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

TL;DR

This work tackles the need for precise absolute sky brightness measurements at low radio frequencies (60–350 MHz) to inform Galactic cosmic-ray lepton models, extragalactic source populations, and potential dark matter signatures. It implements a field-ready, self-calibrating radiometer system (GINAN) coupled to a wideband SKALA4.1 antenna to measure the diffuse sky and compare it to the Global Sky Model, deriving frequency-dependent corrections. The key contributions are a substantial upward revision of GSM brightness across the band, a practical two-parameter GSM correction (offset and scale) validated across the frequency range, and a robust diffuse-sky reference for calibrating the SKA-Low absolute flux density scale. These results impact foreground modeling for CMB analyses, reionisation-era astrophysics, and the interpretation of the extragalactic radio background, while providing a reliable primary calibrator for low-frequency radio astronomy.

Abstract

Precise measurement of the sky radio brightness below 1 GHz and estimation of any unaccounted-for extragalactic brightness is required to understand the Galactic cosmic ray electron spectrum, to constrain populations of nanojansky radio sources, and to constrain dark matter annihilation or decay. The foreground radio brightness must also be accurately accounted for when measuring the cosmic background radiation and departures from its Planck spectrum that trace astrophysical processes in the early Universe, cosmic dark ages, cosmic dawn and the epoch of reionisation. Here we report a new, precision measurement of the sky spectral brightness over radio frequencies from 60 MHz to 350 MHz. Our measurement motivates a significant correction to previous all-sky images made in this band and the Global Sky Model (GSM) that is constructed from these and other sky images made at radio wavelengths. We find that the GSM requires subtraction of an offset exceeding 100K below 100 MHz and scaling up by a factor of approximately 1.2 below 200 MHz rising to a factor of 1.5 at 350 MHz, thus significantly enhancing previous estimates of unaccounted excess in radio sky brightness. Our measurements were made with a new receiver architecture that dynamically self-calibrates for receiver noise and bandpass in situ, while connected to an antenna. We used a single, accurately modelled, wideband logperiodic antenna placed on a 40m diameter ground mesh. Our accurate measurement requires upward revision of sky brightness and motivates revisiting models for source populations and dark matter decay. Additionally, sky models scaled to our measurements serve as a stable reference in calibrating the absolute flux density scale for low-frequency radio telescopes. This will be important for calibration accuracy of the SKA-Low telescope, that will operate at the frequencies of our measurements.

Figures (16)

• Figure 1: The observing system at Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-astronomy Observatory in Australia. A single SKALA4.1 antenna 9107113 at the centre of a 40 ground mesh is connected via a 3.1 coaxial cable to the smaller GINAN receiver box at the base of the antenna that contains a switch, calibration loads, and a noise source. The smaller box is connected via a 19 coaxial cable to the main GINAN receiver box in a tent at the edge of the mesh. See Extended Data Fig. \ref{['fig:receiver_small_box']} for a close-up of the smaller box that is difficult to see here.
• Figure 1: $|$ Smaller GINAN receiver box. The small rectangular shielded box at the base of the antenna contains a mechanical RF switch, a noise source, and four calibration loads. The switch is cycled to connect the antenna, the noise source, then each of the four loads in turn to the main receiver box located near the edge of the ground mesh.
• Figure 2: Distribution of antenna temperature in time-frequency space. Log$_{10}$ of the antenna temperatures in units of kelvin are displayed, with the colour bar on the right providing the scale which is the same for both plots. a, The measured antenna temperatures. b, A prediction made using the GSM.
• Figure 2: $|$ Antenna end caps.c, The original SKALA4.1 antenna end cap with embedded LNA for installation at the vertex of each antenna polarization. b The modified end cap we used for measurements in this work, where the LNA is replaced with a direct microstrip connection to an SMB coaxial RF connector. The antenna may then be connected via coaxial cable to an external receiver. a, Modified end cap that replaces the LNA with a 50 load. This was used to terminate the unused antenna polarization of the antenna for our measurements.
• Figure 3: a, Scatter plot of antenna temperatures showing GINAN measured temperature versus predicted temperature calculated from the Global Sky Model 2017MNRAS.464.3486Z and electromagnetic simulations of expected antenna performance. This plot overlays data for all frequencies and all measurement epochs. All points would lie on the ideal dashed green line of unity slope and zero intercept if our measured data agreed with the GSM prediction. b, Derived corrections for the sky brightness temperatures in the GSM; the model must have the offset subtracted and the remainder scaled up by the scale factor to match our measurements. Offset and scale corrections are solved for independently at each frequency to best fit data for all measurement epochs.