Radio spectra and polarisation properties of a bright sample of Radio-Loud Broad Absorption Line Quasars

TL;DR

This study investigates the radio spectra and polarization of radio-loud BAL quasars to understand their origin, testing orientation versus evolutionary models through multi-frequency radio observations and comparisons with non-BAL quasars.

Contribution

It provides the first comprehensive multi-frequency radio analysis of a well-defined BAL QSO sample, comparing their properties with non-BAL QSOs to evaluate prevailing models.

Findings

BAL and non-BAL QSOs have similar fractions of GHz-peaked sources.

BAL QSOs tend to have steeper spectra, mildly supporting edge-on orientation.

Spectral properties do not strongly favor a polar orientation for BAL QSOs.

Abstract

The origin of broad-absorption-line quasars (BAL QSOs) is still an open issue. Accounting for ~20% of the QSO population, these objects present broad absorption lines in their optical spectra generated from outflows with velocities up to 0.2c. In this work we present the results of a multi-frequency study of a well-defined radio-loud BAL QSO sample, and a comparison sample of radio-loud non-BAL QSOs, both selected from the Sloan Digital Sky Survey (SDSS). We aim to test which of the currently-popular models for the BAL phenomenon - `orientation' or 'evolutionary' - best accounts for the radio properties of BAL quasars. Observations from 1.4 to 43 GHz have been obtained with the VLA and Effelsberg telescopes, and data from 74 to 408 MHz have been compiled from the literature. The fractions of candidate GHz-peaked sources are similar in the two samples (36\pm12% vs 23\pm8%), suggesting that BAL QSOs are not generally younger than non-BAL QSOs. BAL and non-BAL QSOs show a large range of spectral indices, consistent with a broad range of orientations. There is weak evidence (91% confidence) that the spectral indices of the BAL QSOs are steeper than those of non-BAL QSOs, mildly favouring edge-on orientations. At a higher level of significance (\geq97%), the spectra of BAL QSOs are not flatter than those of non-BAL QSOs, which suggests that a polar orientation is not preferred.