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The compact radio source Sagittarius~A$^*$ (Sgr~A$^*$)in the Galactic Centeris the primary supermassive black hole candidate. General relativisticmagnetohydrodynamical (GRMHD) simulations of the accretion flow aroundSgr\,A$^*$ predict the presence of sub-structure at observing wavelengths of$\sim 3$\,mm and below (frequencies of 86\,GHz and above). For very longbaseline interferometry (VLBI) observations of Sgr\,A$^*$ at this frequency theblurring effect of interstellar scattering becomes subdominant, and arrays suchas the High Sensitivity Array (HSA) and the global mm-VLBI Array (GMVA) are nowcapable of resolving potential sub-structure in the source. Such investigationsimprove our understanding of the emission geometry of the mm-wave emission ofSgr\,A$^*$, which is crucial for constraining theoretical models and forproviding a background to interpret 1\,mm VLBI data from the Event HorizonTelescope (EHT). We performed high-sensitivity very long baselineinterferometry (VLBI) observations of Sgr\,A$^*$ at 3\,mm using the Very LongBaseline Array (VLBA) and the Large Millimeter Telescope (LMT) in Mexico on twoconsecutive days in May 2015, with the second epoch including the Green BankTelescope (GBT). We find an overall source geometry that matches previousfindings very closely, showing a deviation in fitted model parameters less than3\% over a time scale of weeks and suggesting a highly stable global sourcegeometry over time. The reported sub-structure in the 3\,mm emission ofSgr\,A$^*$ is consistent with theoretical expectations of refractive noise onlong baselines. However, comparing our findings with recent results from 1\,mmand 7\,mm VLBI observations, which also show evidence for east-west asymmetry,an intrinsic origin cannot be excluded. Confirmation of persistent intrinsicsubstructure will require further VLBI observations spread out over multipleepochs.

Micro-arcsecond structure of Sagittarius A* revealed by high-sensitivity 86 GHz VLBI observations