A Database of Phase Calibration Sources and their Radio Spectra for the Giant Metrewave Radio Telescope

TL;DR

This paper presents the first release of a database of 45 low-frequency calibration sources for the GMRT, including their spectra, models, and images, to aid radio astronomy calibration efforts.

Contribution

It provides a publicly available, detailed database of calibration sources with spectral and imaging data, enhancing calibration accuracy for GMRT observations.

Findings

Calibration sources are evenly distributed across the sky.

Spectra are divided into straight, curved, and complex shapes.

Quasars have flatter and more variable spectra, while radio galaxies have steeper spectra.

Abstract

We are pursuing a project to build a database of phase calibration sources suitable for Giant Metrewave Radio Telescope (GMRT). Here we present the first release of 45 low frequency calibration sources at 235 MHz and 610 MHz. These calibration sources are broadly divided into quasars, radio galaxies and unidentified sources. We provide their flux densities, models for calibration sources, (u,v) plots, final deconvolved restored maps and clean-component lists/files for use in the Astronomical Image Processing System (AIPS) and the Common Astronomy Software Applications (CASA). We also assign a quality factor to each of the calibration sources. These data products are made available online through the GMRT observatory website. In addition we find that (i) these 45 low frequency calibration sources are uniformly distributed in the sky and future efforts to increase the size of the database should populate the sky further, (ii) spectra of these calibration sources are about equally divided between straight, curved and complex shapes, (iii) quasars tend to exhibit flatter radio spectra as compared to the radio galaxies or the unidentified sources, (iv) quasars are also known to be radio variable and hence possibly show complex spectra more frequently, and (v) radio galaxies tend to have steeper spectra, which are possibly due to the large redshifts of distant galaxies causing the shift of spectrum to lower frequencies.