Global Oxygen ENAs sky maps from IBEX-Lo: Implication on the ENA sources

The Interstellar Boundary Explorer (IBEX) spacecraft is orbiting the Earth on a highly elliptical trajectory carrying two highly sensitive single-pixel ENA cameras: IBEX-Lo measures energetic neutral atoms (ENAs) from ~10 eV to 2 keV and IBEX-Hi measures them from ~300 eV to 6 keV. The IBEX-Lo sensor can separate two ENA species (H and O) and we completed all-sky ENA maps over more than three years. The H ENA maps include the interstellar H and He gas flow at low energies (~10 to 135 eV) as well as the ribbon and distributed heliospheric ENAs over the entire energy ranges. In contrast, our current analysis shows that all sky neutral Oxygen maps only contain the interstellar gas flow but no ribbon. There could be several potential reasons as to why the ribbon may not appear in the O ENA maps. We have limited counting statistics as well as a limited energy range. In order to improve counting statistics we overlaid the O maps for three years and we integrated over the upper energy range (~0.2 to 2.3 keV). Based on a commonly accepted value for the charge exchange rate and the neutral density of hydrogen, we estimate the expected fluxes, integrated along the line of sight, and then compare them with the measured O ENA fluxes. That comparison shows that we would have enough counting statistics to see ribbon signature. To constrain the expected energy spectrum of O ENAs from the ribbon, we assume a possible association of the ribbon with the solar wind. Recent models such as Heerikuisen et al.(2010) and Siewert et al. (2012) make this assumption. Since the H ribbon signal peaks at 1.1 keV with the above-mentioned assumption, an O ribbon would appear beyond the maximum energy limit of IBEX-Lo. However, signatures of the ribbon in the H ENA maps are observed at lower energies down to 0.2 keV. This would imply that signatures of the ribbon in the O ENA maps could be expected in the energy range of IBEX-Lo. Although, our current study does not shows the ribbon like feature in the all sky O maps of IBEX-Lo, it will constraint the location and possible production mechanism(s) of ENAs. References: Siewert, M. et al., A&A, 539, A75, 2012. Heerikhuisen, J. et al., ApJ, 708L, 126H, 2010