A Chandra Survey of Broad Absorption Line Quasars

By P. Green, Tom Aldcroft, S. Mathur, B. Wilkes, M. Elvis. 2001

134 citations recorded by the source corpus.

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 (αox 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 αox 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 αox—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 αox > 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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/1a6267784ad3ae2b4905c7ae60e764387a0e2436

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Citations in the corpus

Quasars: the characteristic spectrum and the induced radiative heating (2003, background): “Futher, ∼ 10% of optically selected quasars show broad absorption lines (BALs), arising in material apparently flowing outward from the nucleus with velocities ∼ 104 km s−1, and it appears that most or possibly all quasars contain such BAL outflows (e.g. Green et al. 2001).”
The Frequency and Radio Properties of Broad Absorption Line Quasars (2003, methodology): “Ser. 255, Mass Outflow in Active Galactic Nuclei: New Perspectives (San Francisco: ASP) Foltz, C. B., Chaffee, F. H., Hewett, P. C., MacAlpine, G. M., Turnshek, D. A.,Weymann, R. J., & Anderson, S. F. 1987, AJ, 94, 1423 Foltz, C. B., Chaffee, F. H., Hewett, P. C., Weymann, R. J., Anderson, S. F., &M”
A CATALOG OF BROAD ABSORPTION LINE QUASARS IN SLOAN DIGITAL SKY SURVEY DATA RELEASE 5 (2008, background): “Compared to typical QSOs of the same UV luminosity, BAL QSOs are unusually X-ray weak (e.g., Green et al. 1995; Brandt et al. 2000) due to strong X-ray absorption (e.g., Green et al. 2001; Gallagher et al. 2001, 2002, and references therein).”
Unusual Broad Absorption Line Quasars from the Sloan Digital Sky Survey (2002, background): “The ‘hitchhiking’ gas successfully explains the strong soft X-ray absorption in BAL quasars (NH = 10 23−24 cm−2; e.g., Green et al. 2001a) and absorption from very high ionization lines (up to Neviii, Mgx, and Sixii in SBS 1542+541; Telfer et al. 1998).”
An Exploratory Chandra Survey of a Well-defined Sample of 35 Large Bright Quasar Survey Broad Absorption Line Quasars (2006, methodology): “This result strengthens the conclusion of Green et al. (2001) that LoBAL quasars are significantly X-ray weaker than normal BAL quasars.”
X-Ray Emission from Radio-Quiet Quasars in the Sloan Digital Sky Survey Early Data Release: The αox Dependence upon Ultraviolet Luminosity (2002, background): “Recent studies (e.g., Gallagher et al. 2001, 2002; Green et al. 2001) have shown that, although BALQSOs are probably characterized by the same underlying X-ray continua as the majority of the quasar population, their X-ray emission is often depressed by large amounts of intrinsic absorption.”
X-Ray Spectroscopy of Quasi-Stellar Objects with Broad Ultraviolet Absorption Lines (2001, background): “Subsequently , a few more observations of BAL QSOs with ASCA and Chandra have upheld this result (Green et al. 2001; Mathur et al. 2001; Gallagher et al. 2001b).”
CHANDRA Detects Relativistic Broad Absorption Lines from APM 08279+5255 (2002, methodology): “The present X-ray data for BALQSOs are sparse, and only poor to moderate signal-to-noise ra- tio (S/N) spectra are available (e.g., Chartas et al. 2001; Gallagher et al. 2001; Green et al. 2001; Oshima et al. 2001; Brinkmann et al. 2002; Gallagher et al. 2002).”