Structure Effects for 3417 Celestial Reference Frame Radio Sources

M H Xu,R Heinkelmann,G L Wang,J M Anderson,S Lunz,H Schuh

doi:10.3847/1538-4365/ab16ea

Abstract

Abstract Geodetic/astrometric very long baseline interferometry (VLBI) has been routinely observing using various global networks for 40 yr, and it has produced more than 10 million baseline group delay, phase, and amplitude observables. These group delay observables are analyzed worldwide for geodetic and astrometric applications, for instance, to create the International Celestial Reference Frame (ICRF). The phase and amplitude observables are used in this paper, by means of closure analysis, to study intrinsic source structures and their evolution over time. The closure amplitude rms, CARMS, indicating how far away a source is from being compact in terms of morphology, is calculated for each individual source. The overall structure-effect magnitudes for 3417 ICRF radio sources are quantified. CARMS values larger than 0.3 suggest significant source structures and those larger than 0.4 indicate very extended source structures. The 30 most frequently observed sources, which constitute 40% of current geodetic VLBI observables, are studied in detail. The quality of ICRF sources for astrometry is evaluated by examining the CARMS values. It is confirmed that sources with CARMS values larger than 0.30 can contribute residual errors of about 15 ps to geodetic VLBI data analysis and those with the CARMS values larger than 0.4 generally can contribute more than 20 ps. We recommend CARMS values as an indicator of the astrometric quality for the ICRF sources and the continuous monitoring of the ICRF sources to update CARMS values with new VLBI observations as they become available.

Highlights

Extragalactic radio sources have been routinely observed by geodetic/astrometric5 very long baseline interferometry (VLBI) since 1979 and have been used, for example, to create the International Celestial Reference Frame (ICRF; e.g., Johnston et al 1995; Ma et al 1998) adopted by the International Astronomical Union as the fundamental celestial reference frame
The largest effort to image ICRF sources is a series of Very Long Baseline Array (VLBA) astrometric experiments called Research and Development VLBI sessions (Fey et al 1996; Fey & Charlot 1997, 2000) and the VLBA Calibrator Surveys, a series of six campaigns run on the VLBA from 1994 to 2007 (VCS1–VCS6; Beasley et al 2002; Fomalont et al 2003; Petrov et al 2005, 2006, 2008; Kovalev et al 2007)
The concept of closure amplitude rms (CARMS) based on closure amplitudes is defined and used to quantify the overall structure-effect magnitude in the geodetic/astrometric VLBI data set for each individual source

Summary

Introduction

Extragalactic radio sources have been routinely observed by geodetic/astrometric very long baseline interferometry (VLBI) since 1979 and have been used, for example, to create the International Celestial Reference Frame (ICRF; e.g., Johnston et al 1995; Ma et al 1998) adopted by the International Astronomical Union as the fundamental celestial reference frame. The newly released, third realization of the ICRF (ICRF3;6 Charlot et al 2018) contains 4536 radio sources on the sky. Since ICRF radio sources are too distant to exhibit any detectable proper motion, except apparent proper motions due to the acceleration of the solar system barycenter (see Titov et al 2011; Xu et al 2012; Titov & Lambert 2013), the ICRFs are considered to be global and quasi-inertial celestial reference frames accurate at the submilliarcsecond level.

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Conclusion