The Position/Structure Stability of Four ICRF2 Sources
Four compact radio sources in the International Celestial Reference Frame (ICRF2) catalog were observed using phase referencing with the VLBA at 43, 23, and 8.6-GHz, and with VERA at 23-GHz over a one-year period. The goal was to determine the stability of the radio cores and to assess structure effects associated with positions in the ICRF2. Conclusions are: (1) 43-GHz VLBI high-resolution observations are often needed to determine the location of the radio core. (2) Over the observing period, the relative positions among the four radio cores were constant to 0.02 mas, suggesting that once the true radio core is identified, it remains stationary in the sky to this accuracy. (3) The emission in 0556+238, one of the four sources investigated and one of the 295 ICRF2 defining sources, was dominated by a strong component near the core and moved 0.1 mas during the year. (4) Comparison of the VLBA images at 43, 23, and 8.6-GHz with the ICRF2 positions suggests that the 8-GHz structure is often dominated by a bright non-core component. The measured ICRF2 position can be displaced more than 0.5 mas from the radio core and partake in the motion of the bright jet component.