Gravitational self-lensing of the supermassive black hole binary in NGC1566

It is generally accepted that all massive galaxies host supermassive black holes in their center 1 and that mergers of two galaxies lead to the formation of black hole binaries 2 . The most interesting among them comprise the mergers in their final state, i.e. with pc (3.2 light years) or sub-pc orbital separations. Binary self-lensing is a possibility to detect these systems. Here we report the detection of a central supermassive black hole binary in the active galaxy (AGN) NGC1566 based on a microlensing outburst. The light curve of the outburst - based on observations with the All Sky Automated Survey for SuperNovae - lasted from the beginning of the year 2017 until the beginning of 2020. The shape of the symmetric light curve looks very different with respect to normal random variations of AGN 3,4 . However, it could be easily reproduced with a best-fit standard microlensing light curve 5 . Based on the light curve we derive a characteristic time-scale of 155 days. During the outburst, the continuum as well as the broad line intensities varied - however the narrow emission lines did not. This is an indication that the lensing object orbits the AGN nucleus between the broad line region and the narrow line region, i.e. at a distance on the order of 250 light days. The observed characteristic time-scale depends on the lens mass, the distances between lens and source, and the transverse velocity. The light curve can be reproduced by a lens with a black hole (BH) mass of 5x10 5 M ⊙ . This implies a mass ratio to the central AGN on the order of 1 to 10.