THE CELESTIAL REFERENCE FRAME AT 24 AND 43 GHz. II. IMAGING

摘要

We have measured the submilliarcsecond structure of 274 extragalactic sources at 24 and 43 GHz in order toassess their astrometric suitability for use in a high-frequency celestial reference frame (CRF). Ten sessions ofobservations with the Very Long Baseline Array have been conducted over the course of ～5 years, with a totalof 1339 images produced for the 274 sources. There are several quantities that can be used to characterize theimpact of intrinsic source structure on astrometric observations including the source flux density, the flux densityvariability, the source structure index, the source compactness, and the compactness variability. A detailed analysisof these imaging quantities shows that (1) our selection of compact sources from 8.4 GHz catalogs yielded sourceswith flux densities, averaged over the sessions in which each source was observed, of about 1 Jy at both 24 and43 GHz, (2) on average the source flux densities at 24 GHz varied by 20–25 relative to their mean values,with variations in the session-to-session flux density scale being less than 10, (3) sources were found to bemore compact with less intrinsic structure at higher frequencies, and (4) variations of the core radio emissionrelative to the total flux density of the source are less than 8 on average at 24 GHz. We conclude that thereduction in the effects due to source structure gained by observing at higher frequencies will result in an improvedCRF and a pool of high-quality fiducial reference points for use in spacecraft navigation over the next decade.