Cross-Polarization Gain Calibration of Linearly Polarized VLBI Antennas by Observations of 4C 39.25

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A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä

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2024-04

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en

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Radio Science, Volume 59, issue 4

Abstract

Radio telescopes with dual linearly polarized feeds regularly participate in Very Long Baseline Interferometry. One example is the VLBI Global Observing System (VGOS), which is employed for high-precision geodesy and astrometry. In order to achieve the maximum signal-to-noise ratio, the visibilities of all four polarization products are combined to Stokes I before fringe-fitting. Our aim is to improve cross-polarization bandpass calibration, which is an essential processing step in this context. Here we investigate the shapes of these station-specific quantities as a function of frequency and time. We observed the extra-galactic source 4C 39.25 for 6 hours with a VGOS network. We correlated the data with the DiFX software and analyzed the visibilities with PolConvert to determine the complex cross-bandpasses with high accuracy. Their frequency-dependent shape is to first order characterized by a group delay between the two orthogonal polarizations, in the order of several hundred picoseconds. We find that this group delay shows systematic variability in the range of a few picoseconds, but can remain stable within this range for several years, as evident from earlier sessions. On top of the linear phase-frequency relationship there are systematic deviations of several tens of degrees, which in addition are subject to smooth temporal evolution. The antenna cross-bandpasses are variable on time scales of ∼1 hr, which defines the frequency of necessary calibrator scans. The source 4C 39.25 is confirmed as an excellent cross-bandpass calibrator. Dedicated surveys are highly encouraged to search for more calibrators of similar quality.

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Publisher Copyright: © 2024. The Authors.

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Jaron, F, Martí-Vidal, I, Schartner, M, González-García, J, Albentosa-Ruiz, E, Bernhart, S, Böhm, J, Gruber, J, Modiri, S, Nothnagel, A, Pérez-Díez, V, Savolainen, T, Soja, B, Varenius, E & Xu, M H 2024, ' Cross-Polarization Gain Calibration of Linearly Polarized VLBI Antennas by Observations of 4C 39.25 ', Radio Science, vol. 59, no. 4, e2023RS007892 . https://doi.org/10.1029/2023RS007892