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NuSTAR reveals extreme absorption in z<0.5 type 2 quasars

NuSTAR reveals extreme absorption in z<0.5 type 2 quasars
NuSTAR reveals extreme absorption in z<0.5 type 2 quasars
The intrinsic column density (NH) distribution of quasars is poorly known. At the high obscuration end of the quasar population and for redshifts z < 1, the X-ray spectra can only be reliably characterized using broad-band measurements that extend to energies above 10 keV. Using the hard X-ray observatory NuSTAR, along with archival Chandra and XMM-Newton data, we study the broad-band X-ray spectra of nine optically selected (from the SDSS), candidate Compton-thick (NH > 1.5 × 1024 cm−2) type 2 quasars (CTQSO2s); five new NuSTAR observations are reported herein, and four have been previously published. The candidate CTQSO2s lie at z < 0.5, have observed [O iii] luminosities in the range $8.4\lt \mathrm{log}({L}_{[{\rm{O}}\ {\rm{III}}]}/{L}_{\odot })\lt 9.6$, and show evidence for extreme, Compton-thick absorption when indirect absorption diagnostics are considered. Among the nine candidate CTQSO2s, five are detected by NuSTAR in the high-energy (8–24 keV) band: two are weakly detected at the ≈3σ confidence level and three are strongly detected with sufficient counts for spectral modeling (gsim90 net source counts at 8–24 keV). For these NuSTAR-detected sources direct (i.e., X-ray spectral) constraints on the intrinsic active galactic nucleus properties are feasible, and we measure column densities ≈2.5–1600 times higher and intrinsic (unabsorbed) X-ray luminosities ≈10–70 times higher than pre-NuSTAR constraints from Chandra and XMM-Newton. Assuming the NuSTAR-detected type 2 quasars are representative of other Compton-thick candidates, we make a correction to the NH distribution for optically selected type 2 quasars as measured by Chandra and XMM-Newton for 39 objects. With this approach, we predict a Compton-thick fraction of ${f}_{\mathrm{CT}}={36}_{-12}^{+14}$ %, although higher fractions (up to 76%) are possible if indirect absorption diagnostics are assumed to be reliable.
1538-4357
1-19
Lansbury, G.B.
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Gandhi, P.
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Alexander, D.M.
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Assef, R.J.
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Aird, J.
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Annuar, A.
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Ballantyne, D.R.
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Balokovic, M.
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Bauer, F.E.
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Boggs, S.E.
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Brandt, W.N.
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Brightman, M.
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Christensen, F.E.
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Civano, F.
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Comastri, A.
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Craig, W.W.
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Del Moro, A.
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Grefenstette, B.W.
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Hailey, C.J.
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Harrison, F.A.
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Hickox, R.C.
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Koss, M.
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LaMassa, S.M.
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Luo, B.
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Puccetti, S.
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Stern, D.
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Treister, E.
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Vignali, C.
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Zappacosta, L.
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Zhang, W.W.
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Lansbury, G.B.
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Gandhi, P.
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Alexander, D.M.
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Assef, R.J.
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Aird, J.
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Annuar, A.
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Ballantyne, D.R.
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Balokovic, M.
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Bauer, F.E.
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Boggs, S.E.
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Brandt, W.N.
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Brightman, M.
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Christensen, F.E.
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Civano, F.
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Comastri, A.
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Craig, W.W.
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Del Moro, A.
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Grefenstette, B.W.
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Hailey, C.J.
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Harrison, F.A.
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Hickox, R.C.
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Koss, M.
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LaMassa, S.M.
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Luo, B.
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Puccetti, S.
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Stern, D.
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Treister, E.
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Vignali, C.
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Zappacosta, L.
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Zhang, W.W.
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Lansbury, G.B., Gandhi, P., Alexander, D.M., Assef, R.J., Aird, J., Annuar, A., Ballantyne, D.R., Balokovic, M., Bauer, F.E., Boggs, S.E., Brandt, W.N., Brightman, M., Christensen, F.E., Civano, F., Comastri, A., Craig, W.W., Del Moro, A., Grefenstette, B.W., Hailey, C.J., Harrison, F.A., Hickox, R.C., Koss, M., LaMassa, S.M., Luo, B., Puccetti, S., Stern, D., Treister, E., Vignali, C., Zappacosta, L. and Zhang, W.W. (2015) NuSTAR reveals extreme absorption in z<0.5 type 2 quasars. The Astrophysical Journal, 809 (2), 1-19, [115]. (doi:10.1088/0004-637X/809/2/115).

Record type: Article

Abstract

The intrinsic column density (NH) distribution of quasars is poorly known. At the high obscuration end of the quasar population and for redshifts z < 1, the X-ray spectra can only be reliably characterized using broad-band measurements that extend to energies above 10 keV. Using the hard X-ray observatory NuSTAR, along with archival Chandra and XMM-Newton data, we study the broad-band X-ray spectra of nine optically selected (from the SDSS), candidate Compton-thick (NH > 1.5 × 1024 cm−2) type 2 quasars (CTQSO2s); five new NuSTAR observations are reported herein, and four have been previously published. The candidate CTQSO2s lie at z < 0.5, have observed [O iii] luminosities in the range $8.4\lt \mathrm{log}({L}_{[{\rm{O}}\ {\rm{III}}]}/{L}_{\odot })\lt 9.6$, and show evidence for extreme, Compton-thick absorption when indirect absorption diagnostics are considered. Among the nine candidate CTQSO2s, five are detected by NuSTAR in the high-energy (8–24 keV) band: two are weakly detected at the ≈3σ confidence level and three are strongly detected with sufficient counts for spectral modeling (gsim90 net source counts at 8–24 keV). For these NuSTAR-detected sources direct (i.e., X-ray spectral) constraints on the intrinsic active galactic nucleus properties are feasible, and we measure column densities ≈2.5–1600 times higher and intrinsic (unabsorbed) X-ray luminosities ≈10–70 times higher than pre-NuSTAR constraints from Chandra and XMM-Newton. Assuming the NuSTAR-detected type 2 quasars are representative of other Compton-thick candidates, we make a correction to the NH distribution for optically selected type 2 quasars as measured by Chandra and XMM-Newton for 39 objects. With this approach, we predict a Compton-thick fraction of ${f}_{\mathrm{CT}}={36}_{-12}^{+14}$ %, although higher fractions (up to 76%) are possible if indirect absorption diagnostics are assumed to be reliable.

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Accepted/In Press date: 12 June 2015
Published date: 14 August 2015
Organisations: Astronomy Group

Identifiers

Local EPrints ID: 400046
URI: http://eprints.soton.ac.uk/id/eprint/400046
ISSN: 1538-4357
PURE UUID: 04d1bd67-d922-4ea5-82cd-753595596f93
ORCID for P. Gandhi: ORCID iD orcid.org/0000-0003-3105-2615

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Date deposited: 08 Sep 2016 10:41
Last modified: 04 Feb 2020 01:34

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Contributors

Author: G.B. Lansbury
Author: P. Gandhi ORCID iD
Author: D.M. Alexander
Author: R.J. Assef
Author: J. Aird
Author: A. Annuar
Author: D.R. Ballantyne
Author: M. Balokovic
Author: F.E. Bauer
Author: S.E. Boggs
Author: W.N. Brandt
Author: M. Brightman
Author: F.E. Christensen
Author: F. Civano
Author: A. Comastri
Author: W.W. Craig
Author: A. Del Moro
Author: B.W. Grefenstette
Author: C.J. Hailey
Author: F.A. Harrison
Author: R.C. Hickox
Author: M. Koss
Author: S.M. LaMassa
Author: B. Luo
Author: S. Puccetti
Author: D. Stern
Author: E. Treister
Author: C. Vignali
Author: L. Zappacosta
Author: W.W. Zhang

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