AChandraSurvey of Broad Absorption Line Quasars

Abstract

We have carried out a survey with the Chandra X-Ray Observatory of a sample of 10 bright broad absorption line (BAL) quasars (QSOs). Eight of 10 sources are detected. The six brightest sources have only high-ionization BALs (hiBALs), while the four faintest all show low-ionization BALs (loBALs). We perform a combined spectral fit for hiBAL QSOs (384 counts total; 0.5-6 keV) to determine the mean spectral parameters of this sample. We derive an underlying best-fit power-law slope Γ = 1.8 ± 0.35, which is consistent with the mean slope for radio-quiet QSOs from ASCA, but BAL QSOs require a (rest-frame) absorbing column of 6.5 × 10²² cm^-2, with a partial covering fraction of ~80%. The optical-to-X-ray spectral slope (α_ox from 2500 Å to 2 keV) varies from 1.7 to 2.4 across the full sample, consistent with previous results that BAL QSOs appear to be weak soft X-ray emitters. Removing the absorption component from our best-fit spectral model yields a range of α_ox from 1.55 to 2.28. All six hiBAL QSOs have deabsorbed X-ray emission consistent with non-BAL QSOs of similar luminosity. The spectral energy distributions of the hiBAL QSOs—both the underlying power-law slope and α_ox—provide the first conclusive evidence that BAL QSOs have appeared to be X-ray weak because of intrinsic absorption and that their underlying emission is consistent with non-BAL QSOs. By contrast, the removal of the best-fit absorption column detected in the hiBAL QSOs still leaves the four loBAL QSOs with values of α_ox > 2 that are unusually X-ray faint for their optical luminosities, which is consistent with other evidence that loBALs have higher column density, dustier absorbers. Important questions of whether BAL QSOs represent a special line of sight toward a QSO nucleus or rather an early evolutionary or high-accretion phase in a QSO lifetime remain to be resolved, and the unique properties of loBAL QSOs will be an integral part of that investigation.