Seven-frequency VLBI Observations of the GHz-Peaked-Spectrum Source OQ 208

We present quasi-simultaneous VLBI images of the GHz-Peaked-Spectrum radio source OQ 208 obtained with the Very Long Baseline Array at 1.4, 1.7, 2.3, 5.0, 8.4, 15.4 GHz and the European VLBI Network at 6.7 GHz. The low frequency （1.4, 1.7 and 2.3 GHz） observations reveal a weak and extended steep-spectrum component at about 30 mas away at position angle - 110°, which may be a remnant emission. The radio structure of OQ 208 consists of two mini-lobes at 5.0, 6.7, 8.4 and 15.4 GHz. Our spectral analysis further confirms that the southwest lobe undergoes free-free absorption and finds that the free-free absorption is stronger in the inner region. By fitting the 8.4 GHz images from 1994 to 2005, we obtain a separation speed of 0.0314-0.006 mas yr^-1 between the two mini-lobes. This indicates a jet proper motion of 0.1054-0.020 c and a kinematic age of 219±42 yr for the radio source.

Correlated Radio-Optical Variations on Intraday Timescales

Correlated radio-optical variations on intraday timescales have been observed （e.g. in BLO 0716＋714） and such radio intraday variability is suggested to have an intrinsic origin. Recently, multi-wavelength observations, simultaneous at radio, mm-submm, optical and hard X-rays, of 0716＋714, show that during a period of intraday/interday variations at radio and mm wavelengths, the apparent brightness temperature of the source exceeded the Compton-limit （-10^12 K） by 2-4 orders of magnitude, but no Compton catastrophe （or no high luminosity of inverse-Compton radiation） was detected. It is also found that the intraday/interday variations at mm-submm wavelengths are consistent with the evolutionary behavior of a standard synchrotron source and for the intraday/interday variations at centimeter wavelengths opacity effects can play a significant role, which is consistent with the interpretation suggested previously by Qian et al. Thus the apparent high brightness temperatures may probably be explained in terms of Doppler boosting effects due to bulk relativistic motion of the source. We will argue a scenario to simulate the correlations between the radio and optical variations on intraday timescales observed in BLO 0716＋714 in terms of a relativistic shock propagating through a jet with a dual structure.

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）.

A possible precession period for superluminal ejection in QSO 3C 279

The search for periodic features in fiux variability and superluminal ejection in blazars has been an important subject, which is helpful for providing significant clues to the understanding of the structure and kinematics of relativistic jets （physical processes of acceleration and collimation） and their association with central energy engines （black hole/accretion disk systems）. The wobbling of the ejection position angle of VLBI knots in superluminal sources has been interpreted in terms of the precession of the jet ejection nozzle as one of the alternative interpretations. We study the change with time of the initial position angle of superluminal knots in QSO 3C 279, using VLBI-data collected from the literature spanning more than ～30 yr and propose a jet-nozzle precession model which has very small viewing angles （less than 2°） to fit the long-term trends in both variations in the inner-jet position angle （Chatterjee et al.） and the ejection position angle of the VLBI components. It is shown that an ejection-nozzle precession period of ～25 yr could be appropriate to fit the long-term trend in the variation of the ejection position angle. However, the short-term swings and fluctuations in the ejection position angle cause some uncertainties. We also fit a model to the trajectory of component C4, correcting its non-ballistic motion near the core.

Model-fitting of the kinematics of superluminal components in blazar 3C 279

A precessing jet-nozzle model with a precession period of about 25 yr has been proposed by Qian to interpret the change with time of the ejection position an- gle of the superluminal components observed using very long baseline interferometry （VLBI） in the blazar 3C 279. We discuss the kinematic properties of six superluminal knots （C3, C4, C7a, C8, C9 and C10） and show that their trajectory, core-distance and apparent speed, derived from VLBI observations, can be consistently well fitted by the model. Their intrinsic Lorentz factors of bulk superluminal motion are thus derived, and the evidence shows no relation between Lorentz factor and the precession phase. Interestingly, for the C7a and C8 knots, the fitted core-distance ranges from ,,～0.1 mas to ～0.4 mas and for knots C9 and C10 from ～0.2mas to ～1.0-1.5 mas. For knot C4, its trajectory and apparent velocity are well fitted in the core-distance range from H1 mas to ～5 mas, taking into account a curvature of the trajectory at core-distance larger than ,,～3 mas. The consistent fitting of the kinematics of these components clearly demonstrates that the amplitude function and collimation param- eter adopted in the precession model are appropriate and applicable for both the in- ner and outer parts of the jet in 3C 279, but in some cases the jet curvature in the outer parts （or deviation from the model trajectory） needs to be seriously taken into consideration. With the exception of C4, the ejection position angles derived from the precession model are consistent with the values measured by VLBI observations （within about 3° - 6°）. Undoubtedly, the consistent interpretation of the kinematics in terms of the precession model for these superluminal components, with their ejection time spanning -～24 yr, significantly expands its applicability and implies that regular patterns of trajectories （or rotating channels） could exist in some periods.

Non-ballistic motion and precessing helical trajectory in quasar NRAO 150

NRAO 150 is a very special radio quasar in which prominent non-ballistic superluminal motion has been observed in its inner-jet region. We apply model-fittings to the kinematics of the superluminal knots（trajectory, distance from the core and apparent velocity） in terms of a helical precessing jet-nozzle model.Five cases are considered in which the angle between the jet axis and the line of sight is assumed to be 6？,3？, 1？, 0.6？ and 0.12？, respectively. It is shown that the superluminal components have intrinsic acceleration in the innermost regions（≤0.2 mas from the core）. The phenomenon of precessing nozzle/trajectory can be understood on the basis of relativistic magnetohydrodynamic theories for relativistic jets.