Spectral Evolution of The Parsec-Scale Jet in The Quasar 3C345

The long-term evolution of the synchrotron emission from the parsec-scale jet in the quasar 3C345 is analysed, on the basis of multi-frequency monitoring with very long baseline interferometry (VLBI) and covering the period 1979-1994. We combine the model fits from 44 VLBI observations of 3C345 made at 8 different frequencies between 2.3 and 100GHz. We calculate the turnover frequency, turnover flux density, integrated 4-25GHz flux and 4-25GHz luminosity of the core and the moving features. The core has an estimated mean luminosity of 7.1+/-3.5 * 10^42 erg/s; the estimated total luminosity of 3C345 on parsec scales is ~3*10^43 erg/s (about 1% of the observed luminosity of the source between the radio to infrared regimes). 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.