A Possible Periodicity in the Radio Light Curves of 3C 454.3

During the period 1966.5-2006.2 the 15 GHz and 8 GHz light curves of 3C 454.3 （z = 0.859） show a quasi-periodicity of ,-12.8 yr （-6.9 yr in the rest frame of the source） with a double-bump structure. This periodic behaviour is interpreted in terms of a rotating double-jet model in which the two jets are created from the black holes of a binary system and rotating with the period of the orbital motion. The periodic variations in the radio fluxes of 3C 454.3 are suggested to be mainly due to the lighthouse effects （or the variation in Doppler boosting） of the precessing jets caused by the orbital motion. In addition, variations in the rate of mass accreting onto the black holes may be also involved.

Periodicity of the ejection of superluminal components in 3C345

The search for periodic behavior in Blazars has been an important subject, which is helpful for providing significant clues to the structure and physical processes of their central energy engine. A binary black hole system has recently been suggested for causing precession of relativistic jets and rotation of the ejection position angle of VLBI knots in superluminal sources. It has been suggested that in QSO 3C345, the ejection direction of the superluminal knots rotates due to the precession of the central engine and thus the ejection position angle of the successive knots shows a periodic behavior. Some authors argue for a period of precession being ～5.6 yr （Abraham ＆ Caproni）, ～8-10 yr （Klare et al.） and ～9.5 yr （Lobanov ＆ Roland）. Applying the helical model proposed by Qian et al. and selecting appropriate parameters to fit the initial trajectories （within 0.3 mas） of all the components （C4 to C10）, we derive the relation between the ejection position angle of the components and their precession phase, and thus find a 6.9-year precession period （4.3 yr in the source frame）, which can fit the ejection position angle of all these superluminal knots well. Since the VLBI observations have covered more than two precession periods, confirmation in one or more future periods would be important. In addition, we emphasize that the initial parts of the trajectories of these knots can be fitted by a common helical pattern （channel） through a precessing of its initial phase. This scenario （or helical precessing model） is different from the usual ballistic precessing model in which the individual superluminal knots move along straight-lines after ejection （Tateyama ＆ Kingham）.

Refractive Focusing by Interstellar Clouds and the Rapid Polarization Angle Swing in QSO 1150+812

A very rapid polarization position angle swing of -180° （with a time scale of ,-6 hours） observed at 2 cm in QSO 1150＋812 （z = 1.25） was reported by Kochenov ＆ Gabuzda. This very rare event is difficult to explain. We found a possible interpretation in the framework of a source model consisting of three polarized components, in which two compact polarized components are nearly simultaneously occulted by an interstellar cloud, with consequent focusing-defocusing effects. A specific plasma-lens model is proposed which can reasonably fit the polarized flux density curve with results derived for the two lensed components. Some physical parameters of the plasma-lens and the source components are estimated. The two compact polarized components are estimated to have brightness temperatures of -6 ×10^12 K. Thus a bulk relativistic motion with a Lorentz factor less than 10 is required to meet the inverse-Compton limit.