The source structure of 0642+449 detected from the CONT14 observations
  The CONT14 campaign with state-of-the-art VLBI data has observed the source
0642+449 with about one thousand observables each day during a continuous
observing period of fifteen 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 (ICRF2), is
found to have two point-like components with a relative position offset of -426
microarcseconds in right ascension and -66 microarcseconds in declination. 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
ps to 90 ps by using this two-component model. Closure delays larger than one
nanosecond 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.
