Detections of nearly bias-free core shifts with 5-30 $\mu$as precisions at 8-43 GHz in BL Lacertae

TL;DR

This study measures core shifts in BL Lacertae with unprecedented precision using VLBI across multiple frequencies, providing insights into jet structure and magnetic fields.

Contribution

It presents nearly bias-free, high-precision two-dimensional core shift measurements at multiple frequencies, advancing understanding of jet physics in BL Lacertae.

Findings

Core shift between 8.4 and 43.2 GHz is 250 μas.

Core shifts scale with frequency as ν^{-1/k_r}.

Core-shift index k_r is approximately 1.18, consistent with energy equipartition.

Abstract

When a radio jet is partially optically thick in the launching region, its apparent compact core may display frequency-dependent positional shifts. High-precision astrometric measurements of core shifts enable astronomers to pinpoint the jet's origin and place tight constraints on the magnetic field. BL Lacertae, the archetypal BL Lac object, hosts a highly variable and well-collimated jet. To independently constrain its innermost core shifts, we conducted very long baseline interferometric (VLBI) observations at 8.4, 12.4, 15.2, 23.6, and 43.2 GHz. By exploiting a nearby (13.3 arcmin) steep-spectrum calibrator (NVSS J220340+420839) through inverse phase-referencing VLBI astrometry, we detect nearly unbiased two-dimensional core shift measurements with state-of-the-art precisions of 5-30 $\mu$as, which are significant at $>3\sigma$ confidence. The core shift between 8.4 and 43.2 GHz reaches 250 $\mu$as. The apparent core shifts scale with frequency as $\nu^{-1/k_r}$, implying the existence of an optically thick region in the upstream of jet. The derived core-shift index, $k_r\!=\!1.18^{+0.59}_{-0.34}$, is consistent, within uncertainties, with the canonical $k_r\!=\!1$ expected under energy equipartition between the jet particle and magnetic field energy densities, while allowing for modest deviations given that BL Lacertae was captured in a flaring state.