Spectral Evolution of the Parsec‐Scale Jet in the Quasar 3C 345

The long-term evolution of the synchrotron emission from the parsec-scale jetin the quasar 3C345 is analysed, on the basis of multi-frequency monitoringwith very long baseline interferometry (VLBI) and covering the period1979-1994. We combine the model fits from 44 VLBI observations of 3C345 made at8 different frequencies between 2.3 and 100GHz. We calculate the turnoverfrequency, turnover flux density, integrated 4-25GHz flux and 4-25GHzluminosity of the core and the moving features. The core has an estimated meanluminosity of 7.1+/-3.5 * 10^42 erg/s; the estimated total luminosity of 3C345on parsec scales is ~3*10^43 erg/s (about 1% of the observed luminosity of thesource between the radio to infrared regimes). The spectral changes in the corecan be reconciled with a shock or dense plasma condensation traveling throughthe region where the jet becomes optically thin. We are able to describe theevolution of the core spectrum by a sequence of 5 flare-like eventscharacterized by an exponential rise and decay of the particle number densityof the material injected into the jet. The same model is also capable ofpredicting the changes in the flux density observed in the core. The flaresoccur approximately every 3.5-4 years, roughly correlating with appearances ofnew moving features in the jet.Comment: 50 pages, 15 figures, 7 tables: accepted to The Astrophysical Journal; AASMS4 macro style file