Detection of Above the Limb Cosmic Rays in the Optical Cherenkov Regime Using Sub-Orbital and Orbital Instruments
July 08, 2021
Many upcoming experiments seek to observe high energy cosmic events while observing from either sub-orbital or orbital altitudes, using the Earth atmosphere as an extremely large instrumental volume, thereby increasing the geometric acceptance of ground-based instruments in addition to supplying uniform exposure in both hemispheres. In particular, the planned Extreme Universe Space Observatory aboard a Super Pressure Balloon-2 (EUSO-SPB2) and future Probe Of Extreme Multi-Messenger Astrophysics (POEMMA) experiments will both utilize UV sensitive optical cameras with nanosecond time resolution and observe near the Earth’s limb to attempt to capture the Cherenkov emission produced by upwards going extensive air showers (EASs) sourced from high energy cosmic neutrino interactions in the Earth. In addition, these Cherenkov cameras also have coverage above the Earth limb, allowing access to the Cherenkov emission produced by cosmic rays skimming the Earth atmosphere at high altitudes.
We show that sub-orbital and orbital based optical Cherenkov telescopes are quite sensitive to the above the limb cosmic ray signal, in particular the sub-orbital case being sensitive down to PeV scale energies, allowing for high event rates even for short observation periods. We argue that because the properties of the arriving Cherenkov photons are similar for neutrino induced EASs and those induced by above the limb cosmic rays, the latter provides an excellent benchmark for qualifying the technique for neutrino observation with a well understood and guaranteed signal.
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