仙后座A低频射电复合谱线的LOFAR探测
LOFAR detections of low-frequency radio recombination lines towards Cassiopeia A
摘要：
利用低频阵（LOFAR）的低波段高光谱分辨率望远镜观测仙后座A。这允许沿着该源的视线搜索无线电复合线（RRL）。从两个独立的LOFAR数据集中，在40和50 MHz之间的吸收中检测到五个碳-α RRL，信噪比> 5。在我们的光谱中，从整个超新星遗迹中提取的RRL的线速度（v_LSR ~ -50 km/s）和积分光学深度（~ 13 s^-1）与每个LOFAR数据集以及先前报道的数据一致。这是第一次，我们能够提取光谱对最亮的热点的残余在频率低于330兆赫。这些光谱显示出明显更高的（15- 80%）积分光学深度，表明在残骸表面上的吸收气体分布中存在约1 pc量级的小尺度角结构。我们还对氢和氦RRL的峰值光学深度设定了3 x 10^-4的上限。这些结果表明，LOFAR具有所需的光谱稳定性和灵敏度，以研究微弱的复合线的decliter带。
英文摘要：
Cassiopeia A was observed using the Low-Band Antennas of the LOw Frequency ARray (LOFAR) with high spectral resolution. This allowed a search for radio recombination lines (RRLs) along the line-of-sight to this source. Five carbon-alpha RRLs were detected in absorption between 40 and 50 MHz with a signal-to-noise ratio of > 5 from two independent LOFAR datasets. The derived line velocities (v_LSR ~ -50 km/s) and integrated optical depths (~ 13 s^-1) of the RRLs in our spectra, extracted over the whole supernova remnant, are consistent within each LOFAR dataset and with those previously reported. For the first time, we are able to extract spectra against the brightest hotspot of the remnant at frequencies below 330 MHz. These spectra show significantly higher (15-80 %) integrated optical depths, indicating that there is small-scale angular structure on the order of ~1 pc in the absorbing gas distribution over the face of the remnant. We also place an upper limit of 3 x 10^-4 on the peak optical depths of hydrogen and helium RRLs. These results demonstrate that LOFAR has the desired spectral stability and sensitivity to study faint recombination lines in the decameter band.
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