The 2006 Radio Flare in the Jet of CTA 102

The radio source CTA 102 (z = 1.037) is one of the most observed AGN in the northern sky. Its observational history started in the late 1950s (Harris & Roberts 1960). Flux density variations were reported in the source (Sholomitskii 1965), which lead other authors to suggest that the signal was coming from an extraterrestrial civilization (Kardashev 1964). Later, CTA 102 was identified as a quasar. Due to its strong flux density variability CTA 102 has been the target for numerous observations at different wavelengths. On kpc-scales CTA 102 consists of a central core and two faint lobes (Spencer et al. 1989). The brighter lobe has a flux density of 170 mJy at a distance of 1.6 arcsec from the core at position angle (P. A.) of 143 • (measured from North through East). The other lobe, with a flux density of 75 mJy, is located 1 arcsec from the Send offprint requests to: C. M. Fromm center at P. A. -43 • . The spectral indices of the lobes are -0.7 for the brighter one and -0.3 for the other. High resolution 15 GHz VLBI images show a curved jet with components which exhibit apparent velocities up to 15.4 ± 0.9 c, adopting a cosmology with Ω m = 0.27, Ω Λ = 0.73 and H 0 = 71 km s -1 Mpc -1 . (Lister et al. 2009). Jorstad et al. (2005) and Hovatta et al. (2009) derived bulk Lorentz factors, Γ, between 15 and 17 and Doppler factors, δ, between 15 and 22 from 43 GHz VLBI observations and from single-dish light curves. A major flux density outburst occurred in April 2006, which offers a unique opportunity to study CTA 102 under these conditions (Fromm et al. 2010). For our analysis we used data from the monitoring of CTA 102, performed within the framework of the 15 GHz MOJAVE 1 program (Monitoring of Jets in Active galactic nuclei with VLBA Experiments; Lister et al. (2009)).

The MOJAVE observations of CTA 102 between 2005 and 2008 have been used to analyze kinematic changes in the source during the 2006 radio flare. The raw data were calibrated using standard AIPS procedures. The calibrated data were fitted by several circular Gaussian components using DIFMAP. The fitted components are labeled from C1 to C12, in inverse order of distance to the core. In Using the evolution of the flux density and displacement of the fitted components from the VLBI core we could identify the modeled features between the different epochs. This approach allows us to identify the feature labeled C12 as a possible candidate for a newly ejected The separation from the core of component C12 remains constant until 2006.6, followed by a sharp acceleration and strong decrease in the observed flux density. Lister et al. (2009) derived an apparent speed of 8.5 ± 1.0 c and an ejection time of t ej = 2005.04 ± 0.3 yr including the stationary phase before mid 2006 in the fit. For our analysis we separated the C12 trajectory into a stationary section, C12 a , (be-fore 2006.6) and moving section, C12 b (after 2006.6). For C12 b we derived an apparent velocity of 17.3 ± 0.7 c and an ejection time of t ej = 2006.16 ± 0.05 yr.

There is a flare in 15 GHz observations of CTA 102 around 2005.8 (see increase in total and core flux density in bottom panel of Fig. 2). Furthermore, as already mentioned, the trajectory of component C12 is stationary until 2006.6. In the evolution of the flux density of this component one can see a global maximum around this time (see Fig. 2). A possible scenario which explains the trajectory of C12 and the evolution of the flux density could be an interaction between traveling and standing shocks. The observed flare in CTA 102 could be explained by the propagation of a relativistic shock wave. This wave is generated by pressure mismatches at the jet nozzle and during its way downstream it reaccelerates the underlying jet particles at the shock front. This interaction of the shock with the jet flow leads to an increased emissivity and can be observed with VLBI techniques (Marscher 2009). The stationarity of component C12 a could be associated with a standing shock around 0.15 mas away from the core (see Fig. 2). A standing shock is a stationary wave in nonpressure matched jets, created during the collimation of the jet (Daly & Marscher 1988;Falle 1991). The position of such a shock remains stationary as long as the relative conditions of the jet and ambient medium remain unchanged. Using the trajectory of component C12 b , we derived an ejection time of t ej = 2006.16 ± 0.05 yr. From this result it is not immediately obvious that one can associate the C12 b with a travelling shock wave which triggered the observed flare around 2005.8. This temporal discrepancy between the onset of the flare and the calculated ejection time of component C12 b could be explained in the following way: Due to the resolution limit of the 15 GHz VLBI observations there is a lack of data less than 0.15 mas from the core. By using a linear in-terpolation based on the observed trajectory of component C12 b , we implicitly assumed a constant velocit

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