[1] Russian Acad Sci, Astro Space Ctr, Lebedev Phys Inst, Ul Profsoyuznaya 84-32, Moscow 117997, Russia
[2] Russian Acad Sci, Crimean Astrophys Observ, Nauchnyi 298409, Crimea, Russia
[3] Moscow Phys Tech Inst, Inst Skii Proezd 9, Dolgoprudnyi 141700, Moscow Oblast, Russia
[4] Max Planck Inst Radioastron, 69 Hugel, D-53121 Bonn, Endenich, Germany
[5] Univ Hamburg, Inst Expt Phys, Luruper Chaussee 149, D-22761 Hamburg, Germany
[6] Russian Acad Sci, Inst Appl Astron, Kutuzovskaya Nab 10, St Petersburg 191187, Russia