High-precision Astrometric Millimeter Very Long Baseline Interferometry Using a New Method for Multi-Frequency Calibration

TL;DR

This paper introduces a new multi-frequency calibration method for millimeter VLBI that achieves high-precision astrometric measurements, enabling detailed studies of AGN jet cores and their physical processes.

Contribution

The paper presents a novel calibration approach combining ionospheric and tropospheric corrections for mm-VLBI, improving astrometric accuracy and core-shift measurements.

Findings

Measured core-shift in BL Lacertae at 22-43 GHz is smaller than predicted by simple extrapolation.

Demonstrated that the VLBI core is likely associated with a recollimation shock.

Achieved astrometric alignment enabling the study of frequency-dependent jet structures.

Abstract

In this paper we describe a new approach for mm-VLBI calibration that provides bona-fide astrometric alignment of the mm-wavelength images from a single source, for the measurement of frequency dependent effects, such as `core-shifts' near the black hole of AGN jets. We achieve our astrometric alignment by solving firstly for the ionospheric (dispersive) contributions using wide-band cm-wavelength observations. Secondly we solve for the tropospheric (non-dispersive) contributions by using fast frequency-switching at the target mm-wavelengths. These solutions can be scaled and transferred from the low frequency to the high frequency. To complete the calibration chain one additional step was required to remove a residual constant phase offset on each antenna. The result is an astrometric calibration and the measurement of the core-shift between 22 and 43 GHz for the jet in BL Lacertae to be -8$\pm$5, 20$\pm$6 $\mu$as, in RA and Declination, respectively. By comparison to conventional phase referencing at cm-wavelengths we are able to show that this core shift at mm-wavelengths is significantly less than what would be predicted by extrapolating the low frequency result, which closely followed the predictions of the Blandford \& K\"onigl conical jet model. As such it would be the first demonstration for the association of the VLBI core with a recollimation shock, normally hidden at low frequencies due to the optical depth, which could be responsible for the $\gamma$-ray production in blazar jets.