Revisiting Correlations Between Broad-Line and Jet Emission Variations
  for AGNs: 3C 120 and 3C 273
  We restudy the issue of cross-correlations between broad-line and jet
emission variations, and aim to locate the position of radio (and gamma-ray)
emitting region in jet of active galactic nuclei (AGNs). Considering the radial
profiles of the radius and number density of clouds in a spherical broad-line
region (BLR), we derive new formulae connecting jet emitting position
$R_{\rm{jet}}$ to time lag $\tau_{\rm{ob}}$ between broad-line and jet emission
variations, and BLR radius. Also, formulae are derived for a disk-like BLR and
a spherical shell BLR. The model-independent FR/RSS method is used to estimate
$\tau_{\rm{ob}}$. For 3C 120, positive lags of about 0.3 yr are found between
the 15 GHz emission and the H$\beta$, H$\gamma$ and He II $\lambda 4686$ lines,
including broad-line data in a newly published paper, indicating the line
variations lead the 15 GHz ones. Each of the broad-line light curves
corresponds to a radio outburst. $R_{\rm{jet}}=$1.1--1.5 parsec (pc) are
obtained for 3C 120. For 3C 273, a common feature of negative time lags is
found in the cross-correlation functions between light curves of radio emission
and the Balmer lines, and as well Ly$\alpha \/\ \lambda 1216$ and C~{\sc iv}
$\lambda 1549$ lines. $R_{\rm{jet}}=$ 1.0--2.6 pc are obtained for 3C 273. The
estimated $R_{\rm{jet}}$ are comparable for 3C 120 and 3C 273, and the
gamma-ray emitting positions will be within $\sim$ 1--3 pc from the central
engines. Comparisons show that the cloud number density and radius radial
distributions and the BLR structures only have negligible effects on
$R_{\rm{jet}}$.
