The Scattering Screen in Front of Sagittarius A*: Broadening Budgets From Seven Millimetres to 0.87 Millimetres

Interstellar electrons between the Earth and the Galactic Centre scatter-broaden every image of Sagittarius A* by an amount that grows with the square of the wavelength. Under the published scattering law used here, the major-axis broadening at the standard 1.304 millimetre band is 22.3 microarcseconds, 43 per cent of the 51.8 microarcsecond ring, broadening it in quadrature to 56.4 microarcseconds and requiring a deconvolved intrinsic size of 46.8 microarcseconds if the observed size is taken at face value. At 3.5 millimetres the broadening reaches 160 microarcseconds, swamping the shadow; at 0.87 millimetres it falls to 9.9 microarcseconds, a minor correction. The minor-axis figures are roughly half the major-axis ones throughout, so the screen also manufactures apparent ellipticity. Any Sagittarius A* size, shape, or subring claim that does not carry this budget is incomplete, and at longer wavelengths it is the budget, not the source, that dominates the image. Computed from a stated empirical law; simulated, not measured.