Analysis of above-the-limb Cosmic Rays for EUSO-SPB2

TL;DR

This paper evaluates EUSO-SPB2's Cherenkov Telescope's ability to detect above-the-limb cosmic rays, using simulations and preliminary flight data to estimate event rates and improve detection models for future space-based cosmic ray observations.

Contribution

It provides updated simulation estimates of cosmic ray detection rates for EUSO-SPB2's Cherenkov Telescope, incorporating atmospheric and magnetic field effects, and compares them with initial flight data.

Findings

Estimated event rates for above-the-limb cosmic rays.

Impact of magnetic field modeling on detection thresholds.

Preliminary flight data aligns with simulation predictions.

Abstract

The Extreme Universe Space Observatory on a Super Pressure Balloon 2 (EUSO-SPB2) experiment is a pathfinder mission for future space-based instruments targeting the fluxes of Ultra-High Energy Cosmic Rays (UHECR), with energies exceeding 1EeV and very high energy diffuse and transient neutrinos, with energies exceeding 1PeV. Using two telescope designs: the Fluorescence Telescope (FT) and the Cherenkov Telescope (CT), EUSO-SPB2 made novel observations of the backgrounds relevant for space-based detection. EUSO-SPB2 will launch from Wanaka, NZ in Spring of 2023, for a long duration (up to 100d) flight at a nominal float altitude of 33km. In this contribution, we will focus on the CT's capability to measure cosmic rays from above Earth's limb via the Cherenkov emission produced by the resultant Extensive Air Showers (EAS). Using the EASCherSim optical Cherenkov generation code, we provide an updated estimate of the event rate of above-the-limb cosmic rays for the CT, taking into account updated values for the trigger efficiency as determined during the field testing of the instrument. We take particular care to consider the longitudinal development of EAS in rarefied atmosphere, accounting for the energy dependent elongation rate. In addition, we consider improvements to the magnetic field modeling present in EASCherSim and illustrate their impact on the observed events and detection thresholds. Finally, we compare these simulations to preliminary flight data from EUSO-SPB2.