Detections of Nearly Bias-free Core Shifts with 5-30 μas Precisions at 8-43 GHz in BL Lacertae

Journal article, 2026

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) 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 the jet. The derived core-shift index, k(r)=1.18(-0.34)(+0.59), 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.