SARAS measurement of the Radio Background at long wavelengths

TL;DR

SARAS is a precision correlation spectrometer that measures the radio sky brightness at long wavelengths, providing accurate spectral data between 110 and 175 MHz and improving calibration of all-sky maps.

Contribution

This work presents the first wideband measurement of the radio background at these frequencies with high accuracy, and offers an improved calibration for existing sky maps.

Findings

Measured sky brightness with 1.2% systematic accuracy.

Calibrated the 150-MHz all-sky map reducing offset and scale errors.

Determined the spectral index range of -2.3 to -2.45 across the band.

Abstract

SARAS is a correlation spectrometer connected to a frequency independent antenna that is purpose-designed for precision measurements of the radio background at long wavelengths. The design, calibration and observing strategies admit solutions for the internal additive contributions to the radiometer response, and hence a separation of these contaminants from the antenna temperature. We present here a wideband measurement of the radio sky spectrum by SARAS that provides an accurate measurement of the absolute brightness and spectral index between 110 and 175 MHz. Accuracy in the measurement of absolute sky brightness is limited by systematic errors of magnitude 1.2%; errors in calibration and in the joint estimation of sky and system model parameters are relatively smaller. We use this wide-angle measurement of the sky brightness using the precision wide-band dipole antenna to provide an improved absolute calibration for the 150-MHz all-sky map of Landecker & Wielebinski (1970):subtracting an offset of 21.4 K and scaling by factor 1.05 will reduce the overall offset error to 8 K (from 50 K) and scale error to 0.8% (from 5%). The SARAS measurement of the temperature spectral index is in the range -2.3 to -2.45 in the 110 to 175 MHz band and indicates that the region towards the Galactic bulge has a relatively flatter index.