[论文解读] Intraday variability in x-ray selected BL Lacertae objects

J. Heidt, S. J. Wagner|arXiv (Cornell University)|Sep 12, 1997

Astrophysics and Cosmic Phenomena参考文献 2被引用 5

一句话总结

本研究调查了33个X射线选源BL Lac对象（其中22个来自EMSS，11个来自EXOSAT）的日内X射线变异性，发现仅有不到40%的源表现出变异性，且其占空比≤0.4。典型峰峰值振幅约为10%，特征 timescales 范围为1.3至2.7天，结果表明X射线选源与射电选源BL Lac对象在占空比（相差约2倍）和振幅（相差约3倍）方面存在差异，但相似的timescales暗示了共同的变异性机制，可能与X射线选源中更强的磁场和更大的视线角度有关。

ABSTRACT

We present a study of the intraday variability behaviour of two samples of x-ray selected BL Lac objects, the EMSS and EXOSAT samples consisting of 22 and 11 sources, respectively. In both samples we were able to detect intraday variability in less than 40% of the sources only. The duty cycle (the fraction of time, when a BL Lac object is variable) in x-ray selected BL Lac objects is 0.4 or less. The typical peak-to-peak amplitudes of the variability are 10%. Typical time-scales and an activity parameter for our variable BL Lac objects were inferred from structure function and autocorrelation function analyses. In only 4 BL Lac objects we were able to measure a characteristic time-scale, which was in the range between 1.3 and 2.7 days. Comparison with our previous study of a complete sample of radio-selected BL Lac objects from the 1 Jy catalogue shows that x-ray and radio-selected BL Lac objects differ in their duty cycle by a factor of 2 and the typical peak-to-peak amplitudes by a factor of 3. The observed time-scales are similar. We also found that the same mechanism may be responsible for the observed variability in the x-ray selected and radio-selected BL Lac objects. The expectations of the various schemes linking x-ray selected and radio-selected BL Lac objects have been compared to our observations. Consistency is found for a scenario, where x-ray selected BL Lac objects have on average stronger magnetic fields and are seen under relatively larger viewing angles than the radio-selected BL Lac objects. However, the suggestion that x-ray selected BL Lac objects have decelerating jets and radio-selected BL Lac objects accelerating jets can also not be ruled out.

研究动机与目标

• 调查来自EMSS和EXOSAT样本的X射线选源BL Lac对象的日内X射线变异性。
• 将X射线选源BL Lac对象的变异性特征与1 Jy星表中射电选源BL Lac对象的特征进行比较。
• 确定变异性行为的差异是否反映了喷流特性或视线几何的内在差异。
• 通过比较观测到的变异性模式，检验将X射线选源与射电选源BL Lac对象联系起来的理论模型。

提出的方法

• 分析来自EMSS和EXOSAT巡天的33个BL Lac对象的X射线光 light curves。
• 应用结构函数和自相关函数技术，推断特征timescales和变异性振幅。
• 将占空比计算为源表现出显著日内变异性的时间比例。
• 比较X射线选源与射电选源样本的变异性参数（振幅、timescale、占空比）。
• 基于喷流取向、磁场强度和喷流加速状态，评估理论模型以解释观测到的差异。

实验结果

研究问题

• RQ1 有多少比例的X射线选源BL Lac对象表现出日内X射线变异性，其典型占空比是多少？
• RQ2 X射线选源BL Lac对象的日内变异性峰峰值振幅和特征timescale与射电选源BL Lac对象相比如何？
• RQ3 何种物理机制可以解释X射线选源与射电选源BL Lac对象之间观测到的变异性行为差异？
• RQ4 观测到的变异性是否与两个样本中存在共同的辐射机制一致，还是代表了不同的群体？
• RQ5 哪些理论模型——例如不同的视线角度或喷流加速状态——最能解释观测到的变异性模式？

主要发现

• X射线选源BL Lac对象的日内X射线变异性占空比≤0.4，表明其仅在一小部分时间内表现出变异性。
• X射线波段中典型峰峰值变异性振幅约为10%。
• 仅在四个源中测得特征timescale，范围为1.3至2.7天。
• X射线选源BL Lac对象的占空比约为射电选源BL Lac对象的一半，其典型变异性振幅也约为后者的三分之一。
• 尽管占空比和振幅存在差异，但两个样本的典型timescale相似，暗示存在共同的潜在机制。
• 数据与如下模型一致：X射线选源BL Lac对象的磁场更强且视线角度更大，尽管涉及减速与加速喷流的模型也无法被排除。