SIMBAD references

We present multifrequency spectra of a large number of radio and X-ray selected BL Lacertae objects constructed using non-simultaneous archival data. The data were obtained using the European Space Information System (ESIS) and are from several radio and optical catalogues, the IRAS Faint Source Catalogue, the Einstein and the EXOSAT databases. The sample includes 121 BL Lacs that have been extracted from the 1Jy and the S4 radio surveys (Stickel et al. 1991; Stickel & Kuehr 1994), the Einstein IPC Slew Survey, the Einstein Extended Medium Sensitivity Survey (EMSS), the EXOSAT High Galactic Latitude Survey, and from the compilations of Giommi et al. (1990), and Veron & Veron (1993). We find that the shape of the radio to infra-red spectrum of Radio Selected and X-ray Selected BL Lacs is very similar. The difference between these two classes of objects is instead evident in the optical/X-ray part of the spectrum. The classical radio discovered BL Lacs are characterized by an energy spectrum with a sharp cutoff in the IR/optical band while in most of the X-ray discovered objects the turnover is located near the UV/X-ray band or at higher frequencies. For a given X-ray flux this diversity can give rise to radio fluxes different by a factor of 100 or more. We argue that BL Lac objects may be a single population of sources characterized by a wide range of energy cutoffs. In this scenario BL Lacs discovered in radio surveys are representative of the entire population, while objects characterized by an energy break near the X-ray band, which are abundantly detected at X-ray frequencies, are intrinsically a small minority.