Precise measurement of the absolute sky brightness at 60–350 MHz

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, to constrain nanojansky radio source populations and dark-matter models, to investigate the origins of the diffuse radio background, and to improve the 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 band-pass 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 upwards 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 substantially increases the previously inferred excess radio background, which motivates a review of faint source populations and dark-matter decay models. Sky models scaled to our measurements could be used to set the absolute flux-density scale for SKA-Low and other low-frequency radio telescopes. They could also be used to improve the foreground characterization for cosmic dawn and epoch-of-reionization experiments.