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

Puccetti, S. and Comastri, A. and Bauer, F. E. and Brandt, W. N. and Fiore, F. and Harrison, F. A. and Luo, B. and Stern, D. and Urry, C. M. and Alexander, D. M. and Annuar, A. and Arevalo, P. and Balokovic, M. and Boggs, S. E. and Brightman, M. and Christensen, F. E. and Craig, W. W. and Gandhi, P. and Hailey, C. J. and Koss, M. J. and La Massa, S. and Marinucci, A. and Ricci, C. and Walton, D. J. and 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.


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.

Item Type:Article
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Publisher statement:Reproduced with permission from Astronomy & Astrophysics, © ESO 2016
Date accepted:14 October 2015
Date deposited:12 May 2016
Date of first online publication:14 January 2016
Date first made open access:12 May 2016

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