Accounting for the Structure of a Radio Source in the Processing of Geodetic VLBI Observations Using the 0014+813 Source as an Example

S. L. Kurdubov, A. B. Serbin

Transactions of IAA RAS, issue 66, 18–28 (2023)

DOI: 10.32876/ApplAstron.66.18-28

Keywords: extragalactic radio source, radio source signal delay, group delay, structural delay, radio brightness map, VLBI, ionospheric-free delay combination, Gaussian function

Abstract

This article is devoted to studying the possibilities of taking into account the structure of a radio source when processing VLBI observation data. The group delay of a signal from extragalactic radio sources between two remote radio telescopes, determined from VLBI observations, is the sum of delays due to various effects, including the structural structure of the radio source. Many of the sources included in the International Celestial Reference Frame (ICRF) have a spatially extended structure, so the structural delay must be taken into account when processing the results of VLBI observations. However, in the example of structural structure modeling, there are problems, one of which is the variability of the source structure. The research method is based on constructing a numerical model of the structural delay using the formulas given in the article by Patrick Charlot. As input data, we used FITS files containing radio brightness maps from the http:// astrogeo.org/vlbi_images/ database for 2018–2019 and residuals obtained as a result of processing daily VLBI observation data on the DiFX correlator for the same period of time. In the calculations, we used not the pixels of the radio brightness map contained in the FITS file, but Gaussian functions that approximate the radio brightness map contained in the same FITS file. The written program allows you to simulate radio brightness maps by selecting the brightest structural elements of the source map, taking into account which minimizes the standard deviation of the residuals. At the beginning of the work, a program was written to calculate the structural delay of the signal depending on universal time. To test the program, the obtained dependence for the radio source NRAO140 (0333+321) was compared with the data from the original article. Next, using this program, ionospheric-free combinations of structural delays were calculated depending on sidereal time. When comparing the obtained delays for source 0014+813 with the residuals obtained by the DiFX correlator, the similarity of their dependences on sidereal time was discovered. The standard deviations of the residuals were also calculated after subtracting from them the ionospheric-free combinations of structural delays. It was shown that taking into account all Gaussians of the radio source model does not lead to a significant reduction in the standard deviation for most FITS files. Therefore, a special block was added to the program, using a certain algorithm to select Gaussians, taking into account which best reduces the standard deviation of the residuals. The use of this algorithm led to a significant reduction in the standard deviation of residuals when using all FITS files.

Citation