Degree-scale galactic radio emission at 122 MHz around the North Celestial Pole with LOFAR-AARTFAAC

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700AV Groningen, The Netherlands
e-mail: kbharatgehlot@gmail.com
² ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
³ Department of Physics, Technion, Haifa 32000, Israel
⁴ Institute for Computational Science, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
⁵ Anton Pannekoek Institute, University of Amsterdam, Postbus 94249 1090 GE Amsterdam, The Netherlands
⁶ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 75014 Paris, France
⁷ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 17 December 2021
Accepted: 8 April 2022

Abstract

Aims. Contamination from bright diffuse Galactic thermal and non-thermal radio emission poses crucial challenges in experiments aiming to measure the 21-cm signal of neutral hydrogen from the cosmic dawn (CD) and Epoch of Reionisation (EoR). If not included in calibration, this diffuse emission can severely impact the analysis and signal extraction in 21-cm experiments. We examine large-scale diffuse Galactic emission at 122 MHz around the North Celestial Pole, using the Amsterdam-ASTRON Radio Transient Facility and Analysis Centre (AARTFAAC-) High Band Antenna (HBA) system.

Methods. In this pilot project, we present the first-ever wide-field image produced with a single sub-band of the data recorded with the AARTFAAC-HBA system. We demonstrate two methods, multi-scale CLEAN and shapelet decomposition, to model the diffuse emission revealed in the image. We used angular power spectrum metrics to quantify different components of the emission and compared the performance of the two diffuse structure modelling approaches.

Results. We observed that the point sources dominate the angular power spectrum (ℓ(ℓ + 1)C_ℓ/2π≡Δ²(ℓ)) of the emission in the field on scales of ℓ ≳ 60 (≲3 degree). The angular power spectrum after subtraction of compact sources is flat within the 20 ≲ ℓ ≲ 200 range, suggesting that the residual power is dominated by the diffuse emission on scales of ℓ ≲ 200. The residual diffuse emission has a brightness temperature variance of Δ_ℓ=180² = (145.64 ± 13.61) K² at 122 MHz on angular scales of 1 degree, and it is consistent with a power law following C_ℓ ∝ ℓ^−2.0 in the 20 ≲ ℓ ≲ 200 range. We also find that, in the current set-up, multi-scale CLEAN is suitable to model the compact and diffuse structures on a wide range of angular scales, whereas the shapelet decomposition method better models the large scales, which are of the order of a few degrees and wider.