The long-term evolution of the synchrotron emission from the parsec-scale jet in the quasar 3C 345 is analysed, on the basis of multi-frequency monitoring with very long baseline interferometry (VLBI) and covering the period 1979-1994. We demonstrate that the compact radio structure of 3C 345 can be adequately represented by Gaussian model fits and that the model fits at different frequencies are sufficiently reliable for studying the spectral properties of the jet. We combine the model fits from 44 VLBI observations of 3C 345 made at 8 different frequencies between 2.3 and 100 GHz. This combined database is used for deriving the basic properties of the synchrotron spectra of the VLBI core and the moving features observed in the jet. We calculate the turnover frequency, turnover flux density, integrated 4-25 GHz flux and 4-25 GHz luminosity of the core and the moving features. The core has an estimated mean luminosity L core = (7.1 ± 3.5) · 10 42 h −2 erg s −1 ; the estimated total luminosity of 3C345 on parsec scales is ≈ 3 · 10 43 h −2 erg s −1 (about 1% of the observed luminosity of the source between the radio to infrared regimes). The luminosities of the core and most of the moving features decrease at the average rate of 1.2 · 10 35 h −2 (0.74 ± 0.06) t−1979.0 erg s −2 (t measured in years). The derived luminosity variations require intrinsic acceleration of the moving features. The turnover frequency of one of the moving features reaches a peak during the above period. The combination of the overall spectral and kinematic changes in that feature cannot be reproduced satisfactorily by relativistic shocks, which may indicate rapid dissipation in shocks. The spectral changes in the core can be reconciled with a shock or dense plasma condensation traveling through the region where the jet becomes optically thin. We are able to describe the evolution of the core spectrum by a sequence of 5 flare-like events characterized by an exponential rise and decay of the particle number density of the material injected into the jet. The same model is also capable of predicting the changes in the flux density observed in the core. The flares occur approximately every 3.5-4 years, roughly correlating with appearances of new moving features in the jet, and indicating that a quasi-periodic process in the nucleus may be driving the observed emission and structural evolution of 3C 345.
III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio structure typical of a Seyfert galaxy, but also with an extremely variable, blazar-like radio core. We have now discovered a new radio flare in which the source has brightened more than 20-fold within less than 2 yr. A broadband radio spectrum between 1.4 and 666 GHz shows a textbook-like synchrotron spectrum peaking at 43 GHz, with a self-absorbed synchrotron spectral index ϩ2.5 at frequencies below 43 GHz and an optically thin spectral index Ϫ0.75 at frequencies above 43 GHz. The outburst spectrum can be well fitted by two homogenous, spherical components with equipartition sizes of 0.1 and 0.2 pc at 43 and 15 GHz and with magnetic fields of 0.4 and 1 G. VLBA observations at 43 GHz confirm this double structure and these sizes. Timescale arguments suggest that the emitting regions are shocks which are continuously accelerating particles. This could be explained by a frustrated jet scenario with very compact hot spots. Similar millimeter-peaked spectrum sources could have escaped our attention because of their low flux density at typical survey frequencies and their strong variability.
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