The Position/Structure Stability of Four Icrf2 Sources

Abstract

Four close radio sources in the International Celestial Reference Frame (ICRF) 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 ICRF. Although the four sources were compact at 8.6 GHz, the VLBA images at 43 GHz with 0.3-mas resolution showed that all were composed of several components. A component in each source was identified as the radio core using some or all of the following emission properties: compactness, spectral index, location at the end of the extended emission region, and stationary in the sky. Over the observing period, the relative positions between the four radio cores were constant to 0.02 mas, the phase referencing positional accuracy obtained at 23 and 43 GHz among the sources, suggesting that once a radio core is identified, it remains stationary in the sky to this accuracy. Other radio components in two of the four sources had detectable motion in the radio jet direction. Comparison of the 23 and 43 GHz VLBA images with the VLBA 8.6 GHz images and the ICRF positions suggests that some ICRF positions are dominated by a moving jet component; hence, they can be displaced up to 0.5 mas from the radio core, and may also reflect the motion of the jet component. Future astrometric efforts to determine a more accurate quasar reference frame at 23 and 43 GHz and from the VLBI2010 project are discussed, and supporting VLBA or EVN observations of ICRF sources at 43 GHz are recommended in order to determine the internal structure of the sources. A future collaboration between the radio (ICRF) and the optical frame of Gaia is discussed.

Halometry from astrometry

“…Their light centroids do undergo some proper motion, however, as they are violent and active regions at the centers of galaxies. The Very Long Baseline Array has measured the positions of four compact radio sources in close proximity on the sky, using phase referencing over multiple frequency bands [99]. These high-resolution measurements had an instrumental accuracy of 20 µas.…”

Very Long Baseline Interferometry with the SKA

“…By virtue of the fact that many sources will be used to construct the calibrator frame, cross-checks will be possible allowing the removal of sources which demonstrate discernible structure evolution, mitigating the third error component. This is currently poorly constrained (Fomalont et al 2011), as it is below the error floor for most current observations. The fourth error component is only relevant for certain targets (such as masers) and can be mitigated by compressing the campaign duration.…”

Interferometer Techniques for Astrometry and Geodesy

“…The use of such distant objects to define the reference frame provides a level of astrometric uncertainty at least an order of magnitude better than optical measurements of stars. The ultimate accuracy of this frame may depend on the structural stability of the radio sources involved [see, e.g., Fey and Charlot (1997), Fomalont et al (2011)]. The level of uncertainty in the connection between the radio and optical frames is essentially the uncertainty in optical positions.…”