Optical polarization of high-energy BL Lacertae objects
Optical polarization of high-energy BL Lacertae objects
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DOI：
10.1051/0004-6361/201628974
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发表时间：
2016-12-01
影响因子：
6.5
通讯作者：
Pursimo, T.
中科院分区：
文献类型：
--
作者：
Hovatta, T.;Lindfors, E.;Pursimo, T.
上下文我们使用RoboPol blazar监测计划和北欧光学望远镜的数据研究了高能BL Lac天体的光学偏振特性。我们希望了解是否有BL Lac对象，已被检测到与当前一代TeV仪器和那些对象尚未被检测到的差异。我们使用了最大似然方法来研究这些光源的光学偏振分量及其变化。为了研究偏振位置角的变化，我们计算了电矢量位置角（EVPA）变化的时间导数。我们还研究了Stokes Q/I-U/I平面内的色散和偏振平面内的旋转。TeV检测到的BL拉克的平均极化分数为5%，而非TeV源显示出更高的平均极化分数，为7%。当考虑到宿主星系的非偏振光的稀释作用时，这种偏振分数的差异就消失了。TeV源显示出比非TeV源稍低的分数偏振变化幅度。此外，在Q/I-U/I平面中具有较小扩展的源的分数以及远离原点的点的聚集分布（可能指示优选的偏振角）在TeV中比在非TeV源中更大。TeV和非TeV样品之间的这些差异似乎是由中间和高光谱峰值源之间的差异而不是TeV检测引起的。当研究EVPA变化时，两种样品中的EVPA变化率相似。我们检测到显着的EVPA旋转TeV和非TeV源，显示旋转可以发生在高光谱峰值BL Lac对象时，监测节奏是足够密集的。我们的模拟表明，我们不能排除这些旋转的随机游走原点。这些结果表明，有没有内在的差异，TeV检测和非TeV检测的高能BL Lac对象的偏振特性。这表明偏振特性与TeV检测没有直接关系，而是TeV响度与一般的耀斑活动，红移和同步加速器峰值位置有关。
Context. We investigate the optical polarization properties of high-energy BL Lac objects using data from the RoboPol blazar monitoring program and the Nordic Optical Telescope.Aims. We wish to understand if there are differences between the BL Lac objects that have been detected with the current-generation TeV instruments and those objects that have not yet been detected.Methods. We used a maximum-likelihood method to investigate the optical polarization fraction and its variability in these sources. In order to study the polarization position angle variability, we calculated the time derivative of the electric vector position angle (EVPA) change. We also studied the spread in the Stokes Q/I - U/I plane and rotations in the polarization plane.Results. The mean polarization fraction of the TeV-detected BL Lacs is 5%, while the non-TeV sources show a higher mean polarization fraction of 7%. This difference in polarization fraction disappears when the dilution by the unpolarized light of the host galaxy is accounted for. The TeV sources show somewhat lower fractional polarization variability amplitudes than the non-TeV sources. Also the fraction of sources with a smaller spread in the Q/I - U/I plane and a clumped distribution of points away from the origin, possibly indicating a preferred polarization angle, is larger in the TeV than in the non-TeV sources. These differences between TeV and non-TeV samples seem to arise from differences between intermediate and high spectral peaking sources instead of the TeV detection. When the EVPA variations are studied, the rate of EVPA change is similar in both samples. We detect significant EVPA rotations in both TeV and non-TeV sources, showing that rotations can occur in high spectral peaking BL Lac objects when the monitoring cadence is dense enough. Our simulations show that we cannot exclude a random walk origin for these rotations.Conclusions. These results indicate that there are no intrinsic differences in the polarization properties of the TeV-detected and non-TeV-detected high-energy BL Lac objects. This suggests that the polarization properties are not directly related to the TeV-detection, but instead the TeV loudness is connected to the general flaring activity, redshift, and the synchrotron peak location.