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

H. Vedantham (1) , E. Koopmans (1) , Alexandre de Bruyn , S. Wijnholds (2) , M. Brentjens (2) , F. Abdalla (3) , M. Asad (4, 5) , G. Bernardi (6) , S. Bus (1) , E. Chapman (7) , B. Ciardi (8) , S. Daiboo (1) , R. Fernandez (9) , A. Ghosh (10) , G. Harker (11) , V. Jelic (1) , H. Jensen (12) , S. Kazemi (13) , P. Lambropoulos (1) , O. Martinez-Rubi (1) , G. Mellema (14) , M. Mevius (2) , A. Offringa (1) , V. Pandey (15) , A. Patil (16) , M. Thomas (17) , V. Veligatla (1) , S. Yatawatta (2) , S. Zaroubi (1) , J. Anderson (18) , A. Asgekar (2) , M. Bell (19) , M. Bentum (2) , P. Best (20) , A. Bonafede (21) , F. Breitling (22) , J. Broderick (23) , M. Bruggen (21) , H. Butcher (2) , A. Corstanje (24) , F. de Gasperin (25) , E. de Geus (26) , A. Deller (2) , S. Duscha (2) , J. Eisloffel (27) , D. Engels (25) , H. Falcke (24) , A. Fallows (2) , R. Fender (23) , C. Ferrari (28) , W. Frieswijk (2) , M. Garrett (29, 2) , Jean-Mathias Griessmeier (30, 31) , A. Gunst (2) , E. Hassall (23) , G. Heald (2) , M. Hoeft (27) , J. Horandel (24) , M. Iacobelli (29) , E. Juette (32) , V. I. Kondratiev (2) , M. Kuniyoshi (33) , G. Kuper (2) , G. Mann (22) , S. Markoff (34) , R. Mcfadden (2) , D. Mckay-Bukowski (35) , P. Mckean (2) , D. Mulcahy (36) , H. Munk (2) , A. Nelles (24) , M. Norden (2) , E. Orru (2) , M. Pandey-Pommier (37) , R. Pizzo (2) , A. Polatidis (2) , W. Reich (33) , A. Renting (2) , H. Rottgering (29) , D. Schwarz (38) , A. Shulevski (1) , O. Smirnov (39) , B. W. Stappers (36) , M. Steinmetz (40) , J. Swinbank (34) , Michel Tagger (31) , Y. Tang , C. Tasse (41) , S. ter Veen (42) , S. Thoudam (24) , C. Toribio (2) , C. Vocks (22) , M. W. Wise (34) , O. Wucknitz (43) , P. Zarka (41)

1 Kapteyn Astronomical Institute [Groningen]
2 ASTRON - Netherlands Institute for Radio Astronomy
3 UCL - University College of London [London]
4 LaMCoS - Laboratoire de Mécanique des Contacts et des Structures [Villeurbanne]
5 MULTIMAP - Mécanique multiphysique pour les matériaux et les procédés
6 LPNHE - Laboratoire de Physique Nucléaire et de Hautes Énergies
7 HHMI - Howard Hughes Medical Institute
8 Max Planck Institute for Astrophysics
9 Departamento de Geologıa
10 ACMU - Advanced Computing and Microelectronics Unit [Kolkata]
11 Department of Physics and Astronomy [UCL London]
12 Stockholm University
13 Department of process and energy
14 Stockholm Observatory Department of Astronomy
15 Department of Physics and Astronomy [Ghent]
16 Spatial Ecology and Epidemiology Group
17 Plant Ind, Hort Unit
18 Institute for Mathematics Applied to Geoscience
19 CSIRO Astronomy and Space Science
20 University of Edinburgh
21 Jacobs University [Bremen]
22 AIP - Leibniz-Institut für Astrophysik Potsdam
23 University of Southampton
24 Radboud University [Nijmegen]
25 Hamburger Sternwarte/Hamburg Observatory
26 Metacohorts Consortium
27 TLS - Thüringer Landessternwarte Tautenburg
28 Département de Géologie
29 Leiden Observatory [Leiden]
30 USN - Unité Scientifique de la Station de Nançay
31 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
32 Ruhr-Universität Bochum [Bochum]
33 MPIFR - Max-Planck-Institut für Radioastronomie
34 AI PANNEKOEK - Astronomical Institute Anton Pannekoek
35 University of Oulu
36 Jodrell Bank Centre for Astrophysics
37 CRAL - Centre de Recherche Astrophysique de Lyon
38 Interactions Son Musique Mouvement
39 Rhodes University, Grahamstown
40 DSMZ - Leibniz-Institut DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH / Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures
41 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
42 Department of Astrophysics [Nijmegen]
43 AlfA - Argelander-Institut für Astronomie

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).

Keywords: first stars; methods: observational – techniques: interferometric – Moon – cosmology: dark ages; reionization

Domains: Sciences of the Universe [physics]

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, 2015, 450 (3), pp.2291 - 2305. ⟨10.1093/mnras/stv746⟩. ⟨insu-01182168⟩