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Hard X-ray emission of the luminous infrared galaxy NGC 6240 as observed by NuSTAR

Hard X-ray emission of the luminous infrared galaxy NGC 6240 as observed by NuSTAR
Hard X-ray emission of the luminous infrared galaxy NGC 6240 as observed by NuSTAR
We present a broadband (~0.3?70 keV) spectral and temporal analysis of NuSTAR observations of the luminous infrared galaxy NGC 6240 combined with archival Chandra, XMM-Newton, and BeppoSAX data. NGC 6240 is a galaxy in a relatively early merger state with two distinct nuclei separated by ~1.?5. Previous Chandra observations resolved the two nuclei and showed that they are both active and obscured by Compton-thick material. Although they cannot be resolved by NuSTAR, we were able to clearly detect, for the first time, both the primary and the reflection continuum components thanks to the unprecedented quality of the NuSTAR data at energies >10 keV. The NuSTAR hard X-ray spectrum is dominated by the primary continuum piercing through an absorbing column density which is mildly optically thick to Compton scattering (? ? 1.2, NH ~ 1.5 × 1024 cm-2). We detect moderately hard X-ray (>10 keV) flux variability up to 20% on short (15?20 ks) timescales. The amplitude of the variability is largest at ~30 keV and is likely to originate from the primary continuum of the southern nucleus. Nevertheless, the mean hard X-ray flux on longer timescales (years) is relatively constant. Moreover, the two nuclei remain Compton-thick, although we find evidence of variability in the material along the line of sight with column densities NH ? 2 × 1023 cm-2 over long (~3?15 yr) timescales. The observed X-ray emission in the NuSTAR energy range is fully consistent with the sum of the best-fit models of the spatially resolved Chandra spectra of the two nuclei.
0004-6361
1-14
Puccetti, S.
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Comastri, A.
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Bauer, F.E.
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Brandt, W.N.
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Fiore, F.
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Harrison, F.A.
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Luo, B.
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Stern, D.
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Urry, C.M.
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Alexander, D.M.
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Annuar, A.
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Arévalo, P.
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Baloković, M.
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Boggs, S.E.
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Craig, W.W.
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Gandhi, P.
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Hailey, C.J.
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Koss, M.J.
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La Massa, S.
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Marinucci, A.
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Ricci, C.
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Walton, D.J.
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Zappacosta, L.
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Zhang, W.
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Puccetti, S.
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Comastri, A.
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Bauer, F.E.
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Brandt, W.N.
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Fiore, F.
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Harrison, F.A.
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Luo, B.
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Stern, D.
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Urry, C.M.
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Alexander, D.M.
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Annuar, A.
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Arévalo, P.
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Baloković, M.
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Boggs, S.E.
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Brightman, M.
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Christensen, F.E.
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Craig, W.W.
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Gandhi, P.
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Hailey, C.J.
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Koss, M.J.
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La Massa, S.
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Marinucci, A.
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Ricci, C.
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Walton, D.J.
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Zappacosta, L.
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Zhang, W.
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Puccetti, S., Comastri, A., Bauer, F.E., Brandt, W.N., Fiore, F., Harrison, F.A., Luo, B., Stern, D., Urry, C.M., Alexander, D.M., Annuar, A., Arévalo, P., Baloković, M., Boggs, S.E., Brightman, M., Christensen, F.E., Craig, W.W., Gandhi, P., Hailey, C.J., Koss, M.J., La Massa, S., Marinucci, A., Ricci, C., Walton, D.J., Zappacosta, L. and Zhang, W. (2016) Hard X-ray emission of the luminous infrared galaxy NGC 6240 as observed by NuSTAR. Astronomy & Astrophysics, 585 (A157), 1-14. (doi:10.1051/0004-6361/201527189).

Record type: Article

Abstract

We present a broadband (~0.3?70 keV) spectral and temporal analysis of NuSTAR observations of the luminous infrared galaxy NGC 6240 combined with archival Chandra, XMM-Newton, and BeppoSAX data. NGC 6240 is a galaxy in a relatively early merger state with two distinct nuclei separated by ~1.?5. Previous Chandra observations resolved the two nuclei and showed that they are both active and obscured by Compton-thick material. Although they cannot be resolved by NuSTAR, we were able to clearly detect, for the first time, both the primary and the reflection continuum components thanks to the unprecedented quality of the NuSTAR data at energies >10 keV. The NuSTAR hard X-ray spectrum is dominated by the primary continuum piercing through an absorbing column density which is mildly optically thick to Compton scattering (? ? 1.2, NH ~ 1.5 × 1024 cm-2). We detect moderately hard X-ray (>10 keV) flux variability up to 20% on short (15?20 ks) timescales. The amplitude of the variability is largest at ~30 keV and is likely to originate from the primary continuum of the southern nucleus. Nevertheless, the mean hard X-ray flux on longer timescales (years) is relatively constant. Moreover, the two nuclei remain Compton-thick, although we find evidence of variability in the material along the line of sight with column densities NH ? 2 × 1023 cm-2 over long (~3?15 yr) timescales. The observed X-ray emission in the NuSTAR energy range is fully consistent with the sum of the best-fit models of the spatially resolved Chandra spectra of the two nuclei.

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1510.04477v1.pdf - Accepted Manuscript
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Accepted/In Press date: 14 October 2015
e-pub ahead of print date: 14 January 2016
Published date: January 2016
Organisations: Astronomy Group

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Local EPrints ID: 399910
URI: http://eprints.soton.ac.uk/id/eprint/399910
ISSN: 0004-6361
PURE UUID: c9af1722-f9a2-4da1-9611-ba8a3d4f0d4a
ORCID for P. Gandhi: ORCID iD orcid.org/0000-0003-3105-2615

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Date deposited: 02 Sep 2016 11:29
Last modified: 17 Dec 2019 01:34

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Contributors

Author: S. Puccetti
Author: A. Comastri
Author: F.E. Bauer
Author: W.N. Brandt
Author: F. Fiore
Author: F.A. Harrison
Author: B. Luo
Author: D. Stern
Author: C.M. Urry
Author: D.M. Alexander
Author: A. Annuar
Author: P. Arévalo
Author: M. Baloković
Author: S.E. Boggs
Author: M. Brightman
Author: F.E. Christensen
Author: W.W. Craig
Author: P. Gandhi ORCID iD
Author: C.J. Hailey
Author: M.J. Koss
Author: S. La Massa
Author: A. Marinucci
Author: C. Ricci
Author: D.J. Walton
Author: L. Zappacosta
Author: W. Zhang

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