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 \sim 50 \mu as at 3.4 mm was estimated .
This follows the power-law relationship with wavelength , \propto \lambda ^ { 0.88 } , reported previously at lower frequencies .
These results constrain the core size ( at 3.4 mm ) to a minimum of \thicksim 80 Schwarzschild radii of M81* .