The Power of Simultaneous Multi-Frequency Observations for mm-VLBI: Astrometry up to 130 GHz with the KVN

TL;DR

This paper demonstrates that simultaneous multi-frequency observations with KVN significantly improve coherence times and enable high-precision astrometry at up to 130 GHz, overcoming atmospheric limitations in mm-VLBI.

Contribution

It introduces empirical methods using FPT and SFPR techniques for enhanced coherence and astrometric accuracy at high frequencies in mm-VLBI.

Findings

Coherence time at 130 GHz increased from tens of seconds to about twenty minutes with FPT.

SFPR extended coherence times to many hours, enabling long integrations.

First high-precision astrometry at 130 GHz achieved.

Abstract

Simultaneous observations at multiple frequency bands have the potential to overcome the fundamental limitation imposed by the atmospheric propagation in mm-VLBI observations. The propagation effects place a severe limit in the sensitivity achievable in mm-VLBI, reducing the time over which the signals can be coherently combined, and preventing the use of phase referencing and astrometric measurements. We carried out simultaneous observations at 22, 43, 87 and 130 GHz of a group of five AGNs, the weakest of which is ca. 200 mJy at 130 GHz, with angular separations ranging from 3.6 to 11 degrees, using the KVN. We analysed this data using the Frequency Phase Transfer (FPT) and the Source Frequency Phase Referencing (SFPR) techniques, which use the observations at a lower frequency to correct those at a higher frequency. The results of the analysis provide an empirical demonstration of the increase in the coherence times at 130 GHz from a few tens of seconds to about twenty minutes, with FPT, and up to many hours with SFPR. Moreover the astrometric analysis provides high precision relative position measurements between two frequencies, including, for the first time, astrometry at 130 GHz. Finally we demonstrate a method for the generalised decomposition of the relative position measurements into absolute position shifts for bona fide astrometric registration of the maps of the individual sources at multiple frequencies, up to 130 GHz.