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Extension of the coordinate parameterization of radio sources observed by VLBI (ECORAS)

Subject Area Geodesy, Photogrammetry, Remote Sensing, Geoinformatics, Cartography
Astrophysics and Astronomy
Term from 2014 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 249935073
 
Final Report Year 2024

Final Report Abstract

In project ECORAS we have investigated and determined precise time series and, based on these, a suitable time-variable coordinate model for positions of extra galactic radio sources that assemble the International Celestial Reference Frame, currently ICRF3. We came to the conclusion that a linear spline representation appears to be a good compromise between flexibility and simplicity. Due to the relatively large number of objects observed by geodetic / astrometric VLBI, more than 4000, we finally used MARS (multiple adaptive regression splines), a method that automatically finds significant break points and fits linear splines between these break points. We applied the flexible coordinate model to the analysis of the IVS VLBI data set and found improved stabilisation of the celestial datum and more precise a priori coordinates. In the following, we could improve the Earth Orientation Parameters (EOP) determination. In particular the corrections to the Celestial Pole Offsets (CPO) improved by up to 15%. Furthermore, the flexible coordinate model allows the inclusion of all radio sources into the celestial datum, making the categorization of radio sources into groups, such as defining, special handling, or candidates obsolete. Our flexible radio source coordinate model and the additional experience gained during the last years of intensive studies of VLBI-observed radio sources prepared us to perform detailed comparisons of VLBI and Gaia-derived coordinates and coordinate frames. The latter ESA- mission of which we already received EDR3 (early data release 3) will most likely continue scanning the celestial sphere in optical wavelengths during the next years. Finally, besides other data, Gaia will provide complete astrometry, i.e. positions, proper motions, and parallaxes of a large number of extragalactic objects. For VLBI and astronomy this is a unique historical chance to externally control the ICRF and to detect possible deformations or other effects caused by the imperfections of the VLBI observing system. Since VLBI and Gaia catalogues can be expected to be about at the same quality level, a comparison could also help improving the Gaia data and its reliability. The primary scientific motivation for project ECORAS-2 was and still is the determination and correction of structure-related effects. Radio source structure is the largest unconsidered error source in contemporary VLBI, in particular in VGOS. Currently, structure-induced errors are supposed to be mitigated through an adopted scheduling. Remaining errors are excluded as outliers. The best approach would be to correct for structure. This is not always possible due to the relatively sparse geodetic / astrometric observation of individual radio sources as demanded for the precise correction of atmospheric errors. If otherwise no precise radio images are available, an alternative could be to steer the weights of the radio sources in the equation system. The stationary structure correction works fine assuming enough observations in a limited time interval. Over time scales of months and years, however, the centre of radio emission can shift slightly leading to spurious trends in radio source coordinates. Luckily, Dr. Minghui Xu received an ERC grant from the EU on this topic and hence, we are very likely to proceed significantly with this work. These and other effects are superimposed on each other when it comes to the inter-frequency comparison of radio source positions. Investigations of the dispersive core-shift may lead to radio source ties that could be used to combine celestial positions obtained at various wavelength similar to the local ties for the combination of terrestrial coordinates at co-location sites. Against optical positions, such as the ones obtained by Gaia, the correct identification of counterparts requires additional work. As the Gaia catalogue becomes more and more precise in future, comparisons based on extragalactic objects will improve with the more accurate astrometry. For the bright part of the Gaia catalogue, still a lot can be done for improving the alignment. Up to now, only a small group of radio stars has been observed with enough astrometric precision. In the ECORAS-successor project AGORA, we will mainly concentrate on obtaining more radio star observations with significantly better spatial-temporal distribution. This is necessary to improve the geometry for the determination of rotation and spin parameters and to resolve for orbital and other non-linear motions of multiple star systems. Finally, the consideration of both, bright and faint optical subsets of Gaia objects, has the potential of providing the best alignment of the Gaia frame to the ICRS realizations.

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