Closure Amplitude Analysis of 15, 22 and 43GHz VLBA Observations of Sagittarius A*: Size is Consistent with the Scattering Law

Abstract

High frequency very long baseline interferometry is necessary to avoid the effects of strong interstellar scattering when imaging Sagittarius A*. Unfortunately, the reliability of imaging declines with increasing frequency due to amplitude calibration errors for this low declination source. Closure amplitude analysis provides an unbiased method for estimating source parameters. We analyzed several new and several archival VLBA data sets with the closure amplitude technique. Our results indicate that there is no deviation from the scattering law in the minor axis size at 15, 22 and 43 GHz. There is a slight excess (2σ ∼ 3%) over the scattering law in the major axis size at 43 GHz which can be accounted for by a marginal increase in the error estimate, a slight recalibration of the scattering law or an intrinsic source of size 0.2 mas ∼ 2 AU ∼ 40Rs. The absence of an apparent jet or out.ow allows us to set an upper limit to the velocity of any ballistic components at 0.001c.