Three-Parameter Model Building of the Quasar Structure Based on Geodetic VLBI Observation Residuals

A. B. Serbin, S. L. Kurdubov

Transactions of IAA RAS, issue 72, 37–41 (2025)

DOI: 10.32876/ApplAstron.72.37-41

Keywords: structural delay, extragalactic radio source, group delay, radio brightness map, VLBI, ionosphere-free combination of delays

Abstract

The importance of taking into account the quasar structure is due to the fact that its presence causes a structural delay in VLBI observations. However, the structural delay is not taken into account during processing VLBI observations, so it introduces an error in the parameters being determined. Previously, many authors have shown that the use of classical radio brightness maps does not improve the situation due to the variability of the quasar structure and the features of the algorithms for constructing radio brightness maps. This paper is devoted to studying the possibility of constructing models of the quasar radio brightness structure using measured delays. In this research, when modeling the quasar radio brightness structure, the residuals obtained as the result of processing geodetic VLBI observations are used. The contribution of the structural delay appears in the residuals in the form of significant systematic deviations when constructing a graph of the residual dependence on sidereal time. Accordingly, correct accounting of the structural delay should lead to the elimination of these systematic deviations and a decrease in the standard deviation of the residuals. The criterion for the correctness of the constructed model is the maximum possible decrease in the standard deviation of the residuals after subtracting the constructed model of the structural delay from them. An algorithm based on Monte Carlo and gradient descent methods is proposed, when modelling the quasar structure is represented as a set of delta functions, all of which are characterized by three parameters. It was shown that the parameters of the model constructed from the residuals significantly depend on the baseline length since the angular resolution of the interferometer base is inversely proportional to the length of the baseline.

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