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The NuSTAR Serendipitous Survey: Hunting for the most extreme obscured AGN at >10 keV

The NuSTAR Serendipitous Survey: Hunting for the most extreme obscured AGN at >10 keV
The NuSTAR Serendipitous Survey: Hunting for the most extreme obscured AGN at >10 keV
We identify sources with extremely hard X-ray spectra (i.e., with photon indices of {{Γ }}≲ 0.6) in the 13 deg2 NuSTAR serendipitous survey, to search for the most highly obscured active galactic nuclei (AGNs) detected at > 10 {keV}. Eight extreme NuSTAR sources are identified, and we use the NuSTAR data in combination with lower-energy X-ray observations (from Chandra, Swift XRT, and XMM-Newton) to characterize the broadband (0.5–24 keV) X-ray spectra. We find that all of the extreme sources are highly obscured AGNs, including three robust Compton-thick (CT; {N}{{H}}> 1.5× {10}24 cm‑2) AGNs at low redshift (z<0.1) and a likely CT AGN at higher redshift (z = 0.16). Most of the extreme sources would not have been identified as highly obscured based on the low-energy (<10 keV) X-ray coverage alone. The multiwavelength properties (e.g., optical spectra and X-ray–mid-IR luminosity ratios) provide further support for the eight sources being significantly obscured. Correcting for absorption, the intrinsic rest-frame 10–40 keV luminosities of the extreme sources cover a broad range, from ≈ 5× {10}42 to 1045 erg s‑1. The estimated number counts of CT AGNs in the NuSTAR serendipitous survey are in broad agreement with model expectations based on previous X-ray surveys, except for the lowest redshifts (z<0.07), where we measure a high CT fraction of {f}{CT}{obs}={30}-12+16 % . For the small sample of CT AGNs, we find a high fraction of galaxy major mergers (50% ± 33%) compared to control samples of “normal” AGNs.
galaxies: active, galaxies: nuclei, quasars: general, surveys, X-rays: galaxies
0004-637X
Lansbury, G.B.
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Alexander, D.M.
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Aird, J.
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Gandhi, P.
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Stern, D.
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Koss, M.
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Lamperti, I.
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Ajello, M.
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Annuar, A.
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Assef, R.J.
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Ballantyne, D.R.
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Baloković, M.
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Bauer, F.E.
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Brandt, W.N.
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Brightman, M.
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Chen, C.-T.J.
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Civano, F.
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Comastri, A.
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Del Moro, A.
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Fuentes, C.
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Harrison, F.A.
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Marchesi, S.
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Masini, A.
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Mullaney, J.R.
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Ricci, C.
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Saez, C.
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Tomsick, J.A.
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Treister, E.
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Walton, D.J.
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Zappacosta, L.
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Lansbury, G.B.
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Alexander, D.M.
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Aird, J.
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Gandhi, P.
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Stern, D.
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Koss, M.
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Lamperti, I.
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Ajello, M.
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Annuar, A.
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Assef, R.J.
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Ballantyne, D.R.
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Baloković, M.
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Bauer, F.E.
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Brandt, W.N.
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Brightman, M.
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Chen, C.-T.J.
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Civano, F.
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Comastri, A.
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Del Moro, A.
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Fuentes, C.
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Harrison, F.A.
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Marchesi, S.
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Masini, A.
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Mullaney, J.R.
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Ricci, C.
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Saez, C.
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Tomsick, J.A.
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Treister, E.
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Walton, D.J.
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Zappacosta, L.
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Lansbury, G.B., Alexander, D.M., Aird, J., Gandhi, P., Stern, D., Koss, M., Lamperti, I., Ajello, M., Annuar, A., Assef, R.J., Ballantyne, D.R., Baloković, M., Bauer, F.E., Brandt, W.N., Brightman, M., Chen, C.-T.J., Civano, F., Comastri, A., Del Moro, A., Fuentes, C., Harrison, F.A., Marchesi, S., Masini, A., Mullaney, J.R., Ricci, C., Saez, C., Tomsick, J.A., Treister, E., Walton, D.J. and Zappacosta, L. (2017) The NuSTAR Serendipitous Survey: Hunting for the most extreme obscured AGN at >10 keV. The Astrophysical Journal, 846 (1), [20]. (doi:10.3847/1538-4357/aa8176).

Record type: Article

Abstract

We identify sources with extremely hard X-ray spectra (i.e., with photon indices of {{Γ }}≲ 0.6) in the 13 deg2 NuSTAR serendipitous survey, to search for the most highly obscured active galactic nuclei (AGNs) detected at > 10 {keV}. Eight extreme NuSTAR sources are identified, and we use the NuSTAR data in combination with lower-energy X-ray observations (from Chandra, Swift XRT, and XMM-Newton) to characterize the broadband (0.5–24 keV) X-ray spectra. We find that all of the extreme sources are highly obscured AGNs, including three robust Compton-thick (CT; {N}{{H}}> 1.5× {10}24 cm‑2) AGNs at low redshift (z<0.1) and a likely CT AGN at higher redshift (z = 0.16). Most of the extreme sources would not have been identified as highly obscured based on the low-energy (<10 keV) X-ray coverage alone. The multiwavelength properties (e.g., optical spectra and X-ray–mid-IR luminosity ratios) provide further support for the eight sources being significantly obscured. Correcting for absorption, the intrinsic rest-frame 10–40 keV luminosities of the extreme sources cover a broad range, from ≈ 5× {10}42 to 1045 erg s‑1. The estimated number counts of CT AGNs in the NuSTAR serendipitous survey are in broad agreement with model expectations based on previous X-ray surveys, except for the lowest redshifts (z<0.07), where we measure a high CT fraction of {f}{CT}{obs}={30}-12+16 % . For the small sample of CT AGNs, we find a high fraction of galaxy major mergers (50% ± 33%) compared to control samples of “normal” AGNs.

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e-pub ahead of print date: 28 August 2017
Keywords: galaxies: active, galaxies: nuclei, quasars: general, surveys, X-rays: galaxies

Identifiers

Local EPrints ID: 414425
URI: http://eprints.soton.ac.uk/id/eprint/414425
ISSN: 0004-637X
PURE UUID: 35a2de5a-5eb5-4305-8d60-80425e22527d
ORCID for P. Gandhi: ORCID iD orcid.org/0000-0003-3105-2615

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Date deposited: 28 Sep 2017 16:31
Last modified: 09 Oct 2020 16:36

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Contributors

Author: G.B. Lansbury
Author: D.M. Alexander
Author: J. Aird
Author: P. Gandhi ORCID iD
Author: D. Stern
Author: M. Koss
Author: I. Lamperti
Author: M. Ajello
Author: A. Annuar
Author: R.J. Assef
Author: D.R. Ballantyne
Author: M. Baloković
Author: F.E. Bauer
Author: W.N. Brandt
Author: M. Brightman
Author: C.-T.J. Chen
Author: F. Civano
Author: A. Comastri
Author: A. Del Moro
Author: C. Fuentes
Author: F.A. Harrison
Author: S. Marchesi
Author: A. Masini
Author: J.R. Mullaney
Author: C. Ricci
Author: C. Saez
Author: J.A. Tomsick
Author: E. Treister
Author: D.J. Walton
Author: L. Zappacosta

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