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

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.; Arevalo, P.; Balokovic, 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.; Zhang, W.

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


S. Puccetti

A. Comastri

F.E. Bauer

W.N. Brandt

F. Fiore

F.A. Harrison

B. Luo

D. Stern

C.M. Urry

A. Annuar

P. Arevalo

M. Balokovic

S.E. Boggs

M. Brightman

F.E. Christensen

W.W. Craig

P. Gandhi

C.J. Hailey

M.J. Koss

S. La Massa

A. Marinucci

C. Ricci

D.J. Walton

L. Zappacosta

W. Zhang


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 (τ ≃ 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.


Puccetti, S., Comastri, A., Bauer, F., Brandt, W., Fiore, F., Harrison, F., …Zhang, W. (2016). Hard X-ray emission of the luminous infrared galaxy NGC 6240 as observed by NuSTAR. Astronomy & Astrophysics, 585, Article A157.

Journal Article Type Article
Acceptance Date Oct 14, 2015
Online Publication Date Jan 14, 2016
Publication Date Jan 14, 2016
Deposit Date May 11, 2016
Publicly Available Date May 12, 2016
Journal Astronomy and astrophysics.
Print ISSN 0004-6361
Electronic ISSN 1432-0746
Publisher EDP Sciences
Peer Reviewed Peer Reviewed
Volume 585
Article Number A157
Public URL


Published Journal Article (1.6 Mb)

Copyright Statement
Reproduced with permission from Astronomy & Astrophysics, © ESO 2016

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