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تسجيل مستخدم جديدThe milliarcsecond-scale jet of PKS 0735+178 during quiescence
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تأليف
I. Agudo
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We present polarimetric 5 GHz to 43 GHz VLBI observations of the BL Lacertae object PKS 0735+178, spanning March 1996 to May 2000. Comparison with previous and later observations suggests that the overall kinematic and structural properties of the jet are greatly influenced by its activity. Time intervals of enhanced activity, as reported before 1993 and after 2000 by other studies, are followed by highly superluminal motion along a rectilinear jet. In contrast the less active state in which we performed our observations, shows subluminal or slow superluminal jet features propagating through a twisted jet with two sharp bends of about 90 deg. within the innermost three-milliarcsecond jet structure. Proper motion estimates from the data presented here allow us to constrain the jet viewing angle to values < 9 deg., and the bulk Lorentz factor to be between 2 and 4.
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We present the result of our extensive intranight optical monitoring of the well known low-energy peaked BL Lac (LBL) object PKS 0735+178. This long-term follow-up consists of $R$-band monitoring for a minimum duration of $sim 4$ hours, on 17 nights
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We present multi-epoch 8.4 and 43 GHz Very Long Baseline Array images of the BL Lac object 0735+178. The images confirm the presence of a twisted jet with two sharp apparent bends of 90$^{circ}$ within two milliarcseconds of the core, resembling a he
lix in projection. The observed twisted geometry could be the result of precession of the jet inlet, but is more likely produced by pressure gradients in the external medium through which the jet propagates. Quasi-stationary components are observed at the locations of the 90$^{circ}$ bends, possibly produced by differential Doppler boosting. Identification of components across epochs, since the earliest VLBI observations of this source in 1979.2, proves difficult due to the sometimes large time gaps between observations. One possible identification suggests the existence of superluminal components following non--ballistic trajectories with velocities up to $11.6pm 0.6 h_{65}^{-1} c$. However, in images obtained after mid-1995, components show a remarkable tendency to cluster near several jet positions, suggesting a different scenario in which components have remained nearly stationary in time at least since mid-1995. Comparison with the earlier published data, covering more than 19 years of observations, suggests a striking qualitative change in the jet trajectory sometime between mid-1992 and mid-1995, with the twisted jet structure with stationary components becoming apparent only at the later epochs. This would require a re-evaluation of the physical parameters estimated for 0735+178, such as the observing viewing angle, the plasma bulk Lorentz factor, and those deduced from these.
We present a new centimeter polarimetric VLBI image of the BL Lac object 0735+178. This source exhibits one of the most pronounced curvatures observed in jets of AGNs, with two sharp apparent bends of 90 degrees within the inner 2 milliarcseconds fro
m the core. Through the analysis of the data gathered over the past decades we study whether this curvature is produced by precession of the jet with ejection of ballistic components, or a precession in such a way that components velocity vector are always parallel to the jet axis. These possibilities are also studied by comparison with 3D hydrodynamic relativistic simulations of precessing jets.
New data and results on the optical behaviour of the blazar PKS 0735+178 are presented. In addition the whole historical light curve, and a new photometric calibration of comparison stars are reported. Optical spectral indexes are calculated and stud
ied on years scales, while several methods for time-series analysis are applied to the whole historical series and to each observing season of our data set. This allowed to search and identify optical variability modes, characteristic timescales and the signal power spectrum over 3 decades in time. In the last 10 years the optical flux of PKS 0735+178 exhibited a rather achromatic long-term behaviour and a variability mode resembling the shot-noise. The brightness level was in an intermediate/low state with a mild flaring activity and a superimposition/succession of rapid and slower flares, with no extraordinary/isolated outbursts but, at any rate, characterized by 1 major active phase in 2001. Several mid-term scales (days, weeks) were found, the more common falling into values of about 27-28 days, 50-56 days and 76-79 days. The rapid variability in the historical curve appear to be modulated by a general, slower and rather oscillating trend, where typical timescales of about 4.5, 8.5 and 11-13 years can be identified. This spectral and temporal analysis, accompanying our data publication, suggests the occurrence of distinctive variability signatures at days/weeks scales, that can likely be of transitory nature. On the other hand the possible pseudo-cyclical or multi-component modulations at long times could be more stable, recurrent and correlated to the bimodal radio flux behaviour and the twisted radio structure observed by many years in this blazar.
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