On the influence of radio extended structures on offsets between the optical and VLBI positions of sources in the ICRF2

TL;DR

This study examines how the intrinsic radio structures of extragalactic sources influence positional differences between optical and VLBI measurements in the ICRF2, highlighting the importance of considering source structure in astrometric comparisons.

Contribution

It provides empirical evidence linking source structure to positional offsets and emphasizes the need to account for intrinsic structure in future optical-VLBI position comparisons.

Findings

Offsets can exceed 80 milliarcseconds.

Differences are partly explained by source X-band structure index.

Intrinsic source structure affects optical and VLBI position discrepancies.

Abstract

Context: To investigate the differences between positions, as determined by optical (direct imaging) and Very Long Baseline Interferometry (VLBI) techniques, of extragalactic sources listed in the second realization of the International Celestial Reference Frame (ICRF2). Aims: To verify the influence of the source's intrinsic structure on these differences. Methods: Instruments with mosaics of CCDs were used to acquire the optical positions presented here, leading us to opt for overlapping techniques to build a virtual, continuous CCD over the whole angular size of the respective fields of view, whose translation of the resulting intrumental positions into positions that are consistent with those in the ICRF2 was made with the help of the UCAC2. Results: The differences obtained between the optical and VLBI positions of the observed sources may reach more than 80 milliarcseconds and, taking into consideration that they are hardly explained only by statistical fluctuations or systematic errors in the optical reference frame used here, we argue that these differences can be related to the sources' X-band structure index (8.4 GHz). Conclusions: In this context, the presence of the intrinsic structure should be taken into consideration when comparing the optical and VLBI positions of ICRF2 sources in the future.