Discovery of a binary active galactic nucleus in the ultraluminous infrared galaxy NGC 6240 using Chandra

Ultraluminous infrared galaxies (ULIRGs) are outstanding due to their huge luminosity output in the infrared, which is predominantly powered by superstarbursts and/or hidden active galactic nuclei (AGNs). NGC 6240 is one of the nearest ULIRGs and is considered a key representative of its class. Here we report the first highresolution imaging spectroscopy of NGC 6240 in X-rays. The observation, performed with the ACIS-S detector aboard the Chandra X-Ray Observatory, led to the discovery of two hard nuclei, coincident with the optical-IR nuclei of NGC 6240. The AGN character of both nuclei is revealed by the detection of absorbed, hard, luminous X-ray emission and two strong neutral Fe Kalpha lines. In addition, extended X-ray emission components are present, changing their rich structure in dependence of energy. The close correlation of the extended emission with the optical H emission of NGC 6240, in combination with the softness of its spectrum, clearly indicates its relation to starburst-driven superwind activity.