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

Abstract

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 (AGN). Over a period of about two years, the source exhibits hard X-ray variability of the order of 20 per cent, peaking around 20 keV. When the two AGN are resolved with Chandra, column densities in the range $N_{\text{H}}\sim 1-2\times 10^{24}$ 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 AGN 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 AGN, 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.

12 pages, 9 figures, 4 tables. Accepted for publication on MNRAS

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• Compton-thick AGN in the NuSTAR era III: A systematic study of the torus covering factor
• MICONIC: Dual active galactic nuclei, star formation, and ionised gas outflows in NGC 6240 seen with MIRI/JWST
• Revisiting the complex nuclear region of NGC 6240 with Chandra
• A 6.4yr optical quasi-periodic oscillations in SDSS J075217.84+193542.2: a new candidate for central binary black hole system
• NGC 6240 Supermassive Black Hole Binary dynamical evolution based on Chandra data
• A 3.8yr optical quasi-periodic oscillations in blue quasar SDSS J132144+033055 through combined light curves from CSS and ZTF
• A candidate of binary black hole system in AGN with broad Balmer emission lines having quite different line widths
• Dissecting the Extended X-ray Emission in the Merging Pair NGC 6240
• Detection of the Fe K lines from the binary AGN in 4C+37.11