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

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.

18 pages, 11 Postscript figures, 12 Tables. Accepted for publication in MNRAS

References in corpus (8)

• The Sloan Digital Sky Survey Quasar Catalog IV. Fifth Data Release
• The VLA Low-frequency Sky Survey
• Fast outflows in compact radio sources: evidence for AGN-induced feedback in the early stages of radio source evolution
• On the Fraction of Quasars with Outflows
• Outflows and the Physical Properties of Quasars
• The Physical Nature of Polar Broad Absorption Line Quasars
• VLBA images of High Frequency Peakers
• FIRST-based survey of Compact Steep Spectrum sources, V. Milliarcsecond-scale morphology of CSS objects

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• A Chandra Survey of the X-ray Properties of Broad Absorption Line Radio-Loud Quasars
• Quasar emission lines as probes of orientation: implications for disc wind geometries and unification
• A VLBI survey of compact Broad Absorption Lines (BAL) quasars with BALnicity Index BI=0
• Fraction of broad absorption line quasars in different radio morphologies
• Radio-Loud and Radio-Quiet BAL Quasars: A Detailed Ultraviolet Comparison
• Investigating the radio-loud phase of broad absorption line quasars
• A Giant Metrewave Radio Telescope Survey of Radio-loud Broad Absorption Line Quasars
• VLBA imaging of radio-loud Broad Absorption Line QSOs
• The Nature of LoBAL QSOs: II. HST/WFC3 Observations Reveal Host Galaxies Dominated by Mergers
• Broadband nuclear emission in two radio-loud BAL quasars