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Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre


The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation1. Sagittarius A* (Sgr A*), the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4,000,000 times that of the Sun2,3. A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding Sgr A*, where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering4,5,6,7. Here we report observations at a wavelength of 1.3 mm that set a size of microarcseconds on the intrinsic diameter of Sgr A*. This is less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of Sgr A* emission may not be centred on the black hole, but arises in the surrounding accretion flow.

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Figure 1: Fitting the size of Sgr A* with 1.3 mm wavelength VLBI.
Figure 2: Observed and intrinsic size of Sgr A* as a function of wavelength.

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High-frequency VLBI work at MIT Haystack Observatory is supported by grants from the National Science Foundation. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics. We thank G. Weaver for the loan of a frequency reference from Johns Hopkins University Applied Physics Labs; J. Davis for use of GPS equipment; I. Diegel, R. Vessot, D. Phillips and E. Mattison for assistance with hydrogen masers; the NASA Geodesy Program for loan of the CARMA Hydrogen Maser; D. Kubo, J. Test, P. Yamaguchi, G. Reiland, J. Hoge and M. Hodges for technical assistance; M. Gurwell for SMA calibration data; A. Kerr and A. Lichtenberger for contributions at ARO/SMT; A. Broderick, V. Fish, A. Loeb and I. Shapiro for discussions; and the staff at all participating facilities.

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Correspondence to Sheperd S. Doeleman.

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Doeleman, S., Weintroub, J., Rogers, A. et al. Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre. Nature 455, 78–80 (2008).

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