High Precision Astrometric Millimeter VLBI Using a New Method for Atmospheric Calibration

TL;DR

This paper introduces a novel Source-Frequency Phase Referencing method that combines fast and slow switching techniques to achieve high-precision astrometry in millimeter VLBI, demonstrated with VLBA observations at 86 GHz.

Contribution

The paper presents a new calibration method for millimeter VLBI that enables accurate astrometry and registered imaging, improving upon existing techniques.

Findings

Successful application with VLBA at 86 GHz

Achieved high-precision relative astrometry

Method is broadly applicable to millimeter VLBI

Abstract

We describe a new method which achieves high precision Very Long Baseline Interferometry (VLBI) astrometry in observations at millimeter wavelengths. It combines fast frequency-switching observations, to correct for the dominant non-dispersive tropospheric fluctuations, with slow source-switching observations, for the remaining ionospheric dispersive terms. We call this method Source-Frequency Phase Referencing. Provided that the switching cycles match the properties of the propagation media, one can recover the source astrometry. We present an analytic description of the two-step calibration strategy, along with an error analysis to characterize its performance. Also, we provide observational demonstrations of a successful application with observations using the Very Long Baseline Array at 86 GHz of the pairs of sources 3C274 & 3C273 and 1308+326 & 1308+328, under various conditions. We conclude that this method is widely applicable to millimeter VLBI observations of many target sources, and unique in providing bona-fide astrometrically registered images and high precision relative astrometric measurements in mm-VLBI using existing and newly built instruments.