Improved Measurement of the Spectral Index of the Diffuse Radio Background Between 90 and 190 MHz

TL;DR

This paper presents highly precise measurements of the diffuse radio background spectral index between 90 and 190 MHz, improving accuracy over previous studies and comparing results with sky models.

Contribution

It provides the most accurate spectral index measurements between 90 and 190 MHz, reducing uncertainty from 0.10 to 0.02, and compares these with existing sky models and maps.

Findings

Spectral index between -2.60 and -2.62 across 0-12 h LST

Spectral index flattens to -2.50 at 17.7 h LST during Galactic Centre transit

EDGES instrument shows high stability with night-to-night reproducibility

Abstract

We report absolutely calibrated measurements of diffuse radio emission between 90 and 190 MHz from the Experiment to Detect the Global EoR Signature (EDGES). EDGES employs a wide beam zenith-pointing dipole antenna centred on a declination of -26.7$^\circ$. We measure the sky brightness temperature as a function of frequency averaged over the EDGES beam from 211 nights of data acquired from July 2015 to March 2016. We derive the spectral index, $\beta$, as a function of local sidereal time (LST) and find -2.60 > $\beta$ > -2.62 $\pm$0.02 between 0 and 12 h LST. When the Galactic Centre is in the sky, the spectral index flattens, reaching $\beta$ = -2.50 $\pm$0.02 at 17.7 h. The EDGES instrument is shown to be very stable throughout the observations with night-to-night reproducibility of $\sigma_{\beta}$ < 0.003. Including systematic uncertainty, the overall uncertainty of $\beta$ is 0.02 across all LST bins. These results improve on the earlier findings of Rogers & Bowman (2008) by reducing the spectral index uncertainty from 0.10 to 0.02 while considering more extensive sources of errors. We compare our measurements with spectral index simulations derived from the Global Sky Model (GSM) of de Oliveira-Costa et al. (2008) and with fits between the Guzm\'an et al. (2011) 45 MHz and Haslam et al. (1982) 408 MHz maps. We find good agreement at the transit of the Galactic Centre. Away from transit, the GSM tends to over-predict (GSM less negative) by 0.05 < $\Delta_{\beta} = \beta_{\text{GSM}}-\beta_{\text{EDGES}}$ < 0.12, while the 45-408 MHz fits tend to over-predict by $\Delta_{\beta}$ < 0.05.