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NuSTAR and XMM-Newton observations of luminous, heavily obscured, WISE-selected quasars at z \~ 2.

Stern, D. and Lansbury, G.B. and Assef, R.J. and Brandt, W.N. and Alexander, D.M. and Ballantyne, D.R. and Baloković, M. and Bauer, F.E. and Benford, D. and Blain, A. and Boggs, S.E. and Bridge, C. and Brightman, M. and Christensen, F.E. and Comastri, A. and Craig, W.W. and Del Moro, A. and Eisenhardt, P.R.M. and Gandhi, P. and Griffith, R.L. and Hailey, C.J. and Harrison, F.A. and Hickox, R.C. and Jarrett, T.H. and Koss, M. and Lake, S. and LaMassa, S.M. and Luo, B. and Tsai, C.-W. and Urry, C.M. and Walton, D.J. and Wright, E.L. and Wu, J. and Yan, L. and Zhang, W.W. (2014) 'NuSTAR and XMM-Newton observations of luminous, heavily obscured, WISE-selected quasars at z \~ 2.', Astrophysical journal., 794 (2). p. 102.

Abstract

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 lsim 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 gsim 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.

Item Type:Article
Keywords:Galaxies: active, Quasars: individual (WISEA J181417.29+341224.8, WISEA J220743.82+193940.1, WISEA J235710.82+032802).
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Status:Peer-reviewed
Publisher Web site:http://dx.doi.org/10.1088/0004-637X/794/2/102
Publisher statement:© 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
Date accepted:23 July 2014
Date deposited:20 May 2015
Date of first online publication:October 2014
Date first made open access:No date available

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