Radio spectra and polarisation properties of radio-loud Broad Absorption Line Quasars

TL;DR

This study investigates the radio spectra and polarisation of 15 radio-loud BAL QSOs, revealing their compactness, spectral shapes, and similarities to young radio sources, with implications for their orientation and evolution.

Contribution

It provides the first multi-frequency radio analysis of BAL QSOs, showing their spectral and polarisation properties and comparing them to non-BAL QSOs and young radio sources.

Findings

Most BAL QSOs have convex spectra peaking at 1-5 GHz.

Approximately two-thirds are unpolarised or weakly polarised at 8.4 GHz.

BAL QSOs show properties similar to young radio sources like CSS and GPS.

Abstract

We present multi-frequency observations of a sample of 15 radio-emitting Broad Absorption Line Quasars (BAL QSOs), covering a spectral range between 74 MHz and 43 GHz. They display mostly convex radio spectra which typically peak at about 1-5 GHz (in the observer's rest-frame), flatten at MHz frequencies, probably due to synchrotron self-absorption, and become steeper at high frequencies, i.e., >~ 20 GHz. VLA 22-GHz maps (HPBW ~ 80 mas) show unresolved or very compact sources, with linear projected sizes of <= 1 kpc. About 2/3 of the sample look unpolarised or weakly polarised at 8.4 GHz, frequency in which reasonable upper limits could be obtained for polarised intensity. Statistical comparisons have been made between the spectral index distributions of samples of BAL and non-BAL QSOs, both in the observed and the rest-frame, finding steeper spectra among non-BAL QSOs. However constraining this comparison to compact sources results in no significant differences between both distributions. This comparison is consistent with BAL QSOs not being oriented along a particular line of sight. In addition, our analysis of the spectral shape, variability and polarisation properties shows that radio BAL QSOs share several properties common to young radio sources like Compact Steep Spectrum (CSS) or Gigahertz-Peaked Spectrum (GPS) sources.