@article{42dd7c1438194752947e8d6cf1c9a5ec,
title = "Hard X-ray emission of the luminous infrared galaxy NGC 6240 as observed by NuSTAR",
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 (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.",
author = "S. Puccetti and A. Comastri and Bauer, {F. E.} and Brandt, {W. N.} and F. Fiore and Harrison, {F. A.} and B. Luo and D. Stern and Urry, {C. M.} and Alexander, {D. M.} and A. Annuar and P. Ar{\'e}valo and M. Balokovi{\'c} and Boggs, {S. E.} and M. Brightman and Christensen, {F. E.} and Craig, {W. W.} and P. Gandhi and Hailey, {C. J.} and Koss, {M. J.} and {La Massa}, S. and A. Marinucci and C. Ricci and Walton, {D. J.} and L. Zappacosta and W. Zhang",
note = "Funding Information: This work was supported under NASA Contract NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). A.C., A.M., F.F. and L.Z. acknowledge support from the ASI/INAF grant I/037/12/0 011/13. W.N.B. acknowledges support from Caltech NuSTAR sub-contract 44A-1092750. F.E.B. and C.R. acknowledge support from CONICYTChile grants BasalCATA PFB06/2007 and FONDECYT 1141218. F.E.B., C.R. and P.A. acknowledge support from “EMBIGGEN” Anillo ACT1101. FEB acknowledges support from the Ministry of Economy, Development, and Tourism{\textquoteright}s Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS. M.B. acknowledges support from NASA Headquarters under the NASA Earth and Space Science Fellowship Program, grant NNX14AQ07H. Publisher Copyright: {\textcopyright} ESO, 2016.",
year = "2016",
month = jan,
doi = "10.1051/0004-6361/201527189",
language = "English (US)",
volume = "585",
journal = "Astronomy and Astrophysics",
issn = "0004-6361",
publisher = "EDP Sciences",
}