POEMMA-Balloon with Radio: A multi-messenger, multi-detector balloon payload

A review of the current status of the field of Ultra-High-Energy Cosmic Ray (UHECR) including a summary of remaining open questions was presented in the white paper “Ultra-High Energy Cosmic Rays: at the Intersection of the Cosmic and Energy Frontiers” (Astropart. Phys. 147 (2023) 102794; arXiv:2205.05845). The authors concluded that two types of next-generation detectors are needed to answer these questions: high-accuracy instruments and detectors that maximize exposure at the highest energies. The Probe Of Extreme Multi-Messenger Astrophysics (POEMMA), a proposed dual-satellite observatory, exemplifies the latter class and is designed to increase statistics of the highest-energy cosmic rays and to detect very-high-energy neutrinos following multi-messenger alerts. POEMMA-Balloon with Radio (PBR) implements a compact, balloon-borne version of the POEMMA concept, adapted for a Super-Pressure Balloon flight from Wanaka, New Zealand, with an expected campaign exceeding 20 days. PBR couples a wide field-of-view Schmidt telescope and a hybrid optical focal surface with a dedicated radio instrument to deliver simultaneous, complementary measurements of extensive air showers. The mission will validate the fluorescence detection strategy from space and raise technology readiness for a POEMMA-like space mission by observing UHECR-induced fluorescence light from suborbital altitudes, obtaining the first simultaneous optical Cherenkov and radio observations of high-altitude horizontal air showers above the cosmic-ray knee (E>3PeV), enabling energy-spectrum and composition studies at the PeV scale, and performing follow-ups of multi-messenger alerts to search for very-high-energy neutrinos via upward-going air showers. This paper summarizes the PBR payload and its expected performance.

💡 Research Summary

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The paper presents POEMMA‑Balloon with Radio (PBR), a compact, balloon‑borne implementation of the POEMMA concept designed to serve as a technology‑demonstration and science platform for ultra‑high‑energy cosmic‑ray (UHECR) and very‑high‑energy neutrino research. Building on the roadmap that calls for two complementary next‑generation detector classes—high‑precision instruments and exposure‑maximizing observatories—the authors propose a super‑pressure balloon (SPB) flight from Wanaka, New Zealand, at a nominal altitude of ~33 km for a campaign exceeding 20 days.

The payload integrates a wide‑field Schmidt telescope (≈2.5 m aperture, 45° field of view) with a hybrid focal plane that houses a Fluorescence Camera (9216‑pixel multi‑anode PMT, 1 µs sampling) and a Cherenkov Camera (2048‑pixel SiPM, nanosecond sampling). A dedicated radio instrument covering 60–600 MHz provides an independent measurement channel, while an X/γ detector and an infrared camera add auxiliary monitoring capabilities. The gondola is built from lightweight aluminum‑carbon composites, equipped with a three‑axis gimbal that can point from nadir to the horizon, thereby increasing geometric exposure to the highest‑energy events. Power is supplied by high‑efficiency solar panels and lithium‑ion batteries; on‑board FPGA‑based processing performs real‑time triggering, data compression, and downlink to ground stations.

Three primary science goals drive the mission.

1. UHECR Fluorescence Observation (SG1). By detecting UV fluorescence from extensive air showers (EAS) initiated by cosmic rays above 1 EeV at sub‑orbital altitude, PBR will validate the fluorescence detection strategy planned for space‑based POEMMA. Simulations indicate a detection rate of roughly one UHECR event per 10 hours of observation, a tenfold improvement over previous SPB flights, thanks to enhanced optics, longer flight duration, and flexible pointing. This will raise the technology readiness level (TRL) of critical subsystems to 6–7.

2. High‑Altitude Horizontal Air Showers (HAHA) (SG2). HAHA are EAS that skim the atmosphere at near‑horizontal angles, developing above 20 km where the air is thin. At these altitudes the Cherenkov angle shrinks below 0.1°, and the electron energy threshold for Cherenkov emission rises above 200 MeV, making the early shower development highly sensitive to primary composition. Simulations with CORSIKA show that 3 PeV proton‑ and iron‑induced showers differ by >30 % in on‑axis Cherenkov intensity at a slant depth of ~400 g cm⁻². The radio channel, free from ionospheric dispersion at these altitudes, provides a complementary measurement of the geomagnetic emission. Over a 20‑day flight PBR is expected to collect ≥10³ HAHA events, enabling a composition measurement around the knee (≈2 PeV) with systematic uncertainties below 20 %.

3. Neutrino Searches from Targets of Opportunity (SG3). By rapidly repointing in response to multimessenger alerts (gravitational waves, gamma‑ray bursts, etc.), PBR will search for upward‑going air showers produced by Earth‑skimming ντ interactions that generate τ leptons decaying in the atmosphere. The combined optical and radio trigger dramatically reduces background, improving sensitivity to ντ fluxes above ~10 PeV relative to ground‑based radio arrays.

The paper details the instrument design, including mechanical structure, pointing system, data‑processing pipeline, and auxiliary subsystems. Performance estimates are derived from end‑to‑end Monte Carlo simulations, showing an energy reconstruction accuracy of ~15 % for UHECRs, a Cherenkov‑based composition discrimination accuracy of ~20 % at PeV energies, and a combined optical‑radio trigger efficiency of ~70 % for HAHA.

In summary, PBR offers a low‑cost, low‑risk pathfinder that simultaneously validates space‑based fluorescence detection, pioneers optical‑Cherenkov plus radio measurements of high‑altitude horizontal showers, and demonstrates rapid multimessenger neutrino follow‑up. Successful execution will raise the TRL of key technologies, inform the design of the full POEMMA satellite, and deliver valuable scientific data on the spectrum, composition, and sources of the highest‑energy particles in the universe.