The core-shift of Sagittarius A* as a discriminant between disk and jet emission models with millimeter very long baseline interferometry

¹ Department of Astrophysics/IMAPP, Radboud University P.O. Box 9010 6500 GL Nijmegen, The Netherlands
² ASTRON Oude Hoogeveensedijk 4 7991 PD Dwingeloo, The Netherlands

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 February 2022
Accepted: 25 April 2025

Abstract

Context. The nature of the emission region around Sagittarius A* (Sgr A*), the supermassive black hole at the Galactic center, remains under debate. A prediction of jet models is that a frequency-dependent shift in the position of the radio core (i.e., core-shift) of an active galactic nucleus (AGN) occurs when observing emission dominated by a highly collimated relativistic outflow.

Aims. We use millimeter very long baseline interferometry to study the frequency-dependent position of Sgr A*’s radio core, estimate the core-shift for different emission models, investigate the core-shift evolution as a function of viewing angle and orientation, and study its behavior in the presence of interstellar scattering.

Methods. We simulated images of the emission around Sgr A* for accretion inflow models (disks) and relativistic outflow models (jets). They were based on three-dimensional general relativistic magnetohydrodynamic simulations. We created flux density maps at 22, 43, and 86 GHz sampling different viewing angles and orientations of Sgr A*, examined the effects of scattering, and simulated observations with the Very Long Baseline Array (VLBA).

Results. Jet-dominated models show significantly larger core-shifts than disk-dominated models – in some cases by a factor of 16 – with intermediate viewing angles (i = 30° ,45°) showing the largest core-shifts. Our jet models follow a power-law relation for the frequency-dependent position of Sgr A*’s core. Their core-shifts decrease as the position angle increases from 0° to 90°. Disk models do not fit a power-law relation well and their core-shifts are insensitive to changes in the viewing angle. We obtain a maximum value of 241.65 ± 1.93 μas cm⁻¹ for the core-shift of jet models including refractive scattering in an ideal case. In addition, the same models observed by the VLBA yield a value of 215.57 ± 1.95 μas cm⁻¹.

Conclusions. The core-shift can serve as a tool to discriminate between jet- and disk-dominated models and to constrain the geometry of Sgr A*. Our jet models agree with earlier predictions of AGNs with collimated jets, and the core-shift is retrievable even in the presence of interstellar scattering and when simulating observations.