The injector and variability of the quasar 3C 345

The VLBI technique is used to study the fine structure of the quasar 3C 345 at = 49 cm. It is shown that the low-frequency radio emission from the core is feeble, whereas the bright compact component corresponds to the jet region adjacent to the core, rather than to the core itself. This region is ~6.4 mas in size, and its brightness temperature, Tb near equal 0.6 1012 K, does not go beyond the Compton limit. The core itself is an injector of relativistic particles surrounded by a wall of thermal plasma. Between 1983.9 and 1990.8, the radio flux density and angular size of the compact component increased by a factor of about 2, while its brightness temperature did not change markedly. During this period, the emission at millimeter wavelengths was reduced about twofold; the UV emission also decreased. The low-frequency variability is shown to be a result of variations in the transparency of the wall of a cocoon—the thermal plasma surrounding the jet. The electron density in the cocoon wall near the bright compact component is Ne near equal 105 cm3 and rises toward the core as ~ r 2. The polarized emission of the quasar originates mainly from the same region, where compact sources are seen athigh frequencies. Its rotation measure is RM near equal 29 rad m–2 and increases toward the core as ~ r 3, reaching 8 104 rad m–2. In the vicinity of the core, the longitudinal component of the magnetic field is ~0.1 mG