The RGB sample of intermediate BL Lacertae objects

Combining newly identified and previously known BL Lacertae objects from th e ROSAT All-Sky Survey-Green Bank (RGB) catalog, we present a sample of 127 BL Lacertae objects, the largest ever derived from a single uniform survey . A complete sample of 33 objects brighter than O = 18.0 mag is also presen ted. These samples are compared to other known BL Lac samples and are gener ally found to exhibit properties intermediate between those of the previous ly disparate classes of high- and low-energy-peaked BL Lacertae objects (HB Ls and LBLs, respectively). This result is most dramatic in the distributio n of the X-ray to radio logarithmic flux ratios, where the KGB BL Lacertae objects are shown to peak precisely where the sharp dichotomy between the t wo subclasses was previously seen. The alpha(ro) versus alpha(ox) diagram a lso shows the RGB sample smoothly bridges the gap between the previously di stinct subclasses of LBLs and HBLs. The range of broadband spectral energy distributions (SEDs) exhibited by the RGB objects also shows that, contrary to prior claims, searches based on relatively deep surveys cannot limit fo llow-up spectroscopy to targets with a narrow range of SEDs since BL Lacert ae objects clearly constitute a homogeneous population with a wide range of SEDs. As in results based on the Einstein Extended Medium Sensitivity Surv ey (EMSS) and 1 Jy BL Lac samples, we find a weak but statistically signifi cant correlation between the composite X-ray spectral index alpha(xox) and the radio-optical spectral index alpha(ro). This implies that the more LBL- like RGB BL Lacertae objects have secondary sources of X-ray emission, poss ibly from inverse Compton components. This result, in addition to other cha racteristics of the RGB sample, indicates that the simple unified scheme, w hich postulates that HBLs and LBLs differ solely by orientation, may be in need of revision. We also present both the X-ray and radio log N-log S dist ributions for which the competing HBL/LBL unification scenarios have differ ing predictions. The unknown effects of the triple flux limit inherent ill the RGB Complete sample makes quantitative analysis uncertain, but the char acteristics of the RGB sample compare well both with results obtained from previous samples and with general theoretical predictions based on a simple Monte Carlo simulation. Our analysis indicates that the unimodal distribut ion of BL Lac properties found in the RGB sample likely reliably reflects t he underlying population, while the bimodal distribution found in earlier s tudies arose primarily from observational selection effects. The presence o f not only intermediate but also extreme HBL and LBL objects is the KGB sur vey's unique strength and offers clear avenues for future studies that can undoubtedly address the question of how HBLs and LBLs are related.