The EUSO-SPB2 Fluorescence Telescope for the Detection of Ultra-High Energy Cosmic Rays

TL;DR

The EUSO-SPB2 fluorescence telescope, flown on a balloon in 2023, aimed to detect ultra-high energy cosmic rays from near space, providing valuable data on detector stability and atmospheric UV emissivity despite limited observations.

Contribution

This paper reports on the design, deployment, and initial results of the first near-space fluorescence telescope for UHECR detection, highlighting its instrumentation and operational insights.

Findings

No UHECR events detected during the flight.

The detector demonstrated stability in the near-space environment.

Data provided insights into atmospheric UV emissivity.

Abstract

The Extreme Universe Space Observatory on a Super Pressure Balloon 2 (EUSO-SPB2) flew on May 13$^{\text{th}}$ and 14$^{\text{th}}$ 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 6,912 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 hours 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.