2018ApJ...853L..14J

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 eventually could be 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 millimeter/submillimeter VLBI observations of weaker sources. From the analysis of the M81* visibilities, a core size of ∼50 µas at 3.4 mm was estimated. This follows the power-law relationship with wavelength, ∝λ^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*.