The Deep X-ray Radio Blazar Survey (DXRBS) - II. New identifications

We have searched the archived, pointed ROSAT Position Sensitive Proportional Counter data for blazars by correlating the WGACAT X-ray data base with several publicly available radio catalogues, restricting our candidate list to serendipitous X-ray sources with a flat radio spectrum (alpha (r) less than or equal to 0.70, where S-nu proportional to nu (-alpha)). This makes up the Deep X-ray Radio Blazar Survey (DXRBS), Here we present new identifications and spectra for 106 sources, including 86 radio-loud quasars, 11 BL Lacertae objects, and nine narrow-line radio galaxies. Together with our previously published objects and already-known sources, our sample now contains 298 identified objects: 234 radio-loud quasars [181 flat-spectrum quasars: FSRQ (alpha (r) less than or equal to 0.50) and 53 steep-spectrum quasars: SSRQ], 36 BL Lacs and 28 narrow-line radio galaxies. Redshift information is available for 96 per cent of these. Thus our selection technique is similar to 90 per cent efficient at finding radio-loud quasars and BL Lacs. Reaching 5-GHz radio fluxes similar to 50mJy and 0.1-2.0 keV X-ray fluxes a few x 10(-14) erg cm(-2) s(-1), DXRBS is the faintest and largest flat-spectrum radio sample with nearly complete (similar to 85 per cent) identification. We review the properties of the DXRBS blazar sample, including redshift distribution and coverage of the X-ray-radio-power plane for quasars and BL Lacs. Additionally, we touch upon the expanded multiwavelength view of blazars provided by DXRBS. By sampling for the first time the faint end of the radio and X-ray luminosity functions, this sample will allow us to investigate the blazar phenomenon and the validity of unified schemes down to relatively low powers.