Radio luminosities and classificatory criteria of BL Lacertae objects

Using the sample of radio-selected BL Lacertae objects (RBLs) and X-ray selected BL Lacertae objects (XBLs) presented by Sambruna et al. (1996), we calculated the luminosities in radio, optical and X-ray of each source and made a statistical analysis among the luminosities at different wave-bands, broad-band spectral indices from radio to X-ray (alpha(rx)) and peak frequencies (nu(p)). Our results are as follows: (i) there is a positive correlation between radio luminosity L-r and alpha(rx) and a negative correlation between L-r and nu(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/s) and alpha(rx) >(or less than or equal to)0.75 for log L-r-alpha(rx) plot and those of log L-r less than or equal to 43.25 (erg/s) and log nu(p) > 14.7 for the log L-r-log nu(p) plot; (ii) there is a weak positive correlation between optical luminosity L-o and alpha(rx) and a negative weak correlation between L-o and nu(p); (iii) there is no correlation between X-ray luminosity L-x and alpha(rx) or between L-x and nu(p). From our analysis, we find that synchrotron radiation is the main X-ray radiation mechanism for HBLs while inverse Compton scattering is dominant for LBLs.