Deep uGMRT observations for enhanced calibration of 21 cm arrays -- I. First image and source catalogue

TL;DR

This work demonstrates that deep uGMRT observations of the HERA calibration field GLEAM 02H can produce a high-fidelity sky model with sub-arcsecond astrometry and deeper source counts than existing surveys. By building a 6.9°×6.9° mosaic from seven pointings and a 640-source catalogue, the study delivers precise flux densities, morphology classifications, and spectral indices, enabling improved absolute calibration for redundant 21-cm arrays. Through FDR, visibility-area, and completeness corrections, the resulting Euclidean-normalized source counts extend down to ~20 mJy at ~150 MHz and are consistent with LoTSS, validating the uGMRT catalogue for calibration use. Preliminary sky-model simulations for HERA indicate 10–25% amplitude differences between uGMRT+GLEAM and GLEAM-only models with small phase shifts, highlighting the practical impact on calibration fidelity and motivating broader, wider-band follow-up surveys.

Abstract

Radio-interferometric arrays require very precise calibration to detect the Epoch of Reionization 21-cm signal. A remarkably complete and accurate sky model is therefore needed in the patches of the sky used to perform the calibration. Instruments such as HERA, which use a redundant calibration strategy, also require a reference sky model to fix degenerate gain solutions. We have carried out a deep (20 hours) observation using the upgraded GMRT to make a high-fidelity sky model of one of the HERA calibration fields GLEAM 02H (J0200$-$3053). Here, we present the results from a $16.7\,\rm{MHz}$ bandwidth data centred at $147.4\,\rm{MHz}$. Using multiple GMRT pointings, we have made a $6.9^\circ\times6.9^\circ$ mosaic, which yields a median rms of $3.9^{+3.7}_{-1.4}\,{\rm mJy/beam}$ that reduces to $\sim2\,{\rm mJy/beam}$ at source-free regions. In the overlapping patch, this rms is deeper than the GLEAM catalogue, which is used for HERA calibration. We produce a catalogue of $640$ sources ($26\%$ extended) in the flux range $0.01-19.08\,{\rm Jy}$. The catalogue has a sub-arcsec positional accuracy, and the estimated fluxes are consistent with existing catalogues. The differential source counts are found to be deeper than GLEAM and consistent with LoTSS. Preliminary simulations of the sky models from GLEAM and our catalogue show $\sim 10-25\%$ difference in the visibility amplitude, with relatively small phase difference ($\approx 2^\circ$). Future work is planned for larger survey areas and wider bandwidth to reduce the rms and measure the in-band source spectral indices, which are expected to enhance the fidelity of the HERA calibration model.

Figures (19)

• Figure 1: The radio sky at 150 MHz from the Global Sky Model DeOliveiraCosta2008. The dashed lines show the HERA observation stripe centred at HERA’s latitude of $-30.7^{\circ}$ with a width of $10^{\circ}$, which is the FWHM of the primary beam at 150 MHz. The green solid circles show the three ideal calibration fields currently used for absolute calibration. The inset depicts our strategy to observe the GLEAM 02H field with the uGMRT. Adapted from Kern2020.
• Figure 2: The strategy to observe the field GLEAM 02H with uGMRT. The dashed circles represent the Full Width at Half Maxima (FWHM = $1.82^{\circ}$) at 250 MHz, the highest available frequency of Band 2 ($120-250$ MHz). The positions (RA, Dec) of all seven PCs are in Table \ref{['table:ra_dec']}.
• Figure 3: The primary beam corrected images of all the PCs. The extent of the images is restricted to 30% of the peak primary beam response. The coordinates of each PC, the rms of the images, and the PSFs are given in Table \ref{['table:ra_dec']}.
• Figure 4: The final mosaic image of the HERA calibration field GLEAM 02H. It is made by combining the primary beam corrected images of all the 7 PCs, which are shown in Figure \ref{['fig:fits_plot']}.
• Figure 5: A zoomed-in view of the central region of the mosaic image. The left panel shows the central $3^\circ \times 3^\circ$ region, where we expect relatively uniform sensitivity. The cyan rectangle outlines a $1.5^\circ \times 1.5^\circ$ region, which is shown in more detail in the middle panel. The magenta square in the middle panel highlights a $0.5^\circ \times 0.5^\circ$ region, which is further magnified in the rightmost panel. The bright source at the center of these zoomed regions is the Quasar PKS 0157--31.