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

We present a broad–band (∼0.3–70 keV) spectral and temporal analysis of NuSTAR observations of the luminous infrared galaxy NGC6240, combined with archival Chandra, XMM–Newton and BeppoSAX data. NGC 6240 is a galaxy in a relatively early merger statewith two distinct nuclei separated by ∼1′.′5. Previous Chandra observations have resolved the two nuclei, showing that they are both active and obscured by Compton–thick material. Although they cannot be resolved by NuSTAR, thanks to the unprecedented quality of the NuSTAR data at energies >10 keV, we clearly detect, for the first time, both the primary and the reflection continuum components. 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 moderate hard X–ray (> 10 keV) flux variability up to 20% on short (15 − 20 ksec) timescales. The amplitude of the variability is maximum 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 of the material along the line of sight with column densities NH≤2×1023 cm−2 over long (∼3–15 years) 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.