Astronomy and Astrophysics Radio Luminosities and Classificatory Criteria of Bl Lacertae Objects

Using the sample of radio selected BL Lacer-tae objects (RBLs) and X-ray selected BL Lacertae objects (XBLs) presented by Sambruna et al. (1996), we calculated the luminosities of radio, optical and X-ray of each source and made the statistical analysis among the luminosities at different wave-bands, broad-band spectral indices from radio to X-ray (α rx) and peak frequencies (ν p). Our results are as follows: (i) there is a positive correlation between radio luminosity L r and α rx and a negative correlation between L r and ν p. High-energy peak BL Lacs (HBLs) and low-energy peak BL Lacs (LBLs) can be distinguished very well, the dividing lines are probably those of log L r = 43.25 (erg/sec) and α rx >(or ≤)0.75 for L r-α rx plot and those of log L r ≤ 43.25 (erg/sec) and log ν p > 14.7 for the L r-ν p plot; (ii) there is a weak positive correlation between optical luminosity L o and α rx and a negatively weak correlation between L o and ν p ; (iii) there is no correlation between X-ray luminosity L X and α rx or between L X and ν p. From our analysis, we find that synchrotron radiation is the main X-ray radiation mechanism for HBLs while inverse Compton scattering for LBLs. A&A manuscript no. (will be inserted by hand later) Your thesaurus codes are: missing; you have not inserted them Most BL Lac objects have been identified in either radio survey (radio-selected BL Lac objects (RBLs) or X-ray survey (X-ray-selected BL Lac objects (XBLs)). These two kinds of BL Lac objects exist many significant differences(for example see, Ledden, O'Dell, 1985; Ghisellini et al.1986; Stocke et al. 1991). The main differences between RBLs and XBLs are as follows. First, there is a difference in spectral energy distributions (SEDs), RBL–like objects show α rx > 0.75, while XBL-like objects show α rx ≤ 0.75 (see, It should be pointed out that Padovani & Giommi (1995) introduced the distinction between high-energy peak BL Lacs (HBLs) and low-energy peak BL Lacs (LBLs), for objects which emit most of their synchrotron power at high (UV-soft-X) and low (far-IR, optical) frequencies respectively. However, a quantitative distinction can be drawn on the basis of the ratio between radio and X-ray fluxes. It can also use the broad-band spectral index α rx to distinguish HBLs and LBLs: α rx ≤ 0.75 …