The RGB Sample of Intermediate BL Lacertae Objects

Abstract

Combining newly identiÐed and previously known BL Lacertae objects from the ROSAT All-Sky SurveyÈGreen 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 presented. These samples are compared to other known BL Lac samples and are generally found to exhibit properties intermediate between those of the previously disparate classes of high-and lowenergyÈpeaked BL Lacertae objects (HBLs and LBLs, respectively). This result is most dramatic in the distribution of the X-ray to radio logarithmic Ñux ratios, where the RGB BL Lacertae objects are shown to peak precisely where the sharp dichotomy between the two subclasses was previously seen. The a ro versus diagram also shows the RGB sample smoothly bridges the gap between the previously distinct a ox 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 follow-up spectroscopy to targets with a narrow range of SEDs since BL Lacertae objects clearly constitute a homogeneous population with a wide range of SEDs. As in results based on the Einstein Extended Medium Sensitivity Survey (EMSS) and 1 Jy BL Lac samples, we Ðnd a weak but statistically signiÐcant correlation between the composite X-ray spectral index and the radio-optical a xox spectral index This implies that the more LBL-like RGB BL Lacertae objects have secondary a ro . sources of X-ray emission, possibly from inverse Compton components. This result, in addition to other characteristics of the RGB sample, indicates that the simple uniÐed scheme, which postulates that HBLs and LBLs di †er solely by orientation, may be in need of revision. We also present both the X-ray and radio log N-log S distributions for which the competing HBL/LBL uniÐcation scenarios have di †ering predictions. The unknown e †ects of the triple Ñux limit inherent in the RGB Complete sample makes quantitative analysis uncertain, but the characteristics 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 distribution of BL Lac properties found in the RGB sample likely reliably reÑects the underlying population, while the bimodal distribution found in earlier studies arose primarily from observational selection e †ects. The presence of not only intermediate but also extreme HBL and LBL objects is the RGB surveyÏs unique strength and o †ers clear avenues for future studies that can undoubtedly address the question of how HBLs and LBLs are related.

Spectroscopy of Ultra–steep-Spectrum Radio Sources

“…This would be an obvious interpretation for most of our featureless spectra. However, the radio to optical Ñux ratio is several orders of magnitude higher than seen in BL Lac objects (Laurent-Muehleisen et al 1999), and we therefore reject this interpretation. Fan et al (1999) reported the discovery of SDSSp J53259.96[003944.1, an object at z \ 4.62, which shows all the characteristics of a quasar (unresolved optical morphology and except for the broad emission lines, M B \ [23), which are missing.…”

Multifrequency observations of a sample of very low frequency peaked BL Lacertae objects

“…The source was detected by ROSAT and is included both in the Faint Source Catalog (FSC, Voges et al 2000) and in the RGB sample (Laurent-Muehleisen et al 1999). Reich et al (2000) reported an integrated flux density of (9.66 ± 2.99) × 10 −13 erg cm −2 s −1 in the 0.1−2.4 keV band, with a power law photon index Γ = (2.9 ± 0.8).…”

Properties of optically selected BL Lacertae candidates from the SDSS

“…The former method favored BL Lacs with synchrotron peaks in the infrared range, while the latter favors targets with their synchrotron emission peaking in the UV to X-ray regime, leading to an apparent bimodality in source properties when viewed in the α ox -α ro plane (e.g., Padovani & Giommi 1995). Later surveys employing cross-correlations between radio and X-ray surveys (Laurent-Muehleisen et al 1999;Giommi et al 2005) have found intermediate targets filling the gap, but even these samples may be biased by the shallowness of the respective surveys and do not give the full picture of the BL Lac population. Nowadays, it is clear that the distribution of synchrotron peak frequencies is not bimodal (e.g., Abdo et al 2010), also resulting in continuous coverage in the α ox -α ro plane.…”