The Connection between X-Ray-- and Radio-selected BL Lacertae Objects
The Connection between X-Ray-- and Radio-selected BL Lacertae Objects
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DOI：
10.1086/175631
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发表时间：
1994-12
期刊：
影响因子：
--
通讯作者：
P. Padovani;P. Giommi
中科院分区：
文献类型：
--
作者：
P. Padovani;P. Giommi
我们解释了 X 射线选择的 BL Lacertae 物体的特性，假设它们构成了 BL Lac 种群中的一小部分，能量截止位于 UV/X 射线波段，如它们的多频谱所示。特别是，我们使用不需要任何自由参数的方法从无线电选择的 BL Lacs 开始推导出它们的 X 射线光度函数、log N-log S 和无线电通量分布。我们的假设与对两类 BL Lacs 存在的最常见解释不一致，即 X 射线选择的物体在明显偏离射束轴的情况下被观察，而无线电选择的物体的射流更与视线对齐。事实上，在这种情况下，X射线选择的源本质上应该比在无线电频带中以某个各向同性光度的固定值（例如漫射无线电发射的光度）选择的物体更多。虽然也考虑了当前 X 射线调查中检测到的 BL Lacs 的大部分特性，但后一种情况将预测它们的无线电发射将由扩展（非波束）分量主导，与观测结果相反。我们还解决了 X 射线选择的 BL Lacs 明显表现出的负演化问题，表明识别问题和 X 射线光谱的断裂都不可能是造成这种情况的原因。我们的假设以简单的方式解释了 alpha_ro - alpha_ox 图中 BL Lacs 的双峰分布。它还对 BL Lacs 的 X 射线、无线电和光学特性做出了具体预测，这些特性将通过未来的深度 X 射线勘测进行测试。
We explain the properties of X-ray selected BL Lacertae objects, under the assumption that they constitute the small minority of the BL Lac population with energy cutoff located in the UV/X-ray band, as suggested by their multifrequency spectra. In particular, we derive their X-ray luminosity function, log N-log S, and radio flux distribution starting from radio selected BL Lacs using a method that does not require any free parameters. Our hypothesis is at variance with the most common explanation for the existence of two classes of BL Lacs, namely that X-ray selected objects are viewed significantly off the beaming axis while radio-selected ones have their jets more aligned with the line of sight. In this case, in fact, X-ray selected sources should be intrinsically more numerous than objects selected in the radio band at a fixed value of some isotropic luminosity such as that of the diffuse radio emission. While also accounting for most of the properties of BL Lacs detected in current X-ray surveys, this latter scenario would predict their radio emission to be dominated by the extended (unbeamed) component, in contrast with observations. We also address the question of the negative evolution apparently exhibited by X-ray selected BL Lacs, showing that neither recognition problems nor a break in the X-ray spectrum are likely to be responsible for it. Our hypothesis explains in a simple way the bimodal distribution of BL Lacs in the alpha_ro - alpha_ ox diagram. It also makes specific predictions for the X-ray, radio, and optical properties of BL Lacs which will be tested by future deep X-ray surveys.