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

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 Hydrogen Epoch of Reionization Array (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 h) observation using the upgraded Giant Metrewave Radio Telescope (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 $6.9^\circ \times 6.9^\circ$ mosaic, which yields a median rms of $3.9^{+3.7}_{-1.4}$ mJy beam−1 that reduces to $\sim 2$ mJy beam−1 at source-free regions. In the overlapping patch, this rms is deeper than the GaLactic and Extragalactic All-sky MWA Survey (GLEAM) catalogue, which is used for HERA calibration. We produce a catalogue of 640 sources (26 per cent extended) in the flux range $0.01-19.08$ 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 LOw Frequency ARray Two Meter Sky Survey. Preliminary simulations of the sky models from GLEAM and our catalogue show $\sim 10-25~{{\ \rm per\ cent}}$ 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.