Astrophys. J. 859, 60 (2018)

Using a network of millimetre-wave telescopes spanning Northern and Southern hemispheres, Ru-Sen Lu and collaborators have detected plasma structures around the Galaxy’s supermassive black hole (SMBH) Sagittarius A* at distances of three Schwarzschild radii (3 RS; ~0.3 au). The uv coverage of the six very-long-baseline interferometry (VLBI) stations was not sufficient to determine the details of the structures, but they are consistent with two-component asymmetric models and not with a single symmetric Gaussian model.

The 230 GHz (1.3 mm) data were taken in March 2013 using five telescopes in the United States, and the APEX telescope in Chile — the addition of which almost doubled the length of previous VLBI baselines and delivered a spatial resolution of just 30 microarcsec. An emitting region around Sgr A* was seen on the expected scale of the SMBH’s shadow (5 RS). General relativistic magnetohydrodynamic models of accretion flows around SMBHs exhibit a variety of structures slightly smaller than these size scales: crescents are indications of highly turbulent magnetic fields in the accretion flow (disk-dominated model), whereas compact filaments at jet footprints are indicators of well-ordered, strong magnetic fields (jet-dominated model). Fitting slightly favours a disk-dominated model, but a jet-dominated model cannot be ruled out for Sgr A*. Both models show a brightness asymmetry and a northeast–southwest orientation.

The April 2017 1.3-mm VLBI campaign, which additionally included the ALMA telescopes, should have sufficient uv coverage to distinguish between the two types of model, and probe the physical processes much closer to the black hole boundary.