Anomalous Flux in the Cosmic Optical Background Detected With New Horizons Observations

Understand

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.