Multifrequency studies of bright radio supernova remnants. 2: W49B
Multifrequency studies of bright radio supernova remnants. 2: W49B
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DOI：
10.1086/175033
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发表时间：
1994-12
期刊：
影响因子：
--
通讯作者：
D. Moffett;S. Reynolds
中科院分区：
文献类型：
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作者：
D. Moffett;S. Reynolds
我们报道了利用NRAO的甚大阵列(VLA)在90、20和6厘米处对超新星遗迹W49B进行的射电观测。这项工作继续从3C391年(Moffett&Reynolds，1994)开始对年轻明亮的超新星遗迹(SNR)的性质进行研究。在这里，我们展示了W49B的总强度、偏振和光谱指数的高分辨率图像。在大尺度形态上，它基本上是一个盒子状的残留物，扩展到明显不均匀的介质中，但我们也发现了小尺度的结构、弧形和细丝，它们位于外壳的投影内部。细丝的实际空间位置尚不清楚，尽管我们怀疑它们位于爆炸波的残余物内部，而不是在冲击波的正面或背面。此外，它们在二维上的分布表明它们是螺旋结构的投影。在我们的最高分辨率(4秒)下，贝壳边缘的亮度急剧上升，至少达到了其周长的3/4，仍然没有被分辨出来。这对于我们假设的8kpc的距离，发射出现的区域的宽度小于0.16pc，这表明激波加速的电子的平均自由程较短，高水平的MHD湍流可能导致了散射。我们在90或20 cm处没有发现极化通量，在90 cm(45秒分辨率)和20 cm(5秒分辨率)时的极化强度上限分别为36mJy和7.2mJy，或在这两个尺度上扩展的发射分别为22微Jy/平方弧秒和370微Jy/平方弧秒。在6 cm处存在极化通量，但平均极化分数(总极化通量除以总通量)很低，仅为0.44%+/-0.06%，对于3C391，这远远低于该频率下明亮信噪比的典型值。偏振强度的形态与总强度的形态相关性较差。我们在一些地方看到偏振分数的漂移至少达到10%，但即使在可以看到偏振的地方，典型的能级也只有几个百分点。射电壳层发射区中纠缠或无序的磁场可能是导致射电同步辐射去极化的原因，但也可能发生某种形式的内部法拉第去极化。我们估计前景法拉第旋转量约为-450rad/平方米，与3C391的结果相似，3C391也是银河系平面，距离大致相同。从总强度图像创建的光谱指数图像显示，在明亮区域，W49B的光谱指数是恒定的，约在增量α内，约为0.1。这一结果弱地支持了射电发射电子的共同起源，如在冲击波中，而不是在由于随机(二阶费米)加速过程而具有不同性质的非均匀湍流区。在增量α约为0.1的水平上可以看到变化，但其意义值得怀疑。使用实验性三维成像技术在90厘米处进行的新观测可能会在这一限制上有所改进。
We report radio observations of the supernova remnant W49B using the Very Large Array (VLA) of the NRAO at 90, 20, and 6 cm. This work continues the study of the properties of young, bright supernova remnants (SNRs) begun with 3C 391 (Moffett & Reynolds 1994). Here we present high-resolution images of total intensity, polarization, and spectral index of W49B. In large-scale morphology it is basically a box-shaped remnant expanding into an apparently inhomogeneous medium, but we also find small-scale structures, arcs and filaments, which lie interior to the outer shell in projection. The actual spatial position of the filaments is unknown, though we suspect that they are in the remnant interior rather than on the front or back face of the blast wave. In addition, their distribution in two dimensions suggests the projection of a helical structure. The shell edge, sharply rising in brightness for at least 3/4 of its circumference, still remains unresolved at our highest resolution (4 sec). This for our assumed distance of 8 kpc, the width of the region in which the emission appears is less than 0.16 pc, indicating short mean free paths for shock-accelerated electrons and high levels of MHD turbulence presumably causing the scattering. We find no polarized flux at 90 or 20 cm, with 3 sigma upper limits in polarized intensity of 36 mJy at 90 cm (45 sec resolution) and 7.2 mJy at 20 cm (5 sec resolution), or 22 micro-Jy/sq arcsec and 370 micro-jy/sq arcsec, respectively, for any emission extended on those scales. Polarized flux is present at 6 cm, but at a very low mean polarized fraction (total polarized flux divided by total flux) of 0.44% +/- 0.06%, which, as for 3C 391, is much lower than typical for bright SNRs at this frequency. The morphology in polarized intensity is poorly correlated with that in total intensity. We see excursions in the polarized fraction up to at least 10% in a few locations, but even where polarization is seen, typical levels are a few percent. Tangled or disordered magnetic fields in the emitting region of the radio shell may be responsible for depolarizing the radio synchrotron radiation, but some form of internal Faraday depolarization may also occur. We estimate the foreground Faraday rotation measure to be about -450 rad/sq m, similar to that found for 3C 391, which is also the Galactic plane and just about as distant. Spectral index images created from the total intensity images show that the spectral index across W49B is constant to within about Delta alpha approximately 0.1 in bright regions. This result weakly supports a common origin of the radio-emitting electrons, as in the blast wave, rather than in inhomogeneous turbulent regions of differing properties due to the stochastic (second-order Fermi) acceleration process. Variations at the level of Delta alpha approximately 0.1 are seen, but their significance is doubtful. New observations at 90 cm, using experimental three-dimensional imaging technology, may improve on this limit.