Lunar occultation of the diffuse radio sky: LOFAR measurements between 35 and 80 MHz
H. K. Vedantham, L. V. E. Koopmans, A. G. de Bruyn, S. J. Wijnholds, M. Brentjens, F. B. Abdalla, K. M. B. Asad, G. Bernardi, S. Bus, E. Chapman, B. Ciardi, S. … 展开作者
H. K. Vedantham, L. V. E. Koopmans, A. G. de Bruyn, S. J. Wijnholds, M. Brentjens, F. B. Abdalla, K. M. B. Asad, G. Bernardi, S. Bus, E. Chapman, B. Ciardi, S. Daiboo, E. R. Fernandez, A. Ghosh, G. Harker, V. Jelic, H. Jensen, S. Kazemi, P. Lambropoulos, O. Martinez-Rubi, G. Mellema, M. Mevius, A. R. Offringa, V. N. Pandey, A. H. Patil, R. M. Thomas, V. Veligatla, S. Yatawatta, S. Zaroubi, J. Anderson, A. Asgekar, M. E. Bell, M. J. Bentum, P. Best, A. Bonafede, F. Breitling, J. Broderick, M. Brüggen, H. R. Butcher, A. Corstanje, F. de Gasperin, E. de Geus, A. Deller, S. Duscha, J. Eislöffel, D. Engels, H. Falcke, R. A. Fallows, R. Fender, C. Ferrari, W. Frieswijk, M. A. Garrett, J. Grießmeier, A. W. Gunst, T. E. Hassall, G. Heald, M. Hoeft, J. Hörandel, M. Iacobelli, E. Juette, V. I. Kondratiev, M. Kuniyoshi, G. Kuper, G. Mann, S. Markoff, R. McFadden, D. McKay-Bukowski, D. D. Mulcahy, H. Munk, A. Nelles, M. J. Norden, E. Orru, M. Pandey-Pommier, R. Pizzo, A. G. Polatidis, W. Reich, A. Renting, H. Röttgering, D. Schwarz, A. Shulevski, O. Smirnov, B. W. Stappers, M. Steinmetz, J. Swinbank, M. Tagger, Y. Tang, C. Tasse, S. ter Veen, S. Thoudam, C. Toribio, C. vocks, M. W. Wise, O. Wucknitz, P. Zarka
我们目前的无线电观测月球之间的$35$和$80$ MHz，以证明一种新的技术，干涉测量大规模的扩散发射远远超出了初级光束（全球信号）的第一次。特别是，我们表明：（i）月球在频率为$35<ν<80$ MHz时表现为一个负通量源，因为它比它所掩盖的弥漫的银河系背景“更冷”;（ii）使用（负）通量，我们可以用月球的热辐射作为参考，以及（iii）由于反射的镜面性质，反射的RFI（射频干扰）集中在月盘的中心，并且可以独立地测量。我们的RFI测量表明：（i）月基宇宙黎明实验必须设计优于$80$ dB的地球隔离，以实现RFI温度$<1$ mK，（ii）月反射RFI有助于地球宇宙黎明实验的偶极子温度低于$20$ mK，（iii）人造卫星─如果在$800$ km高度处可见卫星的总截面超过$80$ m $^2$，则反射RFI温度超过$20$ mK，或$5$ m $^2$，高度为$400$ km。目前，我们的漫射背景光谱受到短基线（10-15%水平）旁瓣混淆的限制。我们的技术的进一步改进可能会产生对来自宇宙黎明（$40>z>12$）和再电离时期（$12>z>5$）的红移全球$21$ -cm信号的约束。
英文摘要：
We present radio observations of the Moon between $35$ and $80$ MHz to demonstrate a novel technique of interferometrically measuring large-scale diffuse emission extending far beyond the primary beam (global signal) for the first time. In particular, we show that (i) the Moon appears as a negative-flux source at frequencies $35<ν<80$ MHz since it is `colder' than the diffuse Galactic background it occults, (ii) using the (negative) flux of the lunar disc, we can reconstruct the spectrum of the diffuse Galactic emission with the lunar thermal emission as a reference, and (iii) that reflected RFI (radio-frequency interference) is concentrated at the center of the lunar disc due to specular nature of reflection, and can be independently measured. Our RFI measurements show that (i) Moon-based Cosmic Dawn experiments must design for an Earth-isolation of better than $80$ dB to achieve an RFI temperature $<1$ mK, (ii) Moon-reflected RFI contributes to a dipole temperature less than $20$ mK for Earth-based Cosmic Dawn experiments, (iii) man-made satellite-reflected RFI temperature exceeds $20$ mK if the aggregate cross section of visible satellites exceeds $80$ m$^2$ at $800$ km height, or $5$ m$^2$ at $400$ km height. Currently, our diffuse background spectrum is limited by sidelobe confusion on short baselines (10-15% level). Further refinement of our technique may yield constraints on the redshifted global $21$-cm signal from Cosmic Dawn ($40>z>12$) and the Epoch of Reionization ($12>z>5$).
