Spectral index of the Galactic foreground emission in the 50–87 MHz range with LEDA

Abstract

Total power radiometry with individual meter-wave antennas is a potentially effective means to detect the brightness temperature from the 21 cm line of neutral hydrogen during the Cosmic Dawn. In the process, spectra of integrated sky brightness temperature can be used to quantify properties of foreground emission. In this work we analyze a subset of data from the Large-aperture Experiment to Detect the Dark Age (LEDA) and constrain the spectral index β of foreground emission in the northern sky. We correct for the effect of gain pattern chromaticity and compare estimated absolute temperatures with simulations. We estimate variation in β with local sidereal time (LST) using two widely spaced, zenith-directed radiometers during May and December 2018, and January-May 2019. For times in the 9-12.5h LST window when the Sun is not in the Sky and the Galactic Center is at least 20° below the local horizon’ we estimate β=-2.48\pm 0.07 for the first antenna, and β=-2.57+0.12 for the second. These results are consistent with previous measurements of the southern sky. Anomalously heavy rainstorms moved through the observing site in winter 2019. The coincident timing of episodic shifts in the temperature scales of the two radiometers suggests substantial sensitivity to the the changes in the soil moisture profile, though other variables may also be involved. We extend the analysis December/January data, for which soil conditions are optimal, to an almost 24 h LST range finding results in agreement with simulated sky models.