Phase-referencing on BL Lac

TL;DR

This study used multi-epoch phase-referencing VLBI observations of BL Lac to investigate jet precession, revealing a potential core position trend over two years, but seasonal weather effects dominate observed periodic variations.

Contribution

It presents the first multi-frequency, multi-epoch phase-referencing analysis of BL Lac's jet precession with detailed position measurements.

Findings

Detected a core position trend of ~0.1 mas over two years.

Observed periodic position variations of ~0.15 mas with a 1-year cycle.

Attributed seasonal weather effects to observed periodic variations.

Abstract

We report the results of a phase-referencing study aimed at uncovering precession of the VLBI jet of BL Lac. The observations were conducted at 8, 15, 22, and 43 GHz and consist of seven epochs spanning about two years. We investigated the change in the absolute position of BL Lac's radio core by means of phase-referencing with two nearby sources, 2151+431 and 2207+374. The shift in the position of the core perpendicular to the jet is a signature of precession. However, the periodic variations with an amplitude of ~0.15 mas and a period of 1 year can be attributed to seasonal weather variations. We also detect a trend in position of the core on the scale of ~0.1 mas over two years.