The Viewing Angles of Broad Absorption Line Versus Unabsorbed Quasars

TL;DR

This study uses Monte-Carlo simulations to analyze how the viewing angle influences the presence of broad absorption line features in quasars, revealing orientation as a contributing but not sole factor.

Contribution

It provides a quantitative analysis of the viewing angle differences between BAL and non-BAL quasars using spectral index distributions and simulation models.

Findings

BAL quasars have viewing angles about 10 degrees farther from the jet axis.

Spectral index distributions differ enough to indicate orientation effects.

Orientation influences BAL feature presence but is not the only factor.

Abstract

It was recently shown that there is a significant difference in the radio spectral index distributions of broad absorption line (BAL) quasars and unabsorbed quasars, with an overabundance of BAL quasars with steeper radio spectra. This result suggests that source orientation does play into the presence or absence of BAL features. In this paper we provide more quantitative analysis of this result based on Monte-Carlo simulations. While the relationship between viewing angle and spectral index does indeed contain a lot of scatter, the spectral index distributions are different enough to overcome that intrinsic variation. Utilizing two different models of the relationship between spectral index and viewing angle, the simulations indicate that the difference in spectral index distributions can be explained by allowing BAL quasar viewing angles to extend about 10 degrees farther from the radio jet axis than non-BAL sources, though both can be seen at small angles. These results show that orientation cannot be the only factor determining whether BAL features are present, but it does play a role.