Spectral index of the Galactic foreground emission in the 50-87 MHz range
Abstract
Total-power radiometry with individual meter-wave antennas is a potentially effective way to study the Cosmic Dawn (z20) through measurement of sky brightness arising from the 21~cm transition of neutral hydrogen, provided this can be disentangled from much stronger Galactic and extra-galactic foregrounds. In the process, measured spectra of integrated sky brightness temperature can be used to quantify the foreground emission properties. In this work, we analyze a subset of data from the Large-aperture Experiment to Detect the Dark Age (LEDA) in the range 50-87~MHz and constrain the foreground spectral index β in the northern sky visible from mid-latitudes. We focus on two zenith-directed LEDA radiometers and study how estimates of β vary with local sidereal time (LST). We correct for the effect of gain pattern chromaticity and compare estimated absolute temperatures with simulations. We develop a reference dataset consisting of 14 days of optimal condition observations. Using this dataset we estimate, for one radiometer, that β varies from -2.55 at LST~<6~h to a steeper -2.58 at LST~13~h, consistently with sky models and previous southern sky measurements. In the LST~=13-24~h range, however, we find that β fluctuates between -2.55 and -2.61 (data scatter 0.01). We observe a similar β vs. LST trend for the second radiometer, although with slightly smaller |β|, in the -2.46<β<-2.43 range, over 24~h of LST (data scatter 0.02). Combining all data gathered during the extended campaign between mid-2018 to mid-2019, and focusing on the LST~=9-12.5~h range, we infer good instrument stability and find -2.56<β<-2.50 with 0.09<β<0.12.