2018MNRAS.479.3804S

Dual/binary supermassive black hole (SMBH) systems are the inevitable consequence of the current Lambda cold dark matter cosmological paradigm. In this context, we discuss here the properties of MCG+11-11-032, a local (z = 0.0362) Seyfert 2 galaxy. This source was proposed as a dual active galactic nucleus candidate on the basis of the presence of double-peaked [O III] emission lines in its optical spectrum. MCG+11-11-032 is also an X-ray variable source and was observed several times by the Swift X-ray Telescope (XRT) on time scales from days to years. In this work, we analyze the SDSS-DR13 spectrum and find evidence for double-peaked profiles in all the strongest narrow emission lines. We also study the XRT light curve and unveil the presence of an alternating behavior of the intrinsic 0.3-10 keV flux, while the 123-month Swift BAT light curve supports the presence of almost regular peaks and dips almost every 25 months. In addition, the XRT spectrum suggests for the presence of two narrow emission lines with rest-frame energies of E ∼ 6.16 keV and E ∼ 6.56 keV. Although by considering only the optical emission lines, different physical mechanisms may be invoked to explain the kinematical properties, the X-ray results are most naturally explained by the presence of a binary SMBH in the center of this source. In particular, we evidence a remarkable agreement between the putative SMBH pair orbital velocity derived from the BAT light curve and the velocity offset derived by the rest-frame ΔE between the two X-ray line peaks in the XRT spectrum (i.e. Δv ∼ 0.06c).