Revisiting Correlations Between Broad-Line and Jet Emission Variations for AGNs: 3C 120 and 3C 273

TL;DR

This study analyzes the timing correlations between broad-line and jet emissions in AGNs 3C 120 and 3C 273 to locate gamma-ray emitting regions, deriving new formulas and estimating their positions within a few parsecs of the central black holes.

Contribution

It introduces new formulas linking jet emission positions to observed time lags and BLR properties, and applies them to estimate emission regions in two AGNs.

Findings

Positive lags of ~0.3 yr in 3C 120 indicate line variations lead radio emissions.

Negative lags in 3C 273 suggest radio emissions lead broad-line variations.

Estimated jet emission regions are within 1-3 parsecs from the central engines.

Abstract

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 $R_{\rm{jet}}$.