Precise Measurement of the Absolute Sky Brightness at 60-350 MHz

Abstract

Accurate knowledge of the low-frequency radio sky is essential for modelling foregrounds in experiments targeting cosmic dawn and the epoch of reionization. Measurements below 1 GHz are also needed to understand the Galactic cosmic ray electron spectrum, constrain nanojansky radio source populations and dark matter models, investigate the origins of the diffuse radio background, and improve calibration of long-wavelength radio telescopes. Here we present a precision measurement of the radio sky brightness over 60-350 MHz using a new receiver architecture that self-calibrates its noise contribution and bandpass in situ while connected to an antenna. Our measurement used a log-periodic SKALA4.1 antenna on a 40 m diameter SKA-Low station ground mesh in the Southern Hemisphere to observe approximately half of the celestial sphere. We show that current all-sky maps and the 2016 Global Sky Model (GSM2016) require substantial corrections over this frequency range. GSM2016 must be scaled upward by a factor of 1.2 over 60-200 MHz, increasing to 1.5 at 350 MHz. A smaller offset correction of approximately 100 K is also required below 100 MHz. The revised scaling significantly increases previously inferred excess radio background, motivating review of faint source populations and dark-matter decay models. Sky models scaled to our measurements can set the absolute flux-density scale for SKA-Low and other low-frequency radio telescopes. They can also improve foreground characterisation for cosmic dawn and epoch-of-reionization experiments.