ELAIS-N1 deep field uGMRT Band-2: constraints on diffuse Galactic synchrotron emission power spectrum

ABSTRACT We present high sensitivity, low radio frequency continuum observations of the ELAIS-N1 field with 32 h of observations of the uGMRT (upgraded Giant Meterwave Radio Telescope) Band-2 ($120-250$ MHz) covering $5.86\, \rm{deg}^2$ area, achieving a central off-source RMS noise of $237\, \mu \mathrm{Jy}\,\mathrm{beam}^{-1}$ with a resolution of $11.45\,\mathrm{ arcsec}$ at the central frequency of 183 MHz. A radio source catalogue of 1027 sources statistically matches with similar observations at different frequencies within the sensitivity range of the uGMRT. The calibrated data are further used to characterize the dominant foreground, the diffuse Galactic synchrotron emission (DGSE), in angular scale and frequency regime. We derived the angular power spectrum (APS) of DGSE in two ways: image-based estimator (i-APS) and visibility-based Tapered Gridded Estimator (TGE; hereafter as t-APS). We assess the characteristics of DGSE with a power-law form of $C_{\ell } = A({1000}/{\ell })^{\beta }$. Combining data from Band-2 and earlier Band-3 observations, we derived a spectral variation of $C_{\ell }$ in the form of $C_{\ell } = A{\nu ^{-2{\alpha }}}{\ell ^{-{\beta }}}$. Our result indicates a spectral break at $\nu = 230\, {\pm }\, 5$ MHz, corresponding to a synchrotron age of $t_\rm{syn} = 106\, {\pm }\, 1$ Myr for the cosmic-ray electrons (CRe). This break result suggests a low-energy cut-off in the CRe population, leading to spectral curvature at low frequencies. Using both of the techniques, i-APS and t-APS, we find that the mean spectral index $\alpha$ and power-law index $\beta$ are consistent within the frequency range $120-500$ MHz.