The Kinematics of Quasar Broad Emission Line Regions Using a Disk-wind Model

TL;DR

This paper develops a kinematic disk-wind model for quasar broad emission lines, showing how viewing angle and wind geometry influence line profiles, widths, and velocity shifts, aligning with observational reverberation mapping results.

Contribution

It introduces a simple, orientation-dependent disk-wind model that explains diverse emission line features and reverberation responses in quasars.

Findings

Line profiles vary with viewing angle, becoming asymmetric and blueshifted at face-on orientations.

Emission lines near the disk base are broad and symmetric, with profile shapes depending on wind region.

The model reproduces observed reverberation responses, with faster red or blue side responses depending on wind geometry.

Abstract

The structure and kinematics of the broad line region (BLR) in quasars are still not well established. One popular BLR model is the disk-wind model that offers a geometric unification of a quasar based on the angle of viewing. We construct a simple kinematical disk-wind model with a narrow outflowing wind angle. The model is combined with radiative transfer in the Sobolev, or high velocity, limit. We examine how angle of viewing affects the observed characteristics of the emission line, especially the line widths and velocity offsets. The line profiles exhibit distinct properties depending on the orientation, wind opening angle, and region of the wind where the emission arises. At low inclination angle (close to face-on), we find the shape of the emission line is asymmetric with narrow width and significantly blueshifted. As the inclination angle increases (close to edge-on), the line profile becomes more symmetric, broader, and less blueshifted. Additionally, lines that arise close to the base of the disk wind, near the accretion disk, tend to be broad and symmetric. The relative increase in blueshift of the emission line with increasing wind vertical distance is larger for polar winds compared with equatorial winds. By considering the optical thickness of the wind, single-peaked line profiles are recovered for the intermediate and equatorial outflowing wind. The model is able to reproduce a faster response in either the red and blue sides of the line profile found in reverberation mapping studies. A quicker response in the red side is achieved in the model with a polar wind and intermediate wind opening angle at low viewing angle. The blue side response is faster for an equatorial wind seen at high inclination.