The optical polarization properties of X-ray-selected BL Lacertae objects
The optical polarization properties of X-ray-selected BL Lacertae objects
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DOI：
10.1086/174226
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发表时间：
1994-06
期刊：
影响因子：
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通讯作者：
B. Jannuzi;Paul S. Smith;R. Elston
中科院分区：
文献类型：
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作者：
B. Jannuzi;Paul S. Smith;R. Elston
我们讨论了 X 射线选择的 BL Lacertae 天体 (XSBL) 的光学偏振特性，这是通过对 37 个 BL Lac 天体和候选体的偏振进行 3 年监测而确定的。观测到的物体包括从爱因斯坦扩展介质灵敏度巡天 (EMSS) 中提取的完整 X 射线通量受限样本。我们发现，大多数仅根据其光谱外观进行分类的 XSBL 是 BL Lacertae 天体类别的真正成员，因为它们具有本质上偏振和可变连续体。 XSBL 偏振发射的占空比（偏振度大于 4% 所花费的时间比例）为 44%。观测到的最大偏振百分比约为 10%，远低于射电选定的 BL 蜥蜴天体和耀变体观测到的最大值 (30%-40%)。虽然 XSBL 具有可变的偏振发射，但大多数（约等于 85%）在时间尺度上具有至少长达 3 年的首选偏振位置角。这反映了发射线偏振光发射的区域的几何形状的稳定性。我们描述了偏振度的光谱依赖性，并讨论了产生观察到的特性的一些可能机制。虽然某些天体中确实存在宿主星系星光对偏振发射的稀释，但我们证明，从我们的观测中得出的 XSBL 的平均偏振特性并没有受到这种效应的严重影响。虽然已确认的 BL Lac 天体显示为光度变量，但我们监测的样本中的天体并未显示出大于 1 星等的变化，该变化通常用于表征射电选择的 BL Lacertae 天体或耀变体的光学变异性。
We discuss the optical polarization properties of X-ray-selected BL Lacertae objects (XSBLs) as determined from 3 yr of monitoring the polarization of 37 BL Lac objects and candidates. The observed objects include a complete X-ray flux limited sample drawn from the Einstein Extended Medium Sensitivity Survey (EMSS). We find that the majority of the XSBLs classified solely on the appearance of their optical spectra are true members of the class of BL Lacertae objects since they possess intrinsically polarized and variable continua. The duty cycle of polarized emission (fraction of time spent with the degree of polarization greater than 4%) from XSBLs is 44%. The maximum observed percent polarizations are of order 10%, considerably below the maximum values observed for radio-selected BL Lacertae objects and blazars (30%-40%). While XSBLs have variable polarized emission, the majority (approximately equal to 85%) have preferred polarization position angles on timescales at least as long as 3 yr. This reflects stability in the geometry of the region emitting the linearly polarized optical emission. We describe the spectral dependence of the degree of polarization and discuss some of the possible mechanisms producing the observed characteristics. While dilution of the polarized emission by the host galaxy starlight is certainly present in some objects, we demonstrate that the average polarization properties of XSBLs derived from our observations are not drastically affected by this effect. While the confirmed BL Lac objects are shown to be photometric variables, the objects in our monitored sample did not display the larger than 1 mag variations generally used to characterize the optical variability of radio-selected BL Lacertae objects or blazars in general.