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tokens: We thank Thomas Krichbarum for critically reading the manuscript. T. An thanks the grant support from the 973 programme (No. 2013CB837900). The European VLBI Network is a joint facility of independent European, African, Asian, and North American radio astronomy institutes. Scientific results presented in this publication are derived from the following EVN projects: RP017, EY018, EY019, and EY020. The study has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under grant agreement No. 283393 (RadioNet3). e-VLBI research infrastructure in Europe is supported by the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement number RI-261525 NEXPReS. The WSRT is operated by ASTRON (Netherlands Institute for Radio Astronomy) with support from the Netherlands Organization for Scientific Research. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.