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

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 $35<\nu <80$ 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 ($12>z>5$).

15 pages, 11 figures, 1 table

References in corpus (2)

• Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe
• The LOFAR radio environment

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• Selection between foreground models for global 21-cm experiments
• Probing the primordial Universe with 21-cm line from cosmic dawn/epoch of reionization
• Improvements to the Search for Cosmic Dawn Using the Long Wavelength Array
• On Measuring the 21 cm Global Spectrum of the Cosmic Dawn with an Interferometer Array