The Lunar Occultation Observer (loco) - a Nuclear Astrophysics All-sky Survey Mission Concept

Introduction: The Moon is a unique location for experimental astrophysics. It’s dense regolith and lack of an appreciable atmosphere are just two of the characteristics that can be brought to bare on the challenges of high-energy astrophysics. The Lunar Occultation Observer (LOCO) is a new γ-ray astrophysics mission concept expected to have unprecedented sensitivity in the nuclear regime (~0.02-10 MeV). LOCO will perform an all-sky survey of the Cosmos, and will have the capability to address multiple high-priority science goals in a scaleable and cost effective way. The astrophysics goals include, but are not limited to, Galactic nucleosynthesis, supernovae & novae, potential dark matter annihilation processes, active galactic nuclei, and compact objects. The mission can also be used to perform high-resolution , high-sensitivity orbital geochemistry of the Moon. LOCO will be a pioneering mission in high-energy astrophysics: the first to successfully employ occultation imaging as the principle detection method. Typically, instruments operating in the hard X-ray or nuclear γ-ray regime are either coded-aperture (~10-500 keV) [1] or Compton telescope (~0.5-10 MeV) imagers [2], respectively. Occultation, however, is a powerful yet relatively simple alternative approach, relevant in regimes where traditional imaging approaches are inappropriate, complex, or cost prohibitive. Placed into lunar orbit, LOCO will use the Moon to occult astrophysical sources as they rise and set along the lunar limb. The encoded temporal modulation will then be used to image the sky thereby enabling spectroscopic, time-variability, point& extended-source analyses. The benefits of this concept derive from the innovative imaging technique and the lack of a lunar atmosphere & magnetosphere. By extending the development heritage of planetary orbital-geochemistry investigations with new developments in astrophysical imaging, the LOCO concept will achieve the excellent flux sensitivity, position, and energy resolution required of the nextgeneration nuclear astrophysics mission. Preliminary modeling of its performance show it to meet or exceed the performance necessary to probe this regime. In addition, the concept is cost effective, requires a minimum of technology development, and has a relatively straightforward and scaleable implementation. Occultation Imaging: Conventional imaging techniques, such as the focusing of light with reflecting or refracting optics, are not possible in the hard X-ray or γ-ray regime due to the penetrating nature of the electromagnetic radiation. Imaging, however, has significant advantages over simple measurements of source flux including, but not limited to, the ability to spatially locate astrophysical sources, minimize the contribution of backgrounds, and reduce the effects of source confusion. In short, imaging capabilities translate directly into improved source sensitivity and science return. Occultation imaging requires astrophysical sources to be repeatedly eclipsed by a natural or artificial object, with the temporal modulation effectively encoded on the acquired data. In its simplest form, occultation image generation requires the identification of rising and setting occultation “edge”, as well as knowledge of the ephemerides of the occulting object and detector. Sources can be subsequently identified within the images and analyzed to obtain parameters such as source intensity, variability, and spectrum. A detailed descrip200 400 600 100 200 300 smooth.tmp1_1