Peak frequency of the synchrotron emission and classificatory criteria of BL Lacertae objects

In this short paper, we quantitatively consider how the peak frequency of the synchrotron emission can be used to distinguish RBLs and XBLs. A sample of RBLs and XBLs presented by Sambruna et al. (1996, AAA065.159.120) was employed and the peak frequency, nu(p), of the synchrotron emission of each source was calculated by considering the proper physical origin of the X-ray emission for different classes of BL Lac objects. A plot of alpha(rx)-log nu(p) can be divided into four different regions by lines of alpha(rx) = 0.75 and log nu(p) = 14.7 +/- 10.4. All RBL-like objects, defined by alpha(rx) > 0.75, are located in the upper-left region, while all XBL-like objects, defined by alpha(rx) less than or equal to 0.75, are inside the lower-right region. No sources are in the lower-left and upper-right regions, suggesting that the classificatory criteria in terms of the peak frequency should be log nu(p) = 14.7 +/- 0.4. This provides evidence supporting what Giommi et al. (1995, AAA063.158.163) proposed: RBL-like and XBL-like objects can be distinguished by the difference in the peak frequency of the synchrotron emission. The sometime confusing and observationally dependent RBL-XBL terminology should be replaced by the more physical LBL-HBL one.