2011A&A...532A.150S

For more than 15 years, since the days of the Energetic Gamma-Ray Experiment Telescope (EGRET) on board the Compton Gamma-Ray Observatory (CGRO; 1991-2000), it has remained an open question why the prominent blazar 3C345 was not reliably detected at γ-ray energies ≥20MeV. Recently a bright γ-ray source (0FGLJ1641.4+3939/1FGLJ1642.5+3947), potentially associated with 3C345, was detected by the Large Area Telescope (LAT) on Fermi. Multiwavelength observations from radio bands to X-rays (mainly GASP-WEBT and Swift) of possible counterparts (3C345, NRAO512, B3 1640+396) were combined with 20 months of Fermi-LAT monitoring data (August 2008-April 2010) to associate and identify the dominating γ-ray emitting counterpart of 1FGLJ1642.5+3947. The source 3C345 is identified as the main contributor for this γ-ray emitting region. However, after November 2009 (15 months), a significant excess of photons from the nearby quasar NRAO512 started to contribute and thereafter was detected with increasing γ-ray activity, possibly adding flux to 1FGLJ1642.5+3947. For the same time period and during the summer of 2010, an increase of radio, optical and X-ray activity of NRAO512 was observed. No γ-ray emission from B3 1640+396 was detected.