Nuclear absorption and emission in the AGN merger NGC 6240 : the hard X-ray view

We present the analysis of four NuSTAR observations of the luminous infrared galaxy merger NGC 6240 , hosting a close pair of highly obscured active galactic nuclei (AGNs). Over a period of about 2 yr, the source exhibits hard X-ray variability of the order of 20 per cent, peaking around 20 keV. When the two AGNs are resolved with Chandra , column densities in the range of N_H ∼ 1-2 × 10²⁴ cm^-2 are estimated for both of them. The exact values are hard to determine, as they appear to depend on aspects that are sometimes overlooked in Compton-thick objects, such as the covering factor of the absorber, iron abundance and the contamination in the Fe K band from foreground hot-gas emission. Nearly spherical covering and slightly subsolar iron abundance are preferred in this case. While the southern nucleus is suggested to be intrinsically more powerful, as also implied by the mid-IR and 2-10 keV brightness ratios, solutions involving a similar X-ray luminosity of the two AGNs cannot be ruled out. The observed variability is rather limited compared to the one revealed by the Swift /BAT light curve, and it can be fully explained by changes in the continuum flux from the two AGNs, without requiring significant column density variations. NGC 6240 is hereby confirmed to represent a unique opportunity to investigate the X-ray (and broad-band) properties of massive galaxy mergers, which were much more frequent in the early Universe.