POEMMA-Balloon with Radio (PBR)

• POEMMA-Balloon with Radio (PBR) is a NASA-led ultra-long-duration balloon payload designed for hybrid detection of UHECRs, very-high-energy neutrinos, and high-altitude air showers.
• It integrates a Schmidt telescope, segmented mirror, and advanced optical, radio, and X-γ detectors to enable rapid, multi-messenger observations from the stratosphere.
• By validating cross-calibrated detection techniques in a near-space environment, PBR provides critical insights for future space-based observatories like the full POEMMA mission.

The POEMMA-Balloon with Radio (PBR) is a NASA-led, ultra-long-duration balloon (ULDB) payload developed by the JEM-EUSO Collaboration as the suborbital pathfinder for the dual-satellite Probe of Extreme Multi-Messenger Astrophysics (POEMMA) mission. Launched from Wanaka, New Zealand, PBR will operate at approximately 33 km altitude for up to 100 days, enabling hybrid optical and radio detection of ultra-high-energy cosmic rays (UHECRs), very-high-energy neutrinos (VHENs), and high-altitude horizontal air showers (HAHAs). The platform advances technical readiness for space-based observation by validating integrated fluorescence, Cherenkov, and radio systems in the stratospheric environment, implementing rapid tilt and repointing for target-of-opportunity (ToO) astrophysical events, and providing the first multi-messenger balloon-borne measurements of EAS development above the atmosphere (Eser et al., 4 Sep 2025, Battisti et al., 10 Sep 2024).

1. Scientific Rationale and Mission Objectives

PBR targets three primary science goals:

1. UHECR Fluorescence Measurement: Conducting the first observation of faint ultraviolet fluorescence produced by UHECR-induced air showers from suborbital altitudes using a Schmidt telescope coupled to a highly segmented MAPMT-based focal surface (FC). This validates key POEMMA techniques and improves statistics above 10²⁰ eV (Battisti et al., 27 Aug 2024, Cafagna, 20 Nov 2025).
2. High-Altitude Horizontal Air Showers (HAHAs): Detecting PeV-range cosmic rays skimming the atmosphere, PBR employs coincident optical Cherenkov (CC) and radio (RI) detection to probe shower development in the rarefied stratosphere, providing unique sensitivity to geometry, energy, and composition in the “knee” region (3–10⁶ PeV) (Scotti et al., 19 Nov 2025, Battisti, 15 Nov 2025, Mayotte et al., 7 Sep 2025).
3. VHEN and Multi-Messenger Astrophysics: Following ToO alerts (e.g., GRBs, BNS mergers), PBR rapidly repoints below the horizon to search for Earth-skimming $\nu_\tau \to \tau \to$ EAS events, using combined optical and radio triggers to extend sensitivity to cosmogenic and transient neutrino fluxes (Olinto et al., 3 Jul 2025, Reno et al., 18 Sep 2025, Scotti et al., 19 Nov 2025).

PBR directly addresses the need for increased exposure at the highest energies, hybrid instrument cross-calibration, and fast, wide-area multi-messenger follow-up, while informing both instrument design and mission operations for the full POEMMA satellite observatory (Olinto, 2023, Eser et al., 4 Sep 2025, Battisti et al., 10 Sep 2024).

2. Instrumentation Suite and Design

The PBR payload is anchored by a 1.1 m aperture f/0.75 Schmidt telescope with diamond-turned PMMA corrector plate, a 12-segment spherical primary mirror (3.81 m²), and a tiltable support frame enabling elevation from nadir to +15° above the limb (Mayotte et al., 7 Sep 2025, Wiencke et al., 17 Sep 2025).

Main Instrument Modules

• Fluorescence Camera (FC): Four PDMs, each a 6×6 MAPMT array (64 channels per MAPMT) totaling 9216 pixels, arranged on a 2×2 grid to cover a 24°×24° field. Sensitivity: 300–400 nm with BG3 filtering, 1.05 μs time resolution (GTU), ground-projected pixel size ≈115 m. Readout uses custom SPACIROC-3 ASICs for single-photon and charge-integration modes, with double-pulse resolution ≤10 ns (Battisti et al., 27 Aug 2024, Cafagna, 20 Nov 2025).
• Cherenkov Camera (CC): 2048‐pixel SiPM array (Hamamatsu S13361-3050), 0.2° per pixel, 12°×6° FoV, sensitive from 320–900 nm. Integration time 10 ns, read out with low-power, deep-buffered MIZAR ASIC supporting sub-nanosecond timing and multi-segment derandomization (Scotti et al., 19 Nov 2025, Bertaina et al., 15 Nov 2025).
• Radio Instrument (RI): Two dual-polarization broadband sinuous antennas (PUEO LF heritage), 50–500 MHz, ≈0–5 dBi gain, hemispherical pattern, boresighted to the telescope, mass ≈68 kg. Signals are digitized at ≳500 MS/s with ring buffers. Dedicated electronics implement amplitude threshold and geometric coincidence triggers plus real-time spectral excision for RFI rejection (Mayotte et al., 7 Sep 2025, Filippatos, 5 Sep 2025, Eser et al., 4 Sep 2025).
• X-γ Detector: Array of four scintillator-SiPM modules (NaI(Tl)/CsI(Tl)), covering overlapping bands from 10 keV–4 MeV, anti-coincidence veto for charged particles, 16–30° FoV, timing resolution ≲1 μs. Operates in triggered mode, aligned with the main optical axes for coincident EAS/HAHA detection (Battisti, 15 Nov 2025).
• Auxiliary Systems: IR cloud imager (40° FoV) for atmospheric monitoring, centralized data processing/DAQ with redundant CPUs and GPS-synchronized event timing (sub-μs), and an EMI-shielded “dark box” integrating all electronics (Mayotte et al., 7 Sep 2025, Scotti et al., 19 Nov 2025).

A summary of key system parameters is provided below:

Subsystem	Sensor Type	Pixels/Channels	Band/FoV	Time Res.
FC	MAPMT (M64)	9216	24°×24°, 300–400 nm	1.05 μs
CC	SiPM (S13361-3050)	2048	12°×6°, 320–900 nm	10 ns
RI	Sinuous Antenna	2 (dual-pol), up to 16+	50–500 MHz, ±30°	sub-μs
X-γ	Scint/SiPM × 4	4	16–30°, 10 keV–4 MeV	≲1 μs

3. Detection Techniques and Performance Modeling

PBR advances hybrid EAS detection by combining established fluorescence and Cherenkov imaging with broadband radio and X-ray/gamma-ray monitoring. Key methodologies:

• Fluorescence: Isotropic UV emission from N$_2$ excited by EAS ionization. Signal yield $\propto$ local $dE/dX \cdot Y(\lambda, P, T)$, where $Y$ is the pressure- and temperature-dependent fluorescence yield (Cafagna, 20 Nov 2025, Battisti et al., 27 Aug 2024).
• Cherenkov: Forward-beamed, nanosecond-scale optical emission sampled at 10 ns for imaging horizontal or upward-going EASs. Bi-focalizer optics provide robust charged-particle background rejection (Scotti et al., 19 Nov 2025).
• Radio: Coherent geomagnetic ($\propto |\mathbf{v}\times\mathbf{B}|$) and Askaryan (charge-excess) emission, with detectable electric field amplitude scaling as $E_{\rm radio}\propto E_{\rm shower}^{\alpha}$, $\alpha\sim1$. Detection requires SNR≳5–7 above galactic and thermal noise, implemented by spatial/temporal coincidence across antennas and FPGA-based beamforming (Filippatos, 5 Sep 2025, Battisti et al., 10 Sep 2024).
• X-γ: Prompt synchrotron photons from PeV–EeV e± in HAHAs, with effective area $$A_{\rm eff}(E) = A_{\rm geom}[1-e^{-\mu(E)d}]\cdot{\rm QE}(E)$$; energy resolution ≃7% at 100 keV (Battisti, 15 Nov 2025).

Simulations (using $\nu$SpaceSim, EASCherSim, EUSO-Offline):

• Energy thresholds: Fluorescence: ~1.8 EeV (nadir), Cherenkov: ≳0.5 PeV, Radio: ≃10¹⁶–10¹⁷ eV. Neutrino $\nu_\tau$-induced upgoing EAS: optimized for $10^8$–$10^{10}$ GeV (Battisti et al., 27 Aug 2024, Filippatos, 5 Sep 2025, Reno et al., 18 Sep 2025).
• Geometric aperture: At $E\sim10^8$ GeV, $A_{\rm eff}\sim10^4$ km² sr for radio/optical combined triggers (20 day flight) (Reno et al., 18 Sep 2025, Olinto et al., 3 Jul 2025).
• Event rates: UHECR: 0.18 events/hr above $1\times10^{18.5}$ eV (fluorescence) with full reconstruction, $60$ events/hr HAHAs per 30-day flight cycle, $\sim$65 EAS/hr (Cherenkov at PeV) (Battisti et al., 27 Aug 2024, Scotti et al., 19 Nov 2025). Neutrinos: sensitivity $E_\nu^2\Phi_\text{lim}\sim5\times10^{-8}$ GeV cm⁻² s⁻¹ sr⁻¹ at $E_\nu=10^8$ GeV over 20 days (Reno et al., 18 Sep 2025).

4. Mission Architecture, Pointing, and Data Acquisition

The payload is mounted on a mechanically robust, thermally insulated 6061-T6 aluminum structure. The telescope supports tilt from nadir (–90°, directly downward) to +15° above the horizon via a NEMA 23 stepper/wormgear system, giving a single-axis tilt range of 105°, with absolute pointing precision ≤0.1° (dual gravity-referenced tilt sensors) (Wiencke et al., 17 Sep 2025).

• Shutter system: Motorized, autonomously closes/open to shield the 1.1 m entrance pupil during daytime, triggered by light sensors and PLC logic derived from EUSO-SPB2 heritage (Wiencke et al., 17 Sep 2025).
• Thermal design: Multilayer Mylar, Titan-RF Faraday fabric and polyurethane foam, battery heaters, and low-outgassing epoxies. Survival from –70 °C to +30 °C validated in thermal-vacuum testing (Mayotte et al., 7 Sep 2025).
• EMI Mitigation: >60 dB Faraday cage, single-point ground, double-shielded RF cables, filtered electronic feedthroughs to maintain radio quiet (Mayotte et al., 7 Sep 2025).
• Data processing and synchronization: Centralized trigger/clock board (Xilinx XC7Z SoC) distributes a GPS-locked 1PPS, GTU clock, and supports both joint and independent trigger logic across all instruments (≤1 μs synchronization). Onboard CPUs and SSD arrays provide lossless compression and prioritized storage; telemetry uses Iridium/TDRSS with optional Starlink for bulk data (Scotti et al., 19 Nov 2025).

5. Projected Science Performance and Expected Yields

• UHECRs: Expected to reconstruct $≳100$ events above $1$ EeV for 100-day operation, with instantaneous UHECR exposure exceeding current ground arrays at the highest energies (Battisti et al., 27 Aug 2024, Eser et al., 4 Sep 2025, Battisti et al., 10 Sep 2024).
• HAHAs: Up to $\sim$9000 high-altitude horizontal air showers in a 30-day flight, offering unprecedented statistics for composition and shower development studies near the PeV “knee” (Scotti et al., 19 Nov 2025).
• VHENs: For all-flavor, 90% CL fluence, $E_\nu^2 \Phi_{\text{lim}}\sim1\!-\!2\times10^{-8}$ GeV cm⁻² s⁻¹ sr⁻¹ at $E_\nu\sim10^8$ GeV (20–50 day flight); ToO fluence sensitivity reaches $E^2 F_{\text{sens}}\sim10^{-6}$ GeV cm⁻² in 1000 s for transient events (Olinto et al., 3 Jul 2025, Reno et al., 18 Sep 2025).

PBR’s rapid repointing capability (tilt to desired angle in under a minute), coordinated trigger logic, and sub-μs timing enable optimal follow-up of gravitational wave or neutrino alerts in coordination with observatories such as LIGO, IceCube, and KM3NeT (Olinto et al., 3 Jul 2025). PBR is uniquely positioned to probe Beyond Standard Model effects invoked to reconcile anomalies such as KM3-230213A, with horizon coverage up to 2300 km chord, sensitivity to sterile-active resonance scenarios, and prompt transients (Olinto et al., 3 Jul 2025).

6. Engineering Trade-Offs, Challenges, and Heritage

• Payload mass: Science payload limited to 1361 kg to comply with NASA ULDB constraints; structural solutions (Al6061 body, segmented mirror, composite radio mast) maintain stiffness/margins under 8g launch/thermal loading (Mayotte et al., 7 Sep 2025).
• Thermal/Vacuum operation: All bearings, electronics, and adhesives validated for low-pressure, cyclic –70/+30 °C environment; TEC/heater loops for critical components (Mayotte et al., 7 Sep 2025).
• EMI Control: Multi-layer Faraday/cage design critical for RI sensitivity. External and internal EMI suppression, isolation of analog and digital domains, and board-level filtering (Mayotte et al., 7 Sep 2025).
• Trigger/data bottlenecks: >8500 analog channels with Gb/s peak data rates per instrument. Solution: deep ring buffers, real-time zero-suppression, FPGA-based event selection, and lossless compression with event prioritization (Scotti et al., 19 Nov 2025).
• Operational duty: Optical (fluorescence/Cherenkov) duty cycle ≈20–25% (night, moon avoidance), RI duty up to 80% (day/night, RFI filtered).

The design draws on direct heritage and lessons from Mini-EUSO and EUSO-SPB2 balloon missions, extending and integrating the proven multi-channel, multi-wavelength approach to support hybrid event reconstruction and cross-calibration (Olinto, 2023, Battisti et al., 27 Aug 2024, Filippatos, 5 Sep 2025, Scotti et al., 19 Nov 2025, Cafagna, 20 Nov 2025).

7. Relation to Future Missions and Comparative Context

PBR is a critical Technology Readiness Level (TRL) advancement and risk-reduction step for the POEMMA satellite mission, directly testing wide-FoV, steerable balloon-borne instrumentation, trigger/data systems, and EMI-constrained radio detection in the near-space environment (Olinto, 2023, Mayotte et al., 7 Sep 2025).

Comparison to other platforms:

Platform	Altitude (km)	Effective Area (km² sr)	Technique(s)	Duty Cycle	Coverage
POEMMA Satellites	525	∼10⁵	UV fluo, Cherenkov, radio	>80%	Full-sky
PBR (balloon)	33	∼10³–10⁴	Fluo, Cher, radio, X-γ	20–80%	S. circumpolar (100d)
Ground arrays	1.4 (Auger)	∼10³	Particle, sparse radio	90%	Site-limited

PBR demonstrates multi-messenger detection (optical, radio, X-γ) and real-time ToO astronomy from the stratosphere, maturing the detector, DAQ, and operational framework for space-based UHECR/neutrino astronomy. Its flexible, modular design and robust engineering under balloon constraints provide a direct technological path to the multi-satellite POEMMA and future global observatories (Battisti et al., 10 Sep 2024, Eser et al., 4 Sep 2025, Olinto, 2023).

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References:

(Olinto, 2023, Battisti et al., 27 Aug 2024, Battisti et al., 10 Sep 2024, Olinto et al., 3 Jul 2025, Eser et al., 4 Sep 2025, Filippatos, 5 Sep 2025, Mayotte et al., 7 Sep 2025, Wiencke et al., 17 Sep 2025, Reno et al., 18 Sep 2025, Bertaina et al., 15 Nov 2025, Battisti, 15 Nov 2025, Scotti et al., 19 Nov 2025, Scotti et al., 19 Nov 2025, Cafagna, 20 Nov 2025)