 177 objects brighter than B ~ 18.5 were observed at low spectroscopic resolution. Approximately 60% of these sources are quasars, 15% are radio galaxies and 25% are BL Lacertae objects. We combine our newly discovered objects with previously known objects which match our selection criteria and present a well-defined sample of ~ 60 BL Lacertae objects. These BL Lacertae objects are presented on a LogN-LogS diagram and compared to previous results from other surveys and the implications for beaming models and cosmic evolution are discussed. Gregory & Condon, 1991, ApJS, 75, 1011 Schachter, et al. ApJ, 412, 541

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