Preprint · Zenodo · DOI 10.5281/zenodo.19889593

One source.
Two observers.
Completely different sky.

Why do some Fast Radio Bursts repeat and others don't? The answer might not be the magnetar — it might be the geometry of what's in the way.

Fast Radio Bursts are millisecond flashes of radio energy, each releasing more power than the Sun emits in days. They arrive from galaxies billions of light-years away. Some repeat. Most, apparently, don't.

The standard explanation: repeaters and non-repeaters are fundamentally different kinds of magnetar. This paper proposes something more elegant — they are the same kind of magnetar. What differs is whether you happen to be aligned with the narrow geometric channel that the magnetar's wind nebula has carved through its environment.

The same principle already explains why most gamma-ray bursts are invisible: the jet isn't pointing at us. Fast Radio Bursts may work the same way — not through intrinsic jet collimation, but through the refractive geometry of the surrounding nebula.

Henderson, I.T. (2026) · Geometric Selection Effects in Repeating FRBs 10.5281/zenodo.19889593
§ 1 — The Signal
What a Fast Radio Burst looks like

Each panel below is a frequency-time waterfall plot — the radio sky over a few milliseconds. Repeating FRBs have a characteristic signature: emission drifts downward in frequency over time (the "sad trombone"). This morphology is one of the key puzzles the geometric model explains.

Characteristic downward frequency drift — a propagation signature
§ 2 — The Model
The geometric channel

A young magnetar inflates a wind nebula around itself. Because magnetar emission is intrinsically beamed, the energy injected into the nebula is anisotropic — it carves a preferred low-density channel. The environment (tidal forces, density gradients from a nearby galaxy) shapes which direction that channel points. Only observers aligned with the channel see repeated bursts.

1
Magnetar Engine
All FRBs originate from magnetar activity — crustal failure, magnetospheric reconnection. The engine is the same for repeaters and non-repeaters. We propose no modification.
2
Anisotropic Nebula
The magnetar wind inflates a compact nebula. Beamed energy injection sculpts internal density channels — low-density propagation corridors that act as radio waveguides.
3
Environment Shaping
Tidal forces, density gradients, ram pressure from orbital motion — these break the nebula's symmetry and define which direction the channel points.
Same magnetar · Different observer
What they detect
Repeated bright bursts — narrowband, temporally broadened. This source enters the repeater catalogue. Multi-telescope campaigns begin.
What they conclude
This magnetar is an active repeater. Its environment shows a persistent radio source (PRS) and extreme Faraday rotation. It will be studied for years.
What they detect
One broadband impulsive burst — or nothing at all. If detected, it enters the non-repeater catalogue. No follow-up reveals further emission.
What they conclude
This is a different type of FRB — a cataclysmic, non-repeating event. But the magnetar may still be firing. The channel simply isn't pointing here.
§ 3 — Falsifiability
Five testable predictions

A good geometric model makes specific predictions about what we should find when we look more carefully. Each prediction below is testable with existing data or near-future instruments. Click to expand.

P1
Repeater sightlines should be underdense in foreground matter Existing data
If repeaters are visible because their channel points through low-density environments, their sightlines should show a deficit of foreground galaxies. Split the Takahashi et al. (2026) cross-correlation analysis by repeater status. The null signal at R < 1 h⁻¹ Mpc should be driven by the repeater sub-sample. Testable with published catalogues.
P2
Deconvolving scattering from repeater bursts recovers non-repeater profiles Existing data
If the morphological differences between repeaters and non-repeaters (narrower bandwidth, longer duration) are propagation effects rather than intrinsic source differences, deconvolving the measured scattering kernel should recover broadband impulsive profiles. Best test cases: FRB 20220912A (clean scintillation arc) and FRB 20201124A (measured scintillation bandwidth).
P3
Repeaters with persistent radio sources inhabit more asymmetric environments Near-term
Stronger environmental asymmetry (tidal interactions, merging subhalos, binary systems) should produce stronger nebular channelling, brighter PRS, higher RM, and more extreme repeater morphology. Requires systematic morphological classification of host environments across the localised FRB sample — growing rapidly with EVN and VLBI.
P4
Persistent radio source morphology should show elongation correlated with host geometry Near-term
As VLBI resolution improves, PRS morphology should reveal elongation aligned with the environmental pressure gradient — stretched toward the direction of least resistance. Current PRS detections are unresolved, but future observations at higher frequencies or longer baselines should reveal parsec-scale asymmetry.
P5
Repeater positions within hosts offset toward lower-density directions FRB 20240114A
Within their host galaxies, repeating FRBs should be preferentially offset toward directions of lower environmental density — away from massive companions, toward void boundaries, along tidal streams. FRB 20240114A (localised to a tidally interacting satellite dwarf at z=0.130, 85 kpc from a primary spiral) provides an immediate test: the offset direction should point away from the primary spiral.
§ 4 — What hasn't been done yet
Four critical observational gaps

These analyses haven't been performed despite relevant data existing. They would directly constrain whether geometric selection is contributing to FRB demographics.

Gap 1
Extrinsic scintillation comparison
No published study systematically compares diffractive scintillation bandwidth or timescale between repeating and non-repeating FRB populations. New analytical approaches needed to overcome inherent observational barriers.
Gap 2
Multi-site scintillation correlation
Simultaneous multi-telescope FRB detections exist but no study has cross-correlated diffractive fine structure between sites — which would directly distinguish a coherent structured screen from multi-scale turbulence.
Gap 3
Foreground density by repeater status
The Takahashi et al. (2026) FRB–galaxy cross-correlation has not been performed separately for repeaters and non-repeaters. Straightforward with existing catalogues. Would directly test Prediction 1.
Gap 4
High-frequency PRS polarimetry
No polarimetric measurement of any FRB-associated persistent radio source above ~15 GHz has been performed. Below that frequency, internal Faraday dispersion makes the measurement uninformative. Observations above the depolarisation barrier would reveal whether the nebular magnetic field has an ordered component — the structural signature of anisotropic energy injection.
§ 5 — The Established Precedent
Gamma-ray bursts solved this first

The principle that geometric selection fundamentally biases observed transient populations is not new — it is the foundation of GRB astronomy. Most gamma-ray bursts are invisible: the jet isn't pointing at Earth. The recent detection of the orphan GRB afterglow ASKAP J005512–255834 (Gulati et al. 2026) directly proved this: an event completely missed during its prompt phase, only becoming visible once the off-axis jet decelerated and spread. The FRB geometric channel model applies the same logic through a different physical mechanism — environmental refractive focusing rather than intrinsic jet collimation.

Gamma-ray bursts
Geometric selection via intrinsic jet
The source itself produces a narrow beam. You see the burst only if the jet axis intersects your position. The detected GRB population is a small, directionally biased fraction of all GRBs firing.
Fast Radio Bursts
Geometric selection via extrinsic environment
The source emits broadly but the magnetar wind nebula channels emission along one axis. You see repeated bursts only if the nebular channel points toward you. The mechanism differs — the observational consequence is identical.