Heron-101: Hybrid Neutrino Observatory

• Heron-101 is a guided exposition of the HERON concept, explaining an ultrahigh-energy tau neutrino detector that uses Earth-skimming interactions.
• It combines compact phased radio stations with a widely spaced sparse antenna array to achieve sub-degree pointing and expansive sky coverage.
• A comprehensive simulation chain—from tau propagation to radio signal reconstruction—enables real-time transient detection and improved sensitivity.

In current arXiv usage, “Heron-101” appears as the label of guided technical tours of the Hybrid Elevated Radio Observatory for Neutrinos (HERON), a proposed ultrahigh-energy Earth-skimming tau-neutrino detector. HERON is designed to observe the radio emission from up-going extensive air showers initiated when a $\tau$ lepton, produced by a shallow-angle $\nu_\tau$ interaction in the Earth, escapes into the atmosphere and decays in flight. The observatory concept combines compact phased radio stations with a larger sparse antenna array deployed along a mountain ridge, with the stated goals of sub-degree pointing, wide daily sky coverage, and sensitivity to astrophysical transients above roughly the $10^{16}$–$10^{17}$ eV scale (Zeolla, 12 May 2026, Kotera et al., 6 Jul 2025).

1. Scientific scope and design objectives

HERON is situated within the effort to measure ultra-high-energy neutrinos, identified as the “next frontier of the emerging multi-messenger era.” The stated scientific motivation is to measure the flux and arrival directions of neutrinos with energies above $10^{16}$ eV, test models of cosmic-ray sources such as $\gamma$-ray bursts, active galactic nuclei, and magnetars, and join the broader network of $\gamma$-ray and gravitational-wave observatories (Kotera et al., 6 Jul 2025).

The performance targets are correspondingly explicit. The next leap in sensitivity is described as requiring an instantaneous sensitivity $10\times$ better than current instruments, angular resolution $\Delta\theta\lesssim1^\circ$, and a wide daily field of view $\Omega_{\rm daily}\gtrsim2\ {\rm sr}$. HERON is proposed as a detector optimized for that regime, especially near $\nu_\tau$0 PeV, where the neutrino flux “should be high,” and for transient searches rather than only diffuse-flux accumulation (Kotera et al., 6 Jul 2025).

The observatory is also framed as a hybridization of two radio-detection lineages. One paper states that HERON “combines the complementary features of two radio techniques being demonstrated by the BEACON and GRAND prototypes,” while another describes it as “multiple compact, phased radio arrays embedded within a larger sparse array of antennas, located on the side of a mountain” (Kotera et al., 6 Jul 2025, Zeolla et al., 18 Sep 2025). This architectural choice underlies most of the concept’s claimed advantages: low-threshold triggering from beamforming, large geometric area from elevated horizon viewing, and accurate event reconstruction from sparse-array imaging.

2. Earth-skimming $\nu_\tau$1 detection and radio phenomenology

The central detection channel is the Earth-skimming $\nu_\tau$2 technique. Ultrahigh-energy tau neutrinos with $\nu_\tau$3 PeV enter the Earth at very shallow angles just below the horizon, undergo charged-current interactions in rock, and may generate an escaping $\nu_\tau$4 lepton. One formulation gives the exit probability as

$\nu_\tau$5

where $\nu_\tau$6 is the neutrino interaction length in rock, $\nu_\tau$7 the column depth, $\nu_\tau$8 the chord length through the Earth, and $\nu_\tau$9 the probability that the emerging $10^{16}$0 does not lose all its energy before exit (Zeolla, 12 May 2026).

The emerging lepton carries a substantial fraction of the neutrino energy, with one study quoting $10^{16}$1–$10^{16}$2 (Zeolla, 12 May 2026). A complementary simulation-oriented description emphasizes the long $10^{16}$3 range in rock, “tens of kilometers at 100 PeV,” together with low atmospheric overburden once the particle exits, making the resulting up-going extensive air shower directly observable from a mountainside array (Zeolla et al., 18 Sep 2025).

The radio pulse is produced primarily by the geomagnetic mechanism in air, with an Askaryan or charge-excess contribution also included in full simulations. One paper writes the peak electric field schematically as

$10^{16}$4

with $10^{16}$5, $10^{16}$6 the angle between the shower axis and the geomagnetic field, and $10^{16}$7 encoding coherence and Cherenkov-ring geometry. Since the amplitude scales linearly with shower size, the radiated power obeys the proportionality $10^{16}$8 (Zeolla, 12 May 2026).

Several signal characteristics recur across the HERON papers. The emission is described as impulsive, with durations of $10^{16}$9–$10^{17}$0 ns or $10^{17}$1 in simplified time-domain models, broadband across tens to hundreds of MHz, and strongly forward-beamed into a Cherenkov ring of angular width $10^{17}$2 (Zeolla, 12 May 2026, Zeolla et al., 18 Sep 2025). A common misconception in radio-neutrino discussions is that the emission in air is purely Askaryan; the HERON material explicitly states that in air the geomagnetic mechanism dominates, although both effects are modeled (Kotera et al., 6 Jul 2025).

3. Observatory architecture and mountain-side deployment

HERON adopts a two-tier radio array along elevated terrain. The baseline concept consists of 24 compact phased stations and 360 standalone antennas distributed along a mountain ridge (Zeolla, 12 May 2026, Kotera et al., 6 Jul 2025). The site identified in the preliminary design is in San Juan Province, Argentina, “across a 72 km-long mountain range overlooking a valley,” with ridge altitudes of roughly $10^{17}$3–$10^{17}$4 m and low measured radio-frequency interference in the $10^{17}$5–$10^{17}$6 MHz band (Kotera et al., 6 Jul 2025).

The phased component is optimized for threshold reduction and triggering. In one description, each of the 24 stations hosts 24 antennas in a compact $10^{17}$7 m footprint and forms $10^{17}$8–$10^{17}$9 simultaneous digital beams, maintaining an overall trigger rate of $10^{16}$0 Hz per station (Zeolla, 12 May 2026). Another design study gives the station footprint less as a fixed baseline than as a phasing constraint: antennas must lie within $10^{16}$1 m to preserve phasing efficiency $10^{16}$2, which in turn enables $10^{16}$3 antennas per station “given FPGA limits” (Zeolla et al., 18 Sep 2025). The same literature states that beamforming gain is proportional to $10^{16}$4 and raises the SNR by $10^{16}$5, lowering the energy threshold to $10^{16}$6 eV or, in another estimate, to $10^{16}$7 eV (Kotera et al., 6 Jul 2025).

The sparse component is optimized for imaging and reconstruction. The full-concept paper describes “360 standalone dual-polarization antennas spaced $10^{16}$8 m apart extending several tens of kilometers along the ridge,” while the preliminary-design paper places 15 standalone antennas between each pair of phased stations and notes altitude coverage from $10^{16}$9 to $\gamma$0 m (Zeolla, 12 May 2026, Kotera et al., 6 Jul 2025). Optimization studies also examine a triangular sparse sub-array of 65 antennas with $\gamma$1 km spacing spanning $\gamma$2 km across the mountain side, using simulated interferometry to assess directional reconstruction (Zeolla et al., 18 Sep 2025).

The topography is not incidental. Elevation of $\gamma$3–$\gamma$4 km is said to raise the horizon baseline and produce a geometric area in view of $\gamma$5 per station toward the horizon, while the “encased valley topology” is reported to double the effective area relative to an idealized flat-Earth geometry (Zeolla, 12 May 2026, Kotera et al., 6 Jul 2025). One optimization result further states that choosing $\gamma$6 km, rather than $\gamma$7 km as in BEACON, doubles $\gamma$8 at $\gamma$9 eV by better balancing geometric area and signal strength (Zeolla et al., 18 Sep 2025).

4. Simulation chain and mathematical framework

HERON design studies are based on an end-to-end simulation chain spanning $\gamma$0 propagation, shower development, radio emission, and antenna response. One paper summarizes the toolchain as “MARMOTS+DANTON+ZHAireS-RASPASS” (Zeolla et al., 18 Sep 2025). The purpose of this chain is not only sensitivity estimation but also layout selection, bandwidth optimization, and evaluation of interferometric reconstruction for different sparse-array designs.

For propagation through rock, the $\gamma$1 energy-loss model is written in continuous deterministic form as

$\gamma$2

with $\gamma$3 and $\gamma$4 at the relevant energies, yielding a mean range

$\gamma$5

Once the $\gamma$6 decays, shower development may be modeled with a Gaisser–Hillas profile, and the radio field in the far field is represented schematically in Fourier space by a coherence-limited expression involving the viewing angle, effective charge excess, and propagation distance (Zeolla et al., 18 Sep 2025).

Detector response enters through effective height, gain pattern, and thermal-noise modeling. The induced voltage is written as

$\gamma$7

while the total noise temperature is

$\gamma$8

An example quoted in the design study is an isotropic matched antenna with $\gamma$9 dBi, bandwidth $10\times$0–$10\times$1 MHz, and $10\times$2, which yields $10\times$3–$10\times$4 K across the band (Zeolla et al., 18 Sep 2025).

Effective area is defined in closely related ways across the HERON papers. One formulation writes

$10\times$5

whereas another Monte Carlo study uses

$10\times$6

In both cases the quantity is the convolution of Earth-skimming conversion, shower development, viewing geometry, and trigger response (Zeolla, 12 May 2026, Zeolla et al., 18 Sep 2025).

5. Sensitivity, triggering, and reconstruction performance

Published HERON performance numbers emphasize near-horizon acceptance. At $10\times$7 PeV, one concept paper reports a peak instantaneous effective area $10\times$8 per station and “in excess of $10\times$9” for the full array toward a narrow strip just below the horizon (Zeolla, 12 May 2026). An optimization paper, using MARMOTS for phased-array stations at $\Delta\theta\lesssim1^\circ$0 km altitude, reports a peak single-station effective area of approximately $\Delta\theta\lesssim1^\circ$1 at $\Delta\theta\lesssim1^\circ$2 eV and a point-source sensitivity curve peaking around $\Delta\theta\lesssim1^\circ$3 PeV, with degradation below $\Delta\theta\lesssim1^\circ$4 PeV and above $\Delta\theta\lesssim1^\circ$5 PeV (Zeolla et al., 18 Sep 2025). The coexistence of these numbers suggests that the quoted figures depend on study definition and observing geometry.

Triggering is based on coherent summation and adaptive thresholding. One formulation writes the coherent noise as $\Delta\theta\lesssim1^\circ$6 and sets the trigger criterion through

$\Delta\theta\lesssim1^\circ$7

With $\Delta\theta\lesssim1^\circ$8 and 24 antennas, digital beamforming lowers thresholds to $\Delta\theta\lesssim1^\circ$9 eV, while adaptive or “noise-riding” thresholds keep false-trigger or trigger rates at $\Omega_{\rm daily}\gtrsim2\ {\rm sr}$0 Hz or $\Omega_{\rm daily}\gtrsim2\ {\rm sr}$1 per station (Kotera et al., 6 Jul 2025).

Angular reconstruction is a defining feature of the hybrid design. The sparse-array concept paper projects shower-axis reconstruction at $\Omega_{\rm daily}\gtrsim2\ {\rm sr}$2 or better and $\Omega_{\rm daily}\gtrsim2\ {\rm sr}$3 resolution of $\Omega_{\rm daily}\gtrsim2\ {\rm sr}$4 (Zeolla, 12 May 2026). A simulation study gives a median angular error of $\Omega_{\rm daily}\gtrsim2\ {\rm sr}$5 even with $\Omega_{\rm daily}\gtrsim2\ {\rm sr}$6 and 2 ns timing jitter, and expects improvement to $\Omega_{\rm daily}\gtrsim2\ {\rm sr}$7 when phased-array baselines are included (Zeolla et al., 18 Sep 2025). The preliminary design paper quotes $\Omega_{\rm daily}\gtrsim2\ {\rm sr}$8 (RMS), $\Omega_{\rm daily}\gtrsim2\ {\rm sr}$9 in the main beam, and an anticipated combined performance of $\nu_\tau$00 (Kotera et al., 6 Jul 2025).

Sky coverage is similarly central. Owing to the Earth’s rotation, day-averaged sky coverage is quoted as $\nu_\tau$01 of the celestial sphere, and the preliminary layout states that each station sees a $\nu_\tau$02 azimuthal sector centered on the horizon, covering about $\nu_\tau$03 of the sky per day individually and about $\nu_\tau$04 collectively (Zeolla, 12 May 2026, Kotera et al., 6 Jul 2025). One optimization study further projects a combined annual exposure at $\nu_\tau$05 eV exceeding $\nu_\tau$06 (Zeolla et al., 18 Sep 2025).

6. Science reach, real-time operations, and terminological disambiguation

HERON’s science case is centered on transients. The collaboration papers list direct searches for neutrinos from gamma-ray bursts, blazar flares, binary mergers, and newborn magnetars, together with diffuse-flux studies and point-source identification through sub-degree localization (Zeolla, 12 May 2026). The concept is explicitly presented as complementary to IceCube-Gen2, GRAND, and Auger: those experiments are described as optimizing diffuse-flux limits, whereas HERON “excels at real-time transient follow-up and horizon-skimming geometry” (Zeolla, 12 May 2026).

Event-rate and sensitivity estimates reflect that focus. For a benchmark diffuse flux $\nu_\tau$07 at the current IceCube limit, HERON is said to record $\nu_\tau$08 events per year above $\nu_\tau$09 PeV (Zeolla, 12 May 2026). For cosmogenic models, another paper quotes $\nu_\tau$10 per year, while optimistic GRB-afterglow stacks in the $\nu_\tau$11 PeV–$\nu_\tau$12 EeV range yield $\nu_\tau$13–$\nu_\tau$14 events per 5 years (Kotera et al., 6 Jul 2025). For a bright gamma-ray-burst transient with duration $\nu_\tau$15 min at redshift $\nu_\tau$16, the short-burst sensitivity is quoted as $\nu_\tau$17 better than existing limits and sufficient to detect optimistic burst models; for long transients $\nu_\tau$18 day such as newborn magnetars, the sensitivity “matches or exceeds Auger limits” (Zeolla, 12 May 2026).

Operationally, HERON is described as a real-time transient monitor. Each phased station runs a local beamforming trigger; after a trigger, full waveforms sampled at $\nu_\tau$19 MS/s over a $\nu_\tau$20s window are recorded, and a $\nu_\tau$21–$\nu_\tau$22 ms timestamp is broadcast to nearby standalone antennas. Candidate selection then relies on rapid offline reconstruction of trajectories emerging just below the horizon, linear polarization, and deep $\nu_\tau$23. External alerts are ingested through GCN/AMON, and phased beams can be re-targeted within $\nu_\tau$24 (Kotera et al., 6 Jul 2025).

The term “Heron” is polysemous in the arXiv literature. It also denotes a cross-site software router for AI inferencing to wind-farm data centers, IBM’s Heron-r2 superconducting quantum processor, and generalized Heron problems on Hadamard manifolds addressed by accelerated Douglas–Rachford methods (Reddy et al., 15 May 2025, Mayo et al., 4 Mar 2026, Sahu et al., 28 Sep 2025). The explicit label “Heron-101,” however, is attached to guided expositions of the neutrino-observatory concept: a compact overview of the Earth-skimming detection principle, the hybrid phased-plus-sparse architecture, and the transient-oriented observing strategy of HERON (Zeolla, 12 May 2026, Kotera et al., 6 Jul 2025).