J/A+AS/109/267 J/A+AS/109/267 Radio to X-Ray distribution of BL Lac Objects P Giommi S G Ansari A Micol Astron. Astrophys. Suppl. Ser. 109 267 1995 1995A&AS..109..267G BL Lacertae objects: general radio continuum: galaxies X-rays: galaxies 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.

Name Source name --- Other Other names --- RAh Right ascension 2000.0 h RAm Right ascension 2000.0 min RAs Right ascension 2000.0 s DE- Declination sign --- DEd Declination 2000.0 deg DEm Declination 2000.0 arcmin DEs Declination 2000.0 arcsec Ref References number=1 1 Elvis at al. 1992 2 Gioia et al. 1990 3 Giommi et al., 1990 4 Giommi et al. 1991 5 Schachter et al. 1993 6 Stickel et al.1991 7 Stickel & Kuehr 1994 8 Stocke et al. 1991 9 Veron & Veron, 1993 --- Name Source name --- nu Frequency Hz SX Flux in the X-ray band, where the measurements are often near the instruments sensitivity limit mW/m2 e_SX rms uncertainty on flux mW/m2 Ref References where the original flux (or count rate) can be found number=1 1 Becker, White & Edwards, 1991 2 Bolton Savage & Wright, 1979 3 Elvis et al. 1992 4 Dixon 1976 5 Giommi et al. 1990 6 Giommi et al. 1991 7 Moshir 1991 8 Stocke et al. 1991 9 Stickel & Kuer 1994 10 Veron & Veron 1993 11 White & Becker 1992 12 Einstein IPC database 13 EXOSAT database --- Patricia Bauer CDS 1995Feb24 UNKNOWN UNKNOWN * Table 5 was incomplete before Feb. 24, 1995. J_A+AS_109_267.xml