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Published October 2014 | Published
Journal Article Open

NuSTAR and XMM-Newton Observations of Luminous, Heavily Obscured, WISE-selected Quasars at z ~ 2


We report on a NuSTAR and XMM-Newton program that has observed a sample of three extremely luminous, heavily obscured WISE-selected active galactic nuclei (AGNs) at z ~ 2 across a broad X-ray band (0.1 - 79 keV). The parent sample, selected to be faint or undetected in the WISE 3.4 μm (W1) and 4.6 μm (W2) bands but bright at 12 μm (W3) and 22 μm (W4), are extremely rare, with only ~1000 so-called "W1W2-dropouts" across the extragalactic sky. Optical spectroscopy reveals typical redshifts of z ~ 2 for this population, implying rest-frame mid-IR luminosities of νL ν(6 μm) ~ 6 × 1046 erg s^-1 and bolometric luminosities that can exceed L bol ~ 1014 L_⊙. The corresponding intrinsic, unobscured hard X-ray luminosities are L(2-10 keV) ~ 4 × 1045 erg s-1 for typical quasar templates. These are among the most AGNs known, though the optical spectra rarely show evidence of a broad-line region and the selection criteria imply heavy obscuration even at rest-frame 1.5 μm. We designed our X-ray observations to obtain robust detections for gas column densities N H <= 1024 cm-2. In fact, the sources prove to be fainter than these predictions. Two of the sources were observed by both NuSTAR and XMM-Newton, with neither being detected by NuSTAR (f 3-24 keV <~ 10-13 erg cm^(-2)s^(-1), and one being faintly detected by XMM-Newton (f 0.5-10 keV ~ 5 × 10-15 erg cm-2 s-1). A third source was observed only with XMM-Newton, yielding a faint detection (f 0.5-10 keV ~ 7 × 10-15 erg cm-2 s-1). The X-ray data imply these sources are either X-ray weak, or are heavily obscured by column densities N H >~ 1024 cm-2. The combined X-ray and mid-IR analysis seems to favor this second possibility, implying the sources are extremely obscured, consistent with Compton-thick, luminous quasars. The discovery of a significant population of heavily obscured, extremely luminous AGNs would not conform to the standard paradigm of a receding torus, in which more luminous quasars are less likely to be obscured, and instead suggests that an additional source of obscuration is present in these extreme sources.

Additional Information

© 2014. The American Astronomical Society. Published 2014 September 29. We gratefully acknowledge the suggestions made by the anonymous referee, which have improved this manuscript. This work was supported under NASA Contract No. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, 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 the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. We acknowledge financial support from the Science and Technology Facilities Council (STFC) grants ST/K501979/1 (G.B.L.), ST/I001573/1 (D.M.A. and A.D.M.), and ST/J003697/1 (P.G.), and the Leverhulme Trust (D.M.A. and J.R.M.). R.J.A. was supported by Gemini-CONICYT grant number 32120009. F.E.B. acknowledges support from CONICYT-Chile (Basal-CATA PFB-06/2007, FONDECYT 1141218, and "EMBIGGEN" Anillo ACT1101) and Project IC120009 "Millennium Institute of Astrophysics (MAS)" of Iniciativa Cient´ıfica Milenio del Ministerio de Econom´ıa, Fomento y Turismo. A.C. acknowledges support from ASI-INAF grant I/37/012/0-011/13. R.C.H. acknowledges support from NASA through ADAP award NNX12AE38G and the National Science Foundation through grant number 1211096. M.K. acknowledges support from Swiss National Science Foundation (NSF) grant PP00P2 138979/1.

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August 20, 2023
October 20, 2023