Three Decades of Very Long Baseline Interferometry Monitoring of the Parsec-Scale Jet in 3C 345
aa r X i v : . [ a s t r o - ph . C O ] D ec **FULL TITLE**ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION****NAMES OF EDITORS** Three Decades of Very Long Baseline InterferometryMonitoring of the Parsec-Scale Jet in 3C 345
Frank K. Schinzel, Andrei P. Lobanov, and J. Anton Zensus
Max-Planck-Institut f¨ur Radioastronomie,Auf dem H¨ugel 69, 53121 Bonn, Germany
Abstract.
The 16 th magnitude quasar 3C 345 (redshift z=0.5928) showsstructural and emission variability on parsec scales around a compact unre-solved radio core. For the last three decades it has been closely monitoredwith very long baseline interferometry (VLBI), yielding a wealth of informationabout the physics of relativistic outflows and dynamics of the central regions inAGN. We present here preliminary results for the long-term jet evolution, basedon the 15 GHz monitoring data collected by the MOJAVE survey and variousother groups over the last ∼
14 years and combined with data from earlier VLBIobservations of 3C 345 which started in 1979. We discuss the trajectories, kine-matics, and flux density evolution of enhanced emission regions embedded inthe jet and present evidence for geometrical (e.g. precession) and physical (e.g.relativistic shocks and plasma instability) factors determining the morphologyand dynamics of relativistic flows on parsec scales.
1. The Source
The 16 th magnitude quasar 3C 345 (redshift z=0.5928) has been observed atradio wavelengths for over 30 years, in particular with VLBI (cf., Biretta et al.(1986), Baath et al. (1992), Zensus et al. (1995), Unwin et al. (1997), Lobanov & Zensus(1999), Ros et al. (2000), Lobanov & Roland (2005)). The source still contin-ues to be of special interest due to its complex, helical parsec-scale jet arounda compact unresolved radio core and its pronounced multiwavelength variabil-ity. A likely 8-10 years periodicity of the high activity phases in 3C 345 hasbeen identified (Lobanov & Zensus 1999). Measurements of nuclear opacity andmagnetic field strength (Lobanov 1998) yield a total mass for the central en-gine of (4.0 ± · M ⊙ . We have analyzed VLBI observations of the lastthree decades in order to understand the physics of the relativistic outflow anddynamics of central regions in 3C 345. Preliminary results of this analysis arepresented here, focusing specifically on trajectories, kinematics, and flux densityevolution of enhanced emission regions embedded in the jet.
2. Observations
We made use of a total of 201 observations (see Table 1) that included VeryLong Baseline Array (VLBA) observations obtained from the NRAO Archive,observations by the MOJAVE survey, and published values pre-dating the VLBA1(before 1995) . Archival VLBI observations were calibrated using NRAO’s As-tronomical Imaging Processing System (AIPS). The total intensity and polar-ization data was processed, with corrections applied for atmospheric opacity (ifdeemed necessary), Faraday rotation, and Earth orientation parameters used bythe VLBA correlator. Fringe fitting was used to calibrate the observations forgroup and phase delays. The then calibrated visibility data was imaged usingCaltech’s Difmap (Shepherd et al. 1995). The source structure was modelfittedusing circular Gaussian components. At the end, individual Gaussian compo-nents were cross-identified at different epochs in order to follow the evolution ofindividual bright features in the jet.It should be noted that the physical nature of these features, also referred toas jet components, is still a matter of debate. Presently, the common viewpointis that moving jet features are relativistic shocks in the jet plasma emittingoptically thin synchrotron radiation. Table 1. Overview of VLBI observations used in this work.Time
3. Results3.1. Component C9
The VLBI data collected on the pc-scale jet in 3C 345 reveals 16 bright features(labeled C1-C16, with C1 being the oldest feature) that we are able to repre-sent by circular Gaussian modelfits. Fits representing C1 were ignored in thisanalysis due to a lack of observations at ≥ r ( ν ) = (1 . · ν (GHz) − − . Unwin et al. (1983), Unwin & Wehrle (1992), Biretta et al. (1986), Zensus et al. (1995),Baath et al. (1992), Lobanov (1996), Krichbaum et al. (1993), Ros et al. (2000), Klare (2004) possible acceleration phase before 1998 and a subsequent transition to a con-stant apparent speed. A linear fit to the observations around 1997 yields a timeof zero separation from the core of 1995.95 ± r < − and for a distance r > − .The 15 GHz flux density evolution of the component has a peak of (1.81 ± right ). The time of the peak is 3.43 ± ∼ Figure 1.
Left:
Radial separation from the core plotted over time for theindividual jet feature labeled C9. The gap between 2000 and 2002 is due todifficulties of component identifications that will be resolved in the future.
Right:
Flux density evolution of the jet feature C9 at 15 GHz for a period of13 years.
Figure 2 shows the trajectories of all jet features. The jet is traced up to about15 mas distance from the core. The measurements become more sparse at dis-tances r > ≤ ◦ . Theindividual jet features trace a common channel of ∼ ∼ ∼ ∼ ∼ ∼ Figure 2. Trajectories of all jet features plotted on top of each other.
The component position angles measured at 0.5 mas radial distance from theVLBI core at 15 GHz offer a different way to represent the previously de-scribed deviations from the 90 ◦ position angle (core separation in declination)in the trajectories of subsequent jet features C4-C15 (1983-2007), as shown inFigure 4. We see no clear periodic trends as has been claimed in the past.Lobanov & Roland (2005) and Klare et al. (2003) describe in their work a short-term periodicity of 8-10 years with an underlying long-term trend of 0.4-2.6 ◦ year − .We cannot confirm this here, but we see long-term variability as well as shortterm changes in the position angles since 2000. The reason these short termchanges are not seen in earlier observations is due to the lower sensitivity andsampling of observations pre-dating the VLBA (before 1995). At that time onlythe brightest features were detectable. With recent, more sensitive and morefrequent observations, we are able to see much more structure in the jet andare even able to see the bright edges of the jet. As stated above, we need tocheck whether the observed behavior can be brought into agreement with aprecessing-helical jet model. Apparent velocities β app have been determined for all components. A propermotion of 1 mas year − is translated to a β app of 19.7 h − c in concordance withthe standard ΛCDM (H = 70 km s − Mpc − and Ω Λ = 0.72 = 1 - Ω M , h isthe dimensionless Hubble parameter). We have not been able to determine acommon velocity for separations < β app ≤ − c. For a separation > β app ≈ − c Figure 3. Close-up view of the trajectories for separations < for most of the components, with the exception of two features that show anapparent velocity of ≈ − c.
4. Summary & Outlook
3C 345 shows a complex jet morphology with many bright features observed byVLBI which have been traced for three decades now. We found a consistentsuperluminal motion of moving jet features at distances > Acknowledgments.
Frank Schinzel was supported for this research through astipend from the International Max Planck Research School (IMPRS) for Astronomyand Astrophysics at the Universities of Bonn and Cologne. The National Radio As-tronomy Observatory is a facility of the National Science Foundation operated undercooperative agreement by Associated Universities, Inc. This research has made use ofdata from the MOJAVE database that is maintained by the MOJAVE team (Lister et al.2009).