PKS 2131-021 is a blazar, a type of active galactic nucleus whose relativistic jet points in the direction toward the Earth. Its redshift is 1.285. Observations of its radio emission spanning a 45-year duration show epochs of periodic brightness variations. These nearly sinusoidal brightness changes have been interpreted as evidence of orbital motion of a binary black hole. [1] The orbital separation of the two black holes is inferred to be 200 to 2000 AU. [2] [3] The periodic variability in the light curve indicates that the pair orbit each other about every two years, at a distance so close that they will merge in about 10,000 years (as viewed from the Earth). [4]