A Point-source Catalogue from Direction-dependent Calibrated 21CMA Images
Abstract
We present a deep low-frequency radio point-source catalogue centred on the North Celestial Pole (NCP) and extending 5○ in radius, derived from a single 24-h observation with the 21 CentiMeter Array (21CMA). Covering 75–175 MHz, the catalogue characterises extragalactic foreground point sources for Epoch of Reionization (EoR) studies. Previous 21CMA imaging has been limited by reduced dynamic range caused by direction-dependent effects, particularly the severe grating lobes of ultra-bright sources such as 3C 61.1 and ionospheric phase distortions. Here we apply 21CMAcali, a direction-dependent calibration and imaging pipeline, to mitigate these effects. The angular resolution ranges from 2.3 arcmin at 100 MHz to 1.4 arcmin at 168.75 MHz. The resulting maps show reduced artefacts and background noise, enabling more reliable recovery of faint sources. We construct a catalogue of 1246 radio point sources. Cross-matching with LoTSS-DR3, WENSS, and NVSS enables broad-band spectral fitting, while an independent in-band analysis yields a median spectral index of α = −0.825 (where S∝να). The Euclidean-normalised differential source counts at 150 MHz, derived from a dedicated 50-MHz-wide multi-frequency synthesis (MFS) image, are broadly consistent with existing measurements and semi-empirical models at the bright end, whereas the deficit at lower flux densities (S ≲ 0.04 Jy) mainly reflects limited sensitivity and incompleteness. This catalogue improves foreground point-source characterisation in the 21CMA NCP field, demonstrates the value of direction-dependent calibration, and provides a reference for deeper catalogues based on multi-day observations.