Removing the Effect of Source Structure From Correlated VLBI Data

Abstract

Recent studies suggest that in geodetic very long baseline interferometry (VLBI) the assumption of sources being point-like results in a significant residual error introduced by the effect of source structure. Our aim is to remove the impact of source structure from geodetic VLBI data right after the correlation step. For this purpose, we have developed a dedicated software tool that takes models of the brightness distribution to correct the interferometric phases of the correlator output. The corrected data are then further processed following conventional procedures. Images obtained from corrected visibilities appear more compact, by up to 43%, compared to the original ones. The distribution of closure delays is significantly improved, by up to 27 ps, for the structure-corrected data, provided that the image information is accurate. The WRMS of post-fit-delay residuals is reduced by up to 5 ps per corrected source. The positive effect of the correction is greatly reduced when applied to data with a large temporal offset to the images used for its configuration. We conclude that the images used for any structure correction should have a time difference of not more than a few weeks to the data to which they are applied.