[论文解读] Ginga observations of Cygnus X-2

R. Wijnands, M. van der Klis+3 more|arXiv (Cornell University)|Apr 23, 1997

Astrophysical Phenomena and Observations被引用 3

一句话总结

本研究分析了来自天鹅座X-2（一个Z型低质量X射线双星）的四年Ginga X射线数据，揭示了其Z轨道上强度依赖的变异性模式。研究发现，快速X射线变异性（包括准周期振荡（QPOs）和极低频噪声）显著随整体亮度水平变化，这挑战了摆动吸积盘模型，并将磁极处的磁场强度限制在≤8.5×10⁹ G以内。

ABSTRACT

We analysed all X-ray data on the low-mass X-ray binary Cygnus X-2 obtained with Ginga. The analysis of the spectral and fast timing behaviour of these 4 years of data provides new insights in the behaviour of this Z source. We confirm the previously observed recurrent patterns of behaviour in the X-ray colour-colour and hardness-intensity diagrams (HIDs) consisting of shifts and shape changes in the Z track. The source behaviour is correlated with overall intensity. When the overall intensity increases, the mean velocity of the motion along the normal branch (NB) of the Z track increase, as well as the width of the NB in the HID. Contrary to previous results we find that, during different observations, when the source is at the same position in the NB of the Z track the rapid X-ray variability differs significantly. During the Kuulkers et al. (1996) ``medium'' level, a normal branch quasi-periodic oscillation (QPO) is detected, which is not seen during the ``high'' overall intensity level. During the high overall intensity level episodes the very-low frequency noise on the lower NB is very strong, whereas during the medium overall intensity level episodes this noise component at the same position in the Z track is much weaker. The explanation of the different overall intensity levels with a precessing accretion disk is difficult to reconcile with our data. Furthermore, the frequency of the horizontal branch QPO decreases when Cyg X-2 enters the upper NB, giving a model dependent upper limit on the magnetic field strength at the magnetic equator of ~8.5x10^9 G. We also report 5 bursts, with durations of ~5 s, whose occurrence are uncorrelated with location in the Z track, overall intensity level or orbital phase. The burst properties indicate that they are not regular type I bursts.

研究动机与目标

• 理解天鹅座X-2在四年Ginga观测期间的光谱与时间行为。
• 研究源的亮度、Z轨道形态与快速X射线变异性之间的相关性。
• 将摆动吸积盘模型与观测到的变异性模式进行对比检验。
• 利用Z轨道上QPO频率演化，约束天鹅座X-2中的磁场强度。

提出的方法

• 分析过去四年从天鹅座X-2收集的所有Ginga X射线数据。
• 构建硬度-强度图（HIDs）和颜色-颜色图，以追踪源状态的转变。
• 在不同亮度水平下识别并表征准周期振荡（QPOs）和极低频噪声。
• 比较在高亮度和中等亮度状态下，同一正常分支（NB）位置处的QPO行为。
• 利用上正常分支上QPO频率的演化，推导出磁场强度的上限。
• 检测并分析爆发事件，评估其与Z轨道位置、亮度或轨道相位的相关性。

实验结果

研究问题

• RQ1 天鹅座X-2的快速X射线变异性如何随整体源亮度变化？
• RQ2 为何在中等亮度状态下检测到正常分支QPO，而在高亮度状态下未检测到？
• RQ3 观测到的变异性模式能否由摆动吸积盘模型解释？
• RQ4 基于QPO频率演化，可对天鹅座X-2中的磁场强度施加多大的上限？
• RQ5 观测到的5秒爆发是否为规则的I型X射线爆发，且是否与Z轨道位置或轨道相位相关？

主要发现

• 沿Z轨道正常分支（NB）的平均运动速度随整体源亮度增加而提高。
• 在硬度-强度图中，NB的宽度随亮度水平升高而变宽。
• 在中等亮度状态下检测到正常分支准周期振荡（QPO），但在高亮度状态下未检测到。
• 在相同Z轨道位置处，高亮度时段下正常分支下部的极低频噪声显著强于中等亮度时段。
• 随着天鹅座X-2向上的正常分支移动，水平分支QPO的频率降低，由此得出磁极处磁场强度的上限约为~8.5×10⁹ G。
• 检测到五次5秒爆发，其与Z轨道位置、亮度水平或轨道相位均无相关性，表明其并非规则的I型爆发。