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We report on the first VLBI image of the M81 nucleus (M81*) at a wavelengthof 3.4 mm, obtained with the source-frequency phase-referencing (SFPR)technique. Thanks to the SFPR calibration, the coherent integration time couldbe eventually increased by more than an order of magnitude, which enabled thedetection of fringes at the level of 45 mJy beam-1 with a dynamic range higherthan 130:1. This paves the way toward future mm/sub-mm VLBI observations ofweaker sources. From the analysis of the M81* visibilities, a core size of ~50uas at 3.4 mm was estimated. This follows the power-law relationship withwavelength lambda^0.88, reported previously at lower frequencies. These resultsconstrain the core size (at 3.4 mm) to a minimum of ~80 Schwarzschild radii ofM81*.

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