Degree-Scale Galactic Radio Emission at 122 MHz around the North
  Celestial Pole with LOFAR-AARTFAAC
  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 and Epoch of
Reionization. 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 AARTFAAC-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. We demonstrate two methods: multiscale
CLEAN and shapelet decomposition, to model the diffuse emission revealed in the
image. We use angular power spectrum metrics to quantify different components
of the emission and compare the performance of the two diffuse structure
modelling approaches. Results: We observe that the point sources dominate the
angular power spectrum ($\ell(\ell+1)C_{\ell}/2\pi\equiv \Delta^2(\ell)$) of
the emission in the field on scales $\ell\gtrsim 60$ ($\lesssim 3$~degree). The
angular power spectrum after subtraction of compact sources is flat within
$20\lesssim \ell \lesssim200$ range, suggesting that the residual power is
dominated by the diffuse emission on scales $\ell\lesssim200$. The residual
diffuse emission has a brightness temperature variance of $\Delta^2_{\ell=180}
= (145.64 \pm 13.61)~{\rm K}^2$ at 122~MHz on angular scales of 1~degree, and
is consistent with a power-law following $C_{\ell}\propto \ell^{-2.0}$ in
$20\lesssim \ell \lesssim200$ range. We also find that, in the current setup,
the multiscale 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.
