Characterizing the astrometric instability of extragalactic radio source positions measured with geodetic VLBI

C. Gattano,P. Charlot

doi:10.1051/0004-6361/202140377

Abstract

Context. Geodetic very long baseline interferometry (VLBI) has been used to observe extragalactic radio sources for more than 40 yr. The absolute source positions derived from the VLBI measurements serve as a basis to define the International Celestial Reference Frame (ICRF). Despite being located at cosmological distances, an increasing number of these sources are found to show position instabilities, as revealed by the accumulation of VLBI data over the years. Aims. We investigate how to characterize the astrometric source position variations, as measured with geodetic VLBI data, in order to determine whether these variations occur along random or preferential directions. The sample of sources used for this purpose is made up of the 215 most observed ICRF sources. Methods. Based on the geodetic VLBI data set, we derived source coordinate time series to map the apparent trajectory drawn by the successively measured positions of each source in the plane of the sky. We then converted the coordinate time series into a set of vectors and used the direction of these vectors to calculate a probability density function (PDF) for the direction of variation of the source position. For each source, a model that matches the PDF and that comprises the smallest number of Gaussian components possible was further adjusted. The resulting components then identify the preferred directions of variation for the source position. Results. We found that more than one-half of the sources (56%) in our sample may be characterized by at least one preferred direction. Among these, about three-quarters are characterized by a unique direction, while the remaining sources show multiple preferred directions. The analysis of the distribution of these directions reveals an excess along the declination axis that is attributed to a VLBI network effect. Whether single or multiple, the identified preferred directions are likely due to source-intrinsic physical phenomena.

Highlights

For nearly 30 yr the primary celestial reference system, the International Celestial Reference System (ICRS, Arias et al 1995), has been extragalactic
We developed a method for characterizing very long baseline interferometry (VLBI) astrometric position variations and for identifying the preferred direction(s) of variability for the radio sources observed with geodetic VLBI
A probability density function on the direction of variation is calculated for each source, and a model with a minimum number of Gaussian components is fitted to this function to unveil the underlying preferred directions

Summary

Introduction

For nearly 30 yr the primary celestial reference system, the International Celestial Reference System (ICRS, Arias et al 1995), has been extragalactic It is a kinematic system defined based on fiducial marks representing stable directions in the sky. This system is realized from a set of celestial objects whose positions on the sky are obtained from observation. To tend to the ideal representation, sources with astrometric characteristics that match as closely as possible the above requirements are desired.

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