  We present a new method of time-dependent instrumental polarization
calibration for Very Long Baseline Interferometry (VLBI). This method has been
implemented in the recently developed polarization calibration pipeline GPCAL.
Instrumental polarization, also known as polarimetric leakage, is a
direction-dependent effect, and it is not constant across the beam of a
telescope. Antenna pointing model accuracy is usually dependent on time,
resulting in off-axis polarimetric leakages that can vary with time. The method
is designed to correct for the off-axis leakages with large amplitudes that can
severely degrade linear polarization images. Using synthetic data generated
based on real Very Long Baseline Array (VLBA) data observed at 43 GHz, we
evaluate the performance of the method. The method was able to reproduce the
off-axis leakages assumed in the synthetic data, particularly those with large
amplitudes. The method has been applied to two sets of real VLBA data and the
derived off-axis leakages show very similar trends over time for pairs of
nearby sources. Furthermore, the amplitudes of the off-axis leakages are
strongly correlated with the antenna gain correction factors. The results
demonstrate that the method is capable of correcting for the off-axis leakages
present in VLBI data. By calibrating time-dependent instrumental polarization,
the rms-noise levels of the updated linear polarization images have been
significantly reduced. The method is expected to substantially enhance the
quality of linear polarization images obtained from existing and future VLBI
observations.

Instrumentation and Methods for Astrophysics