Liu, HT (reprint author), Chinese Acad Sci, Yunnan Astron Observ, Natl Astron Observ, Kunming 650011, Yunnan, Peoples R China.
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, upsilon(d) and theta, where R-BLR is the size of the broad-line region, upsilon(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 do 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 discrimination between the two cases. The measured lags are of 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 yr for the 37 GHz emission, which represents the lines lagging the radio emission. The positive lags have tau(ob) = 3.20 yr, which represents the lines leading 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 less than or similar to R-radio, we have R-gamma less than or similar to 0.40-2.62 pc for the negative lags. For the positive lags, 4.67-30.81 < R-gamma less than or similar to 9.43-62.31 pc. These estimated R-gamma are consistent with those of other studies. These agreements confirm the reliability of the method and assumptions. The method may also be applicable to BL Lacertae objects, in which broad lines were detected.