OJ 287

OJ 287 is a BL Lac object located 3.5 billion light-years away that has produced quasi-periodic optical outbursts going back approximately 120 years, as first apparent on photographic plates from 1891. Seen on photographic plates since at least 1887, it was first detected at radio wavelengths during the course of the Ohio Sky Survey. It is a supermassive black hole binary. The intrinsic brightness of the flashes corresponds to over a trillion times the Sun's luminosity, greater than the entire Milky Way galaxy's light output.
Characteristics.
Its central supermassive black hole is among the largest known, with a mass of 18.35 billion solar masses, more than six times the value calculated for the previous largest object. Its Schwarzschild radius is ~362 AU, about 12 and 0.75 times the semimajor axes of the orbits of Neptune and dwarf planet Sedna, respectively.
The optical light curve shows that OJ 287 has a periodic variation of 11–12 years with a narrow double peak at maximum brightness. This kind of variation suggests that it is a binary supermassive black hole. The double-burst variability is thought to result from the smaller black hole punching through the accretion disc of the larger black hole twice in every 12 years.
The smaller supermassive black hole with a mass of "only" 150 million orbits the larger one with an observed orbital period of ~12 years and a calculated eccentricity of ~0.65. The maximum brightness is obtained when the minor component moves through the accretion disk of the supermassive component at perinigricon. The perinigricon and aponigricon of its orbit are ~3,250 and ~17,500 AU, or about 9 and 48 times the primary's Schwarzschild radius; the latter is also ~0.275 light-year and ~0.085 parsec.
The mass was calculated in 2008 by a team led by Mauri Valtonen of Tuorla Observatory in Finland. The timing of these outbursts allows the precession of the companion's elliptical orbit to be measured (39° per orbit), which allows the mass of the central black hole to be calculated using Einstein's principles of general relativity (see Kepler problem in general relativity). The timings also provide a test of the black hole no-hair theorem, which so far is consistent with the results.
In order to reproduce all the known outbursts, the rotation of the primary black hole has to be 38% of the maximum allowed rotation for a Kerr black hole.
The companion's orbit is decaying via the emission of gravitational radiation and it is expected to merge with the central black hole within approximately 10,000 years.