Voytsik, Petr

Apparent frequency dependence shift of ultra-compact AGN cores.

Author list: Petr Voytsik, Yuri Kovalev

In VLBI images of extragalactic jets, the apparent position of the compact, bright emitting region at the narrow end of the jet (the core) depends on the observing frequency and is fundamentally determined by absorption in the radio emitting plasma (synchrotron self-absorption) and/or in the material surrounding the flow. This dependency provides a tool to probe absolute geometry and physical conditions in the vicinity of the core. It also must be taken into account in order to achieve the highest accuracy of radio reference frame and tie it accurately to the future optical GAIA frame. However, the widely used jet self-referencing method cannot be applied to highly compact ICRF objects with weak or no jets. Relative astrometry is the only tool to detect and measure the core shift for them. In order to do so, we used phase-referencing EVN observations to measure the frequency-dependent core shift between 1.4, 2.3, 5, and 8.4 GHz in a sample of eight ultra-compact ICRF objects. First results of this project are presented and discussed.