| Summary: | International audience Most of the radio sources observed by VLBI, some used as defining sources in the International Celestial Reference Frame [ICRF2], show instabilities in position. These instabilities may be caused by astrophysical phenomena occurring in the central VLBI region of these objects (i.e., active galactic nuclei). On this basis, we have begun to characterize the signal included in the available VLBI position time series. Often, position instabilities happen along a preferred direction. There are cases, however, where two directions are distinguishable. The first scenario is consistent with a regular emergence of jet components from the VLBI core; hence, causing shifts of the radio emission centroid. On the other hand, the second scenario may give clues to the presence of a second black hole within the system that has its own activity offset from that of the first black hole. Comparing these directions with the orientation derived from radio-optical position offset brings further insights into astrophysical phenomena within active galactic nuclei.
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