出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：We present the results of a multi-frequency analysis of the enigmatic BL Lac object S4 0954+658. We have analyzed three epochs of dense VLBA observations with full polarization at 22 GHz and 8 GHz and one epoch at four frequencies 5 GHz, 8 GHz, 15 GHz, and 22 GHz taken in 1996- 1998. We found that the jet is of a prominent helical shape and that the jet ridge line changes dramatically with time. The jet ridge line becomes wider with time and the position of the jet ridge line bend shifts along the jet with a speed of 1.5±0.1 c. The Kelvin-Helmholtz instability can explain such changes of the jet ridge line. We estimate the sound speed of the S4 0954+658 jet ajt = 0.5±0.1c and the Mach number Mjt = 10±0.7.
