The flat-spectrum radio quasar 3C 345 from the high to the low emission state

Abstract: We report simultaneous observations at different energy bands in radio, optical, UV, X-rays and $\gamma$ rays of the flat-spectrum radio-quasar 3C 345. We built the light curve of the source at different frequencies from 2008, the beginning of the \textit{Fermi} all-sky survey, to 2016, using new data and public archives. In particular we obtained several optical spectra, to study the behavior of emission lines and the continuum in different activity states and to derive the black hole mass. 3C 345 showed two flaring episodes in 2009, which occurred simultaneously in $\gamma$ ray, optical/UV and X-rays, and were later followed in radio. The source shows an inverse Compton dominated spectral energy distribution, which moved from higher to lower frequencies from the high to the low state. The reverberation of emission lines during one outburst event allowed us to constrain the location of production of $\gamma$ rays very close to the broad-line region, and possibly in the jet-base. We report the observation of an increased accretion after the outburst, possibly induced by the decrease of magnetic field intensity with respect to the low state.