We propose a new method to locate the \gamma -ray–emitting positions R _ { \gamma } from the measured time lags \tau _ { ob } of \gamma -ray emission relative to broad emission lines .
The method is also applicable to lower frequencies . R _ { \gamma } depends on parameters \tau _ { ob } , R _ { BLR } , v _ { d } and \theta , where R _ { BLR } is the size of broad-line region , v _ { d } is the travelling speed of disturbances down the jet and \theta is the viewing angle of the jet axis to the line of sight .
As \tau _ { ob } = 0 , \tau _ { ob } < 0 or \tau _ { ob } > 0 , the broad lines zero-lag , lag or lead the \gamma -rays , respectively .
It is applied to 3C 273 , in which the lines and the radio emission have enough data , but the \gamma -rays have not .
We find \tau _ { ob } < 0 and \tau _ { ob } > 0 for the 5 , 8 , 15 , 22 and 37 GHz emission relative to the broad lines H \alpha , H \beta and H \gamma .
The lag may be positive or negative , however current data do not allow to discriminate between the two cases .
The measured lags are on the order of years .
For a given line , \tau _ { ob } generally decreases as radio frequency increases .
This trend most likely results from the radiative cooling of relativistic electrons .
The negative lags have an average of \tau _ { ob } = -2.86 years for the 37 GHz emission , which represents that the lines lag the radio emission .
The positive lags have \tau _ { ob } = 3.20 years , which represents that the lines lead the radio emission .
We obtain the radio emitting positions R _ { radio } = 0.40 –2.62 pc and R _ { radio } = 9.43 –62.31 pc for the negative and positive lags , respectively .
From the constraint of R _ { \gamma } \la R _ { radio } ( e.g . 26 ; 44 ) , we have R _ { \gamma } \la 0.40 –2.62 pc for the negative lags .
For the positive lags , 4.67– 30.81 < R _ { \gamma } \la 9.43 –62.31 pc .
These estimated R _ { \gamma } are consistent with those of other researches .
These agreements confirm the reliability of the method and assumptions .
The method may be also applicable to BL Lacertae objects , in which broad lines were detected .