2016AJ....152..151X

The CONT14 campaign with state-of-the-art very long baseline interferometry (VLBI) data has observed the source 0642+449 with about 1000 observables each day during a continuous observing period of 15 days, providing tens of thousands of closure delays-the sum of the delays around a closed loop of baselines. The closure delay is independent of the instrumental and propagation delays and provides valuable additional information about the source structure. We demonstrate the use of this new "observable" for the determination of the structure in the radio source 0642+449. This source, as one of the defining sources in the second realization of the International Celestial Reference Frame, is found to have two point-like components with a relative position offset of -426 microarcseconds (µas) in R.A. and -66 µas in decl. The two components are almost equally bright, with a flux-density ratio of 0.92. The standard deviation of closure delays for source 0642+449 was reduced from 139 to 90 ps by using this two-component model. Closure delays larger than 1 ns are found to be related to the source structure, demonstrating that structure effects for a source with this simple structure could be up to tens of nanoseconds. The method described in this paper does not rely on a priori source structure information, such as knowledge of source structure determined from direct (Fourier) imaging of the same observations or observations at other epochs. We anticipate our study to be a starting point for more effective determination of the structure effect in VLBI observations.