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Supermassive black hole (SMBH) binaries with masses of ~10^8--10^9 Msun areexpected to dominate the contribution to the as-yet undetected gravitationalwave background (GWB) signal at the nanohertz frequencies accessible to PulsarTiming Arrays (PTA). We currently lack firm empirical constraints on theamplitude of the GWB due to the dearth of confirmed SMBH binaries in therequired mass range. Using HST/WFC3 images, we have discovered a z~0.2 quasarhosted in a merger remnant with two closely separated (0.13'' or ~430pc)continuum cores at the heart of the galaxy SDSSJ1010+1413. The two cores arespatially coincident with two powerful [OIII]-emitting point sources withquasar-like luminosities (L_AGN ~ 5x10^46 erg/s, suggesting the presence of abound SMBH system, each with M_BH > 4x10^8 Msun. We place an upper limit on themerging timescale of the SMBH pair of 2.5 billion years, roughly the Universelookback time at z~0.2. There is likely a population of quasar binaries similarto SDSSJ1010+1413 that contribute to a stochastic GWB that should be detectedin the next several years. If the GWB is not detected this could indicate thatSMBHs merge only over extremely long timescales, remaining as close separationbinaries for many Hubble times, the so-called `final-parsec problem'.

Discovery of a close-separation binary quasar at the heart of a z~0.2 merging galaxy and its implications for low-frequency gravitational waves