The EUSO-SPB2 fluorescence telescope for the detection of Ultra-High Energy Cosmic Rays

Abstract

The Extreme Universe Space Observatory on a Super Pressure Balloon 2 (EUSO-SPB2) flew on May 13th and 14th of 2023. Consisting of two novel optical telescopes, the payload utilized next-generation instrumentation for the observations of extensive air showers from near space. One instrument, the fluorescence telescope (FT) searched for Ultra-High Energy Cosmic Rays (UHECRs) by recording the atmosphere below the balloon in the near-UV with a $1\mu s$ time resolution using 108 multi-anode photomultiplier tubes with a total of 6912 channels. Validated by pre-flight measurements during a field campaign, the energy threshold was estimated around 2 EeV with an expected event rate of approximately 1 event per 10 h of observation. Based on the limited time afloat, the expected number of UHECR observations throughout the flight is between 0 and 2. Consistent with this expectation, no UHECR candidate events have been found. The majority of events appear to be detector artifacts that were not rejected properly due to a shortened commissioning phase. Despite the earlier-than-expected termination of the flight, data were recorded which provide insights into the detectors stability in the near-space environment as well as the diffuse ultraviolet emissivity of the atmosphere, both of which are impactful to future experiments.