Properties of Low Radio Luminosity RGB BL Lac Objects

Abstract

The ROSAT All-Sky Survey-Green Bank (RGB) BL Lac object sample presented by Laurent-Muehleisen et al. includes not only interesting intermediate but also extreme HBL and LBL sources. These unique characteristics make it possible to unambiguously address the question of how HBLs and LBLs are related. In this paper, we study the relations between the radio-X-ray spectral index (alpha(rx)) and each of the luminosities (L-r, L-o, L-x), as well as the relation between the X-ray spectral index ( alpha x) and the radio luminosity ( Lr) for this sample. Our analysis results indicate that (1) plots of (alpha(rx), L-i) ( i denotes r, o, or x) all exhibit the continuity of alpha(rx); ( 2) of 71 low-L-r RGB BL Lac objects (L-5 GHz < 10(25) W Hz(-1)), 24% have alpha(rx) steeper than 0.75, while 79% are distributed in the region 0: 6 < alpha(rx) < 0: 9 and have such a broad scatter in alpha(rx) that alpha(rx) can range from 0.55 to 0.8 for any given value of Lr; ( 3) of 60 low-Lr RGB BL Lac objects ( with alpha(x) available), 20% have alpha(x) flatter than 1, and no correlation is present for alpha(x) versus alpha(x). In addition, no correlation is found between L-o and alpha(rx) for the RGB BL Lac sample. These characteristics seem inconsistent with those derived from the earlier classic combined surveys. From our analysis, however, we also found that (1) there is a significant anticorrelation for L-x versus alpha(rx), which agrees with that reported by Mei et al., and ( 2) more importantly, alpha(rx) is well correlated with Lr, which is consistent with the result expected in the blazar sequence but in contradiction with that derived from the DXRBS sample. All of these seemingly odd results show the importance of sampling enough parameter space in order to obtain an unbiased view or model of blazar properties.