The sedentary multifrequency survey - I. Statistical identification and cosmological properties of high-energy peaked BL Lacs

We have assembled a multifrequency data base by cross-correlating the NRAO VLA Sky Survey (NVSS) catalogue of radio sources with the RASSBSC list of s oft X-ray sources, obtaining optical magnitude estimates from the Palomar a nd UK Schmidt surveys as provided by the Automated Plate Measurement (APM) and COSMOS on-line services. By exploiting the nearly unique broad-band pro perties of high-energy peaked BL Lacs (HBL), we have statistically identifi ed a sample of 218 objects that is expected to include about 85 per cent of BL Lacs and that is therefore several times larger than all other publishe d samples of HBLs. Using a subset (155 objects) that is radio-flux-limited and statistically well-defined, we have derived the VVm distribution and th e log N-log S of extreme HBLs (f(x)f(r) greater than or equal to 3 x 10(-10 ) erg cm(-2) s(-1) Jy(-1)) down to 3.5 mJy. We find that the log N-log S fl attens around 20 mJy and that [VVm] = 0.42 +/- 0.02. This extends to the ra dio band earlier results, based on much smaller X-ray-selected samples, abo ut the anomalous cosmological observational properties of HBLs. A compariso n with the expected radio log N-log S of all BL Lacs (based on a beaming mo del) shows that extreme HBLs make up roughly 2 per cent of the BL Lac popul ation, independently of radio flux. This result, together with the flatness of the radio log N-log S at low fluxes, is in contrast with the prediction s of a recent model that assumes an anticorrelation between peak frequency and bolometric luminosity. This scenario would in fact result in an increas ing dominance of HBLs at lower radio fluxes; an effect that, if at all pres ent, must start at fluxes fainter than our survey limit. The extreme f(x)f( r) flux ratios and high X-ray fluxes of these BL Lacs makes them good candi date TeV sources; some of the brighter (and closer) ones are possibly detec table with the current generation of Cerenkov telescopes. Statistical ident ification of sources based on their location in multiparameter space, of th e kind described here, will have to become commonplace with the advent of t he many large, deep surveys at various frequencies currently scheduled or u nder construction.