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Published October 10, 2014 | Submitted + Published
Journal Article Open

Weak Hard X-Ray Emission from Broad Absorption Line Quasars: Evidence for Intrinsic X-Ray Weakness

Abstract

We report NuSTAR observations of a sample of six X-ray weak broad absorption line (BAL) quasars. These targets, at z = 0.148–1.223, are among the optically brightest and most luminous BAL quasars known at z < 1.3. However, their rest-frame ≈2 keV luminosities are 14 to >330 times weaker than expected for typical quasars. Our results from a pilot NuSTAR study of two low-redshift BAL quasars, a Chandra stacking analysis of a sample of high-redshift BAL quasars, and a NuSTAR spectral analysis of the local BAL quasar Mrk 231 have already suggested the existence of intrinsically X-ray weak BAL quasars, i.e., quasars not emitting X-rays at the level expected from their optical/UV emission. The aim of the current program is to extend the search for such extraordinary objects. Three of the six new targets are weakly detected by NuSTAR with ≲45 counts in the 3–24 keV band, and the other three are not detected. The hard X-ray (8–24 keV) weakness observed by NuSTAR requires Compton-thick absorption if these objects have nominal underlying X-ray emission. However, a soft stacked effective photon index (Γ_eff ≈ 1.8) for this sample disfavors Compton-thick absorption in general. The uniform hard X-ray weakness observed by NuSTAR for this and the pilot samples selected with <10 keV weakness also suggests that the X-ray weakness is intrinsic in at least some of the targets. We conclude that the NuSTAR observations have likely discovered a significant population (≳33%) of intrinsically X-ray weak objects among the BAL quasars with significantly weak <10 keV emission. We suggest that intrinsically X-ray weak quasars might be preferentially observed as BAL quasars.

Additional Information

© 2014. The American Astronomical Society. Received 21 May 2014, accepted for publication 14 August 2014. Published 24 September 2014. We acknowledge support from the California Institute of Technology (Caltech) NuSTAR subcontract 44A-1092750 (B.L., W.N.B.), NASA ADP grant NNX10AC99G (B.L., W.N.B.), NASA Postdoctoral Program (S.H.T.), CONICYT-Chile FONDECYT 1140304 (P.A.) and 1141218 (F.E.B.), "EMBIGGEN" Anillo ACT1101 (P.A., F.E.B.), Basal-CATA PFB-06/2007 (F.E.B.), Project IC120009 "Millennium Institute of Astrophysics (MAS)" of Iniciativa Científica Milenio del Ministerio de Economía, Fomento y Turismo (F.E.B.), ASI/INAF grant I/037/12/0-011/13 (A.C.), STFC grant ST/J003697/1 (P.G.), and the Swiss National Science Foundation (NSF) grant PP00P2 138979/1 (M.K.). We thank K. Forster for help with the observation planning, and we thank T. Yaqoob for helpful discussions. We thank the referee for carefully reviewing the manuscript and providing helpful comments. This work was supported under NASA Contract No. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by Caltech, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has made use of NuSTARDAS jointly developed by the ASI Science Data Center (ASDC, Italy) and Caltech (USA).

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Published - 0004-637X_794_1_70.pdf

Submitted - 1408.3633v1.pdf

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August 22, 2023
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