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On the size of the galactic centre compact radio source: diameter <20 AU


The existence of a compact nonthermal radio source at the galactic centre was first suggested by Lynden-Bell and Rees1 as the possible signature of a black hole. Such a source was detected unambiguously by Balick and Brown2.Very long baseline interferometry (VLBI) observations of the structure of this source, hereafter referred to as Sgr A*, have been made for several years3,4. In all previous experiments, the visibility amplitudes, measured over a restricted (U, V) range, could be fitted only to the simplest model of brightness distribution, that is, to a circular gaussian. We have re-observed Sgr A* at λ3.6 cm and λ 1.35 cm with many baselines, using the more sensitive receivers and VLBI recording terminals now available. These observations set an upper limit of 20 AU (3 × l014 cm) to the diameter of Sgr A* at λ 1.35 cm and reveal for the first time an elongated structure at λ 3.6 cm, with the position angle of the long axis almost parallel to the rotation axis of the Galaxy. Sgr A* is unique in our Galaxy, but resembles most closely the compact radio sources at the centre of external galaxies. Observations of the central 4 arc s (0.2 pc) at radio and other wavelengths are best explained by a single massive collapsed object at the galactic centre.

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Lo, K., Backer, D., Ekers, R. et al. On the size of the galactic centre compact radio source: diameter <20 AU. Nature 315, 124–126 (1985).

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