Free-free absorption in the gravitational lens JVAS B0218+357
Free-free absorption in the gravitational lens JVAS B0218+357
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DOI：
10.1051/0004-6361:20066127
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发表时间：
2006-07
影响因子：
6.5
通讯作者：
R. Mittal;R. Porcas;O. Wucknitz
中科院分区：
文献类型：
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作者：
R. Mittal;R. Porcas;O. Wucknitz
我们解决了双像引力透镜 JVAS B0218+357 中出现的异常像通量比问题。从最近的一项研究（Mittal 等人，2006 年，A&A，447, 515）中提出的多频观测结果以及其他作者之前的一些观测结果来看，这种异常现象是显而易见的，即在 15 GHz 至 1.65 GHz 的观测频率范围内，图像通量密度比 (A/B) 从 3.9 降至 2.0。在米塔尔等人中。 (2006)，作者研究了背景无线电源的频率相关结构与相对图像放大率梯度之间的相互作用是否可以解释异常现象。图像质心随频率的偏移不足导致他们放弃了上述效应作为异常原因。在本文中，我们首先通过更详细地评估背景源扩展和透镜平面中的放大梯度的综合效果来进一步进行分析。这是通过直接使用每个图像观察到的 VLBI 通量分布来估计透镜模型中不同频率下的图像通量密度比来完成的。本次调查的结果是，该机制无法解释该异常情况。接下来，我们分析了本质上非引力机制对 B0218+357 中像通量比的影响。这些是自由吸收和散射，并且假设是在沿着图像 A 的视线存在于透镜星系中的分子云的假设下发生的。我们表明，由于 1.65 GHz 下覆盖图像 A 整个结构的命中区域而产生的自由吸收可以解释图像通量比异常。我们还讨论了从我们的分析中得出的具有物理参数的 H II 区域是否与在银河系和河外 H II 区域中观察到的一致。
We address the issue of anomalous image flux ratios seen in the double-image gravitational lens JVAS B0218+357. From the multifrequency observations presented in a recent study (Mittal et al. 2006, A&A, 447, 515) and several previous observations made by other authors, the anomaly is well-established in that the image flux-density ratio (A/B) decreases from 3.9 to 2.0 over the observed frequency range from 15 GHz to 1.65 GHz. In Mittal et al. (2006), the authors investigated whether an interplay between a frequency-dependent structure of the background radio-source and a gradient in the relative image-magnification can explain away the anomaly. Insufficient shifts in the image centroids with frequency led them to discard the above effect as the cause of the anomaly. In this paper, we first take this analysis further by evaluating the combined effect of the background source extension and magnification gradients in the lens plane in more detail. This is done by making a direct use of the observed VLBI flux-distributions for each image to estimate the image flux-density ratios at different frequencies from a lens-model. As a result of this investigation, this mechanism does not account for the anomaly. Following this, we analyze the effects of mechanisms which are non-gravitational in nature on the image flux ratios in B0218+357. These are free-free absorption and scattering, and are assumed to occur under the hypothesis of a molecular cloud residing in the lens galaxy along the line-of-sight to image A. We show that free-free absorption due to an Hit region covering the entire structure of image A at 1.65 GHz can explain the image flux ratio anomaly. We also discuss whether H II regions with physical parameters as derived from our analysis are consistent with those observed in Galactic and extragalactic H II regions.