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 broad line region ( 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 suggests that the \gamma -ray emitting region may be located outside the BLR .
Assuming the magnetic field strength in the \gamma -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 ) \simeq B _ { 1 pc } ( R / 1 pc ) ^ { -1 } , R is the distance from the central engine and B _ { 1 pc } is the magnetic field strength at 1 pc ) , we further calculate the location of the \gamma -ray emitting region , R _ { \gamma } , for these blazars .
We find that R _ { \gamma } \sim 2 \times 10 ^ { 4 } R _ { S } \simeq 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 .