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Lunar occultation of the diffuse radio sky: LOFAR measurements between 35 and 80 MHz

H. Vedantham 1 E. Koopmans 1 Alexandre de Bruyn 2 S. Wijnholds 3 M. Brentjens 3 F. Abdalla 4 M. Asad 5, 6 G. Bernardi 7 S. Bus 1 E. Chapman 8 B. Ciardi 9 S. Daiboo 1 R. Fernandez 10 A. Ghosh 11 G. Harker 12 V. Jelic 1 H. Jensen 13 S. Kazemi 14 P. Lambropoulos 1 O. Martinez-Rubi 1 G. Mellema 15 M. Mevius 3 A. Offringa 1 V. Pandey 16 A. Patil 17 M. Thomas 18 V. Veligatla 1 S. Yatawatta 3 S. Zaroubi 1 J. Anderson 19 A. Asgekar 3 M. Bell 20 M. Bentum 3 P. Best 21 A. Bonafede 22 F. Breitling 23 J. Broderick 24 M. Bruggen 22 H. Butcher 3 A. Corstanje 25 F. de Gasperin 26 E. de Geus 27 A. Deller 3 S. Duscha 3 J. Eisloffel 28 D. Engels 26 H. Falcke 25 A. Fallows 3 R. Fender 24 C. Ferrari 29 W. Frieswijk 3 M. Garrett 30, 3 Jean-Mathias Grießmeier 31, 32 A. Gunst 3 E. Hassall 24 G. Heald 3 M. Hoeft 28 J. Horandel 25 M. Iacobelli 30 E. Juette 33 V. I. Kondratiev 3 M. Kuniyoshi 34 G. Kuper 3 G. Mann 35 S. Markoff 36 R. Mcfadden 3 D. Mckay-Bukowski 37 P. Mckean 3 D. Mulcahy 38 H. Munk 3 A. Nelles 25 M. Norden 3 E. Orru 3 M. Pandey-Pommier 39 R. Pizzo 3 A. Polatidis 3 W. Reich 34 A. Renting 3 H. Rottgering 30 D. Schwarz 40 A. Shulevski 1 O. Smirnov 41 B. W. Stappers 38 M. Steinmetz 42 J. Swinbank 36 Michel Tagger 32 Y. Tang C. Tasse 43 S. ter Veen 44 S. Thoudam 25 C. Toribio 3 C. Vocks 35 M. W. Wise 36 O. Wucknitz 45 P. Zarka 43
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Abstract : 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).
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H. Vedantham, E. Koopmans, Alexandre de Bruyn, S. Wijnholds, M. Brentjens, et al.. Lunar occultation of the diffuse radio sky: LOFAR measurements between 35 and 80 MHz. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2015, 450 (3), pp.2291 - 2305. ⟨10.1093/mnras/stv746⟩. ⟨insu-01182168⟩

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