2018ApJ...852...45W

We model simultaneous or quasi-simultaneous multi-band spectral energy distributions (SEDs) for a sample of 25 blazars that have radio core-shift measurements, where a one-zone leptonic model and Markov chain Monte Carlo technique are adopted. In the SED fitting for 23 low-synchrotron-peaked (LSP) blazars, the seed photons from the broad-line (BLR) and molecular torus are considered respectively in the external Compton process. We find that the SED fitting with the seed photons from the torus are better than those utilizing BLR photons, which suggest that the γ-ray emitting region may be located outside the BLR. Assuming the magnetic field strength in the γ-ray emitting region as constrained from the SED fitting follows the magnetic field distribution as derived from the radio core-shift measurements (i.e., B(R)≃B_1pc(R/1pc)^–1, where R is the distance from the central engine and B_1pc is the magnetic field strength at 1 pc), we further calculate the location of the γ-ray emitting region, R_γ, for these blazars. We find that R_γ∼2×10⁴R_S≃10 R_BLR (R_S is the Schwarzschild radius and R_BLR is the BLR size), where R_BLR is estimated from the broad-line luminosities using the empirical correlations obtained using the reverberation mapping methods.