Millimeter-VLBI observations of low-luminosity active galactic nuclei with source-frequency phase-referencing

TL;DR

This study uses millimeter-VLBI with source-frequency phase-referencing to image and analyze low-luminosity active galactic nuclei M 84 and M 87 at up to 88 GHz, revealing core properties and jet structures.

Contribution

First millimeter-VLBI imaging of M 84 at 88 GHz and detailed core-shift measurements of M 87 using source-frequency phase-referencing.

Findings

Detected the weak VLBI core of M 84 at 88 GHz.

Measured the core-shift of M 87 at multiple frequencies.

Estimated the magnetic field strength near M 87's event horizon.

Abstract

We report millimeter-VLBI results of low-luminosity active galactic nuclei (M 84 and M 87) up to 88 GHz with source-frequency phase-referencing observations. We detected the weak VLBI core and obtained the first image of M 84 at 88 GHz. The derived brightness temperature of M 84 core was about 7.2$\times$10$^9$ K, which could serve as a lower limit as the core down to 30 Schwarzschild radii was still un-resolved in our 88 GHz observations. We successfully determined the core-shifts of M 87 at 22-44 GHz and 44-88 GHz through source-frequency phase-referencing technique. The jet apex of M 87 could be deduced at about 46 $\mu$as upstream of the 43 GHz core from core-shift measurements. The estimated magnetic field strength of the 88 GHz core of M 87 is 4.8$\pm$2.4 G, which is at the same magnitude of 1-30 G near the event horizon probed by the Event Horizon Telescope.