[论文解读] Initial LOFAR observations of Epoch of Reionization windows: II. Diffuse polarized emission in the ELAIS-N1 field

Vibor Jelić, A. G. de Bruyn+107 more|UvA-DARE (University of Amsterdam)|Jul 8, 2014

Radio Astronomy Observations and Technology参考文献 39被引用 52

一句话总结

本研究展示了LOFAR HBA波段对ELAIS-N1场的初步观测，检测到在法拉第深度-10至+13 rad m⁻²范围内，平均亮度温度约为4 K的弥散极化辐射。研究结果揭示了高银纬区域复杂的极化发射结构，表明存在显著的前景污染，对21厘米宇宙学实验构成挑战，且需要达到亚百分之一水平的仪器极化校准，以将泄漏抑制至约4–8 mK的水平。

ABSTRACT

This study aims to characterise the polarized foreground emission in the ELAIS-N1 field and to address its possible implications for the extraction of the cosmological 21-cm signal from the Low-Frequency Array - Epoch of Reionization (LOFAR-EoR) data. We use the high band antennas of LOFAR to image this region and RM-synthesis to unravel structures of polarized emission at high Galactic latitudes. The brightness temperature of the detected Galactic emission is on average 4 K in polarized intensity and covers the range from -10 to +13rad m^-2 in Faraday depth. The total polarized intensity and polarization angle show a wide range of morphological features. We have also used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. The LOFAR and WSRT images show a similar complex morphology, at comparable brightness levels, but their spatial correlation is very low. The fractional polarization at 150 MHz, expressed as a percentage of the total intensity, amounts to 1.5%. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies. The wide frequency range, good angular resolution and good sensitivity make LOFAR an exquisite instrument for studying Galactic polarized emission at a resolution of 1-2 rad m^-2 in Faraday depth. The different polarised patterns observed at 150 MHz and 350 MHz are consistent with different source distributions along the line of sight wring in a variety of Faraday thin regions of emission. The presence of polarised foregrounds is a serious complication for Epoch of Reionization experiments. To avoid the leakage of polarized emission into total intensity, which can depend on frequency, we need to calibrate the instrumental polarization across the field of view to a small fraction of 1%.

研究动机与目标

• 利用LOFAR对高银纬的ELAIS-N1场进行弥散极化前景发射的表征。
• 评估极化前景对21厘米宇宙再电离时代（EoR）信号探测的影响。
• 评估RM合成在解析弥散发射中复杂法拉第深度结构方面的性能。
• 量化来自极化弥散发射的仪器泄漏效应，这些效应可能模拟或掩盖EoR信号。
• 确立LOFAR在低频极化测量中研究磁化星际介质的可行性。

提出的方法

• 利用LOFAR的高带宽天线对ELAIS-N1场进行低频（通常约30–80 MHz）成像。
• 应用RM合成技术，将极化发射分解为不同法拉第深度的成分，解析-10至+13 rad m⁻²范围内的结构。
• 测量极化强度和亮度温度，发现该场内平均值约为4 K。
• 将LOFAR结果与先前在350 MHz波段的WSRT和GMRT观测进行比较，评估形态和强度上的一致性。
• 通过估算0.1–0.2%的泄漏可能在Stokes I中产生约4–8 mK的虚假信号，评估仪器泄漏效应。
• 使用SAGEcal进行源减除，以降低Stokes I图像中的混淆限制噪声，旨在提高泄漏分析的动态范围。

实验结果

研究问题

• RQ1 在高银纬的ELAIS-N1场中，弥散极化发射的形态和强度分布如何？
• RQ2 ELAIS-N1场中极化发射的法拉第深度结构与WSRT在350 MHz波段观测到的结构相比如何？
• RQ3 来自极化弥散发射的仪器泄漏在多大程度上污染了Stokes I功率谱，并可能模拟宇宙学21厘米信号？
• RQ4 RM合成能否有效解析LOFAR频段中复杂且扩展的极化发射特征？
• RQ5 为将泄漏抑制至可接受水平（<4–8 mK）以满足EoR实验需求，需要达到何种水平的仪器极化校准？

主要发现

• 在ELAIS-N1场中，于-10至+13 rad m⁻²的宽法拉第深度范围内检测到弥散极化发射。
• 检测到的极化发射平均亮度温度约为4 K，显著高于早期WSRT和GMRT观测所预期的水平。
• LOFAR数据中的形态特征，特别是东北和东部区域的条纹状图案，与350 MHz波段WSRT观测到的结构存在初步相关性，尽管信噪比和波束失配限制了明确结论。
• 在数K量级的极化发射存在复杂的空间与法拉第深度结构，对EoR实验构成重大挑战。
• 来自极化弥散发射的仪器泄漏可能在Stokes I中产生约4–8 mK的虚假信号，与预期的宇宙学21厘米信号幅度相当。
• 当前Stokes I图像中的噪声水平仍高于泄漏阈值，需要进一步进行源减除和波束建模，以降低混淆并提高动态范围。