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
  Supermassive black hole (SMBH) binaries with masses of ~10^8--10^9 Msun are
expected to dominate the contribution to the as-yet undetected gravitational
wave background (GWB) signal at the nanohertz frequencies accessible to Pulsar
Timing Arrays (PTA). We currently lack firm empirical constraints on the
amplitude of the GWB due to the dearth of confirmed SMBH binaries in the
required mass range. Using HST/WFC3 images, we have discovered a z~0.2 quasar
hosted 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 are
spatially coincident with two powerful [OIII]-emitting point sources with
quasar-like luminosities (L_AGN ~ 5x10^46 erg/s, suggesting the presence of a
bound SMBH system, each with M_BH > 4x10^8 Msun. We place an upper limit on the
merging timescale of the SMBH pair of 2.5 billion years, roughly the Universe
lookback time at z~0.2. There is likely a population of quasar binaries similar
to SDSSJ1010+1413 that contribute to a stochastic GWB that should be detected
in the next several years. If the GWB is not detected this could indicate that
SMBHs merge only over extremely long timescales, remaining as close separation
binaries for many Hubble times, the so-called `final-parsec problem'.
