tips us to a BBC article on the quasar OJ287, a binary object containing largest black hole yet discovered
, weighing in at 18 billion times the mass of Sol. Researchers were able to estimate its mass due to the presence of a smaller black hole in orbit around it. When the smaller companion's orbit intersects OJ287's accretion disk, once every 12 years, it triggers a burst of radiation that was detected by the Spitzer Space Telescope. More detail and a diagram
are available on the Turku University site.