On radio source selection to define a stable celestial frame

Abstract

Context. The most accurate realization of a quasi inertial reference frame, the International Celestial Reference Frame (ICRF), is made of 212 defining extragalactic radio sources whose coordinates are determined using VLBI observations. Recent studies demonstrated however that using other sets of sources could improve the frame stability. Aims. This study examines a simple radio source selection scheme to define celestial reference frame axes more stable than the ones as currently defined by the ICRF. Methods. After having derived source coordinate time series from 26 years of VLBI observations, we select the most suitable sources on the basis of their positional variability (rms and slope), and observational history. We determine the axis stability of the frame defined by the selection, as well as its suitability for global geodetic VLBI analyses, i.e., determination of Earth orientation parameters and source and station coordinates. Results. We select four frames made of 196, 200, 262, and 269 sources, respectively, showing a satisfactory sky coverage in both hemispheres. Our selections provide a frame stability improved by up to 40% with respect to the ICRF, and by 20% with respect to the frame as defined by the 247 stable sources of Feissel-Vernier et al. (2006, A&A, 452, 1107). Reanalysis of data with respect to this frame gives astrometric catalogues aligned to the ICRF-Ext.2 within 17 μas. Effects on the Earth orientation parameter estimates and terrestrial reference frame determination remain marginal. Conclusions. In view of the generation of the next ICRF, we recommend that such an algorithm be taken into consideration along with other criteria based on individual characteristics of the radio sources.