Frequency-Dependent Core Shifts in Ultracompact Quasars

TL;DR

This study demonstrates a new method to measure frequency-dependent core shifts in ultracompact quasars using multi-frequency VLBI observations and relative astrometry, expanding the ability to analyze core positions in compact sources.

Contribution

A novel approach for detecting core shifts in ultracompact sources using relative astrometry with VLBI triplets, overcoming limitations of traditional methods.

Findings

Successfully measured core shifts in 24 AGN jets.

Developed a new method applicable to ultra-compact sources.

Identified key conditions for accurate core shift detection.

Abstract

Results of a pilot project with the participation of the "Kvazar-KVO" radio interferometry array in observations carried out with the European VLBI Network are presented. The aim of the project was to conduct and analyze multi-frequency (1.7, 2.3, 5.0, 8.4 GHz) observations of the parsec-scale jets of 24 active galactic nuclei. Three observing sessions were successfully carried out in October 2008. Maps of the radio intensity distributions have been constructed in all four frequencies using phase referencing. A method for measuring the frequency-dependent shift of the position of the VLBI core by applying relative astrometry to observations of close triplets of radio sources has been developed. The fundamental possibility of detecting core shifts in ultra-compact sources for which traditional methods based on the achromatic positions of optically thin regions of the jet are not suitable is demonstrated. The conditions for successful measurement of this shift are discussed; these are determined by the closeness of the calibrator used, the effective resolution of the system, the quality of the filling of the $uv$ plane, the relative orientations of the jets in the triplets, and the brightnesses of the sources.