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

H. Vedantham 1 E. Koopmans 1 A. 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 15, 16 O. Martinez-Rubi 1 G. Mellema 17 M. Mevius 3 A. Offringa 1 V. Pandey 18 A. Patil 19 M. Thomas 20 V. Veligatla 1 S. Yatawatta 3 S. Zaroubi 1 J. Anderson 21 A. Asgekar 3 M. Bell 22 M. Bentum 3 P. Best 23 A. Bonafede 24 F. Breitling 25 J. Broderick 26 M. Bruggen 24 H. Butcher 3 A. Corstanje 27 F. de Gasperin 28 E. de Geus 29 A. Deller 3 S. Duscha 3 J. Eisloffel 30 D. Engels 28 H. Falcke 27 A. Fallows 3 R. Fender 26 C. Ferrari 31 W. Frieswijk 3 M. Garrett 32, 3 Jean-Mathias Grießmeier 33, 34 A. Gunst 3 E. Hassall 26 G. Heald 3 M. Hoeft 30 J. Horandel 27 M. Iacobelli 32 E. Juette 35 V. I. Kondratiev 3 M. Kuniyoshi 36 G. Kuper 3 G. Mann 37 S. Markoff 38 R. Mcfadden 39 D. Mckay-Bukowski 40 P. Mckean 39 D. Mulcahy 41 H. Munk 39 A. Nelles 27 M. Norden 3 E. Orru 3 M. Pandey-Pommier 42 R. Pizzo 3 A. Polatidis 3 W. Reich 36 A. Renting 3 H. Rottgering 32 D. Schwarz 43 A. Shulevski 1 O. Smirnov 44 B. W. Stappers 41 M. Steinmetz 45 J. Swinbank 38 Michel Tagger 34 Y. Tang C. Tasse 46 S. Ter Veen 47 S. Thoudam 27 C. Toribio 3 C. Vocks 37 M. W. Wise 38 O. Wucknitz 48 P. Zarka 49
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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, A. 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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