Fermi Gamma-ray Space Telescope

Six-years of Fermi-LAT and Multi-wavelength Monitoring of the Broad Line Radio Galaxy 3C 120

Yasuyuki Tanaka
A. Doi, Y. Inoue, C.C.Cheung, Y. Fukazawa, L. Stawarz, M. A. Gurwell

Abstract:

We present multi-wavelength monitoring results for the broad-line radio galaxy (BLRG) 3C 120 in 43 GHz, sub-millimeter, and MeV/GeV gamma-ray bands over six-years. A Fermi-LAT detection of 3C 120 was previously reported (Abdo et al. 2010) and an inner jet origin for the gamma-rays was discussed based on the possible variable GeV flux and strong nuclear radio flux among BLRGs (Kataoka et al. 2011). Here we show through multi-wavelength data that the high-energy gamma-ray emission originated inside the 43 GHz core, rather than pc-scale knots or larger scale jet, at least for the flare in late 2012. Sub-millimeter Array (SMA) monitoring revealed that 3C 120 showed multiple sub-mm flares after the launch of Fermi-LAT in 2008 June. Corresponding to some of the flares, Fermi-LAT detected enhanced MeV/GeV gamma-rays from this object. Very Long Baseline Array monitoring at 43 GHz in 2012-2013 revealed that the radio core brightened after the MeV/GeV flares, suggesting that the high-energy gamma-ray emission took place inside the 43 GHz core. If the distance from the 43 GHz core to the SMBH is ~0.5 pc, as previously estimated based on the time lag between X-ray dips and knot ejections (Marscher et al. 2002, Chatterjee et al. 2009), our result indicates that jet dissipation occurred at sub-parsec distances from the central SMBH. Given the bright accretion disk, we infer that the MeV/GeV gamma-rays are emitted via inverse Compton scattering of external photons coming from broad line region or dusty torus, rather than from the synchrotron self-Compton process.