Anomalous Flux in the Cosmic Optical Background Detected With New
  Horizons Observations
  We used New Horizons LORRI images to measure the optical-band
($0.4\lesssim\lambda\lesssim0.9{\rm\mu m}$) sky brightness within a high
galactic-latitude field selected to have reduced diffuse scattered light from
the Milky Way galaxy (DGL), as inferred from the IRIS all-sky $100~\mu$m map.
We also selected the field to significantly reduce the scattered light from
bright stars (SSL) outside the LORRI field. Suppression of DGL and SSL reduced
the large uncertainties in the background flux levels present in our earlier
New Horizons COB results. The raw total sky level, measured when New Horizons
was 51.3 AU from the Sun, is $24.22\pm0.80{\rm ~nW ~m^{-2} ~sr^{-1}}.$
Isolating the COB contribution to the raw total required subtracting scattered
light from bright stars and galaxies, faint stars below the photometric
detection-limit within the field, and the hydrogen plus ionized-helium
two-photon continua. This yielded a highly significant detection of the COB at
${\rm 16.37\pm 1.47 ~nW ~m^{-2} ~sr^{-1}}$ at the LORRI pivot wavelength of
0.608 $\mu$m. This result is in strong tension with the hypothesis that the COB
only comprises the integrated light of external galaxies (IGL) presently known
from deep HST counts. Subtraction of the estimated IGL flux from the total COB
level leaves a flux component of unknown origin at ${\rm 8.06\pm1.92 ~nW
~m^{-2} ~sr^{-1}}.$ Its amplitude is equal to the IGL.
