VLBI imaging of M81* at 3.4 mm with source-frequency phase-referencing

TL;DR

This paper presents the first VLBI image of M81* at 3.4 mm using source-frequency phase-referencing, achieving high sensitivity and resolution, and constrains the core size relative to the black hole's Schwarzschild radius.

Contribution

It demonstrates the successful application of SFPR technique at 3.4 mm, enabling detection of weaker sources and precise core size measurement at this wavelength.

Findings

Detected M81* at 3.4 mm with high dynamic range

Estimated core size of ~50 microarcseconds at 3.4 mm

Core size follows a lambda^0.88 power-law relationship

Abstract

We report on the first VLBI image of the M81 nucleus (M81*) at a wavelength of 3.4 mm, obtained with the source-frequency phase-referencing (SFPR) technique. Thanks to the SFPR calibration, the coherent integration time could be eventually increased by more than an order of magnitude, which enabled the detection of fringes at the level of 45 mJy beam-1 with a dynamic range higher than 130:1. This paves the way toward future mm/sub-mm VLBI observations of weaker sources. From the analysis of the M81* visibilities, a core size of ~50 uas at 3.4 mm was estimated. This follows the power-law relationship with wavelength lambda^0.88, reported previously at lower frequencies. These results constrain the core size (at 3.4 mm) to a minimum of ~80 Schwarzschild radii of M81*.