Variability analysis of a sample of southern calibrators observed using MeerKAT

Abstract

MeerKAT is a state-of-the-art radio telescope array in South Africa and a precursor to the revolutionary Square Kilometre Array (SKA). Given the critical role of calibration in ensuring that radio interferometers provide accurate scientific products, this thesis investigates the flux stability of a sample of 33 southern sky calibrators observed by MeerKAT. Our work builds upon a previous study of potential southern calibrators observed by the Karoo Array Telescope (KAT-7), a precursor to MeerKAT. The MeerKAT sample was first characterized by creating polynomial spectral models (which included archival data: 80MHz - 4.8GHz) using the SPECFIT package. We found that 26% of the calibrators had a peak, while 15% had a trough in their spectra, and 59% were fit with a linear power law. The flux variability analysis involved Long-Term Variability (LTV), corresponding to approximately a 2 year timescale, and Short-Term Variability (STV), corresponding to an order of hours timescale, with a threshold for potential variability set at variability metrics exceeding 3σ. Our LTV analysis across the three MeerKAT bands (U-, L-, and S- band) concluded that the majority of sources were stable below 1.283GHz. However, PKS J0854+2006, a well-known blazar, exhibited variability at frequencies > 1.283GHz. For the STV component, we performed a detailed statistical comparison on PKS J2152-2828, utilizing two distinct variability indices: VIAller, which provides a measure of the maximum fractional variation of the flux density and VIdebias, which takes into account the measurement uncertainties and is normalized by the mean flux. We specifically analysed two annular regions (within 0.5◦, and between 0.5◦ - 1.2◦ from the phase center) to ensure any variability observed was intrinsic to the source. We concluded that PKS J2152-2828 exhibits no significant STV, confirming its reliability as a phase calibrator for similar duration observations.