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We present a broad-band (~0.3-70 keV) spectral and temporal analysis ofNuSTAR observations of the luminous infrared galaxy NGC 6240, combined witharchival Chandra, XMM-Newton and BeppoSAX data. NGC 6240 is a galaxy in arelatively early merger state with two distinct nuclei separated by ~1."5.Previous Chandra observations have resolved the two nuclei, showing that theyare both active and obscured by Compton-thick material. Although they cannot beresolved by NuSTAR, thanks to the unprecedented quality of the NuSTAR data atenergies >10 keV, we clearly detect, for the first time, both the primary andthe reflection continuum components. The NuSTAR hard X-ray spectrum isdominated by the primary continuum piercing through an absorbing column densitywhich is mildly optically thick to Compton scattering (tau ~ 1.2, N_H ~ 1.5 x10^(24) cm^-2). We detect moderate hard X-ray (> 10 keV) flux variability up to20% on short (15-20 ksec) timescales. The amplitude of the variability ismaximum at ~30 keV and is likely to originate from the primary continuum of thesouthern 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 of the material along the line ofsight with column densities N_H <~ 2 x 10^(23) cm-2 over long (~3-15 years)timescales. The observed X-ray emission in the NuSTAR energy range is fullyconsistent with the sum of the best-fit models of the spatially resolvedChandra spectra of the two nuclei.

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