Exotic PeVatrons as sources of ultra-high-energy gamma rays

We explore novel classes of exotic astrophysical sources capable of producing ultra-high-energy gamma rays extending beyond the PeV scale, motivated by quantum gravity scenarios and dark matter phenomenology. These sources include: ultra-spinning black hole vortex-string systems; exotic compact objects such as boson star, axion star and Q-ball. Such Exotica generate powerful magnetic fields through interactions with millicharged dark matter, enabling particle acceleration mechanisms that surpass the energy limits of conventional astrophysical sources like pulsar wind nebulae and supernova remnants. We demonstrate that such exotic PeVatrons could be distributed throughout our Galaxy and may be detectable by current (LHAASO, HAWC) and next-generation (CTA) gamma-ray observatories.

💡 Research Summary

The paper addresses the recent discovery of ultra‑high‑energy (UHE) gamma‑ray sources extending beyond 100 TeV by wide‑field observatories such as LHAASO and HAWC. Conventional Galactic PeVatrons—pulsar wind nebulae (PWNe) and supernova remnants—are limited by synchrotron and inverse‑Compton losses, and their magnetic fields and spin‑down powers cannot easily accelerate particles to the 10 PeV regime. To explain possible detections above 10 PeV, the authors propose two classes of “exotic PeVatrons” (E‑PeVatrons) that invoke new physics beyond the Standard Model.

1. Exotic Winds (EWinds) – The first class consists of ultra‑spinning black holes (BHs) surrounded by clouds of ultra‑light bosons (e.g., dark photons or scalar dark matter). Superradiant instabilities amplify these boson clouds, leading to the formation of Nielsen‑Olesen vortex strings threading the BH horizon. If the vortex couples to millicharged dark matter (charge q ≪ 1), a quantized magnetic flux Φ = 2πn/q_e is generated. The power radiated by the rotating BH–vortex system scales as P ∼ Φ²Ω², where Ω is the BH angular velocity. For stellar‑mass BHs (1–100 M⊙) with near‑extremal spin (a ≈ 1), Ω can reach ≈10⁵ Hz, two orders of magnitude faster than millisecond pulsars. Because Φ grows as 1/q, even a tiny millicharge (q ≈ 10⁻⁴–10⁻⁶) yields enormous fluxes, allowing P ≈ 10³⁷ erg s⁻¹ and particle energies well above 10 PeV. The authors illustrate this parameter space in Figure 1, showing that modest variations in Ω and q can produce the required power.

2. Exotic Novae (Enovae) – The second class involves exotic compact objects (ECOs) such as boson stars, axion stars, and Q‑balls. When these solitonic objects rotate, they develop vortex structures that also trap quantized magnetic flux Φ = n q_e. During spin‑down, the vortex can eject millicharged particles, converting rotational energy into high‑energy particles. The emission power follows P ≈ Φ²Ω² ∝ n⁴ q_e² m² R⁴, where m is the constituent boson mass, R the star radius, and n the winding number. Using the Gross‑Pitaevskii‑Poisson (GPP) framework, the authors derive a mass‑radius relation for rotating boson stars, M ∝ n (λ/G)¹ᐟ² m⁻³ᐟ², with λ the self‑interaction coupling. By choosing realistic parameters (m ≈ 10⁻⁶–10⁻³ eV, λ ≈ 10⁻⁴–10⁻², q ≈ 10⁻⁴–10⁻⁶, n ≈ 1–10), they find that ECOs of 1–10 M⊙ and radii of a few kilometers can emit UHE gamma rays up to tens of PeV.

The paper revisits the Hillas criterion for particle acceleration, emphasizing that the required magnetic field strength (B < 10 µG) and flow velocities (U > 10⁴ km s⁻¹) are naturally achieved in the BH‑vortex and ECO‑vortex environments, unlike in ordinary PWNe. The authors also discuss the Aharonov‑Bohm effect, which guarantees the stability of the quantized flux, and argue that the resulting gamma‑ray spectra would be markedly harder than those from standard hadronic or leptonic processes.

Observationally, the authors note that LHAASO has already catalogued dozens of Galactic sources above 100 TeV, many without clear counterparts. They argue that the proposed exotic PeVatrons could account for these “orphan” sources. The upcoming Cherenkov Telescope Array (CTA), with its superior sensitivity and energy resolution, should be capable of detecting gamma rays above 10 PeV, measuring spectral cut‑offs, and possibly distinguishing the hard spectra expected from magnetic‑flux‑driven acceleration. Detection of such signatures would provide indirect evidence for millicharged dark matter and for exotic compact objects predicted by quantum‑gravity‑inspired models.

In summary, the paper presents a coherent theoretical framework linking quantum‑gravity motivated ultra‑light bosons, millicharged dark matter, and rotating compact objects to generate magnetic‑flux‑driven accelerators that surpass the energy limits of conventional astrophysical sources. It offers concrete predictions for gamma‑ray observatories and opens a novel avenue for probing exotic physics through high‑energy astrophysics.