Reorienting Our Perspective of Broad Absorption Line Quasars

TL;DR

This study uses multi-frequency radio observations to reveal that BAL quasars tend to have steeper spectral indices, indicating they are viewed from larger angles relative to the jet axis, which advances understanding of their orientation.

Contribution

First direct measurement showing BAL quasars have different spectral index distributions, supporting orientation-based models of BAL phenomena.

Findings

BAL quasars have more steep-spectrum sources

Spectral index differences suggest larger viewing angles for BAL quasars

Monte-Carlo simulations quantify the viewing angle difference

Abstract

New multi-frequency radio observations of a large sample of radio-selected BAL quasars, along with a very well matched sample of normal quasars, are presented. The observations were made one immediately after the other at 4.9 and 8.4 GHz with the goal of measuring the radio spectral index of each source. We have identified, for the first time, a significant difference in the spectral index distributions of BAL versus non-BAL quasars, with BAL sources showing an overabundance of steep-spectrum sources. This is the first direct observation suggesting that BAL quasars are more likely to be seen farther from the radio jet axis, although a range of orientations is needed to explain the width of the distribution. Utilizing a few different relationships between spectral index and viewing angle, we have also performed Monte-Carlo simulations to quantify the viewing angle to these sources. We find that the difference in the distributions of spectral index can be explained by allowing the BAL sources to have viewing angles extending about 10 degrees farther from the jet axis than non-BAL quasars.