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

TL;DR

The paper proposes Exotic PeVatrons as sources of ultra-high-energy gamma rays that exceed conventional acceleration limits by exploiting ultra-spinning black hole vortex systems and rotating solitons (boson stars, gauged axion stars, and Q-balls) interacting with millicharged dark matter. It develops concrete mechanisms—vortex flux around BHs, and spin-down of solitons with quantized flux—to achieve PeVatron-level particle acceleration, with emission powers up to about $10^{37}$ erg s$^{-1}$ and potential photon energies in the PeV range. The authors show distinct scaling of the spin-down power with winding number $n$ and angular momentum $a$, and they outline multi-messenger signatures including gravitational waves in the $1$–$100$ Hz band that could accompany UHE gamma-ray emission. Detectability by current and next-generation facilities (LHAASO, HAWC, CTA) alongside GW detectors could test these scenarios, linking high-energy astrophysics to quantum gravity and light millicharged DM, while highlighting model uncertainties such as the Galactic distribution of ECOs and observational constraints from Auger and KM3NeT.

Abstract

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.

Figures (6)

• Figure 1: Emission power $P = 10^{37}$ erg/s in BH parameter space. The parameter $\Omega$ denotes the angular velocity and $q$ the charge of the millicharge particle. Different color represents different combination of $\Omega$ and $q$.
• Figure 2: Contour plot of the spin-down emission power $P = 10^{37}\,\mathrm{erg/s}$ for rotating BSs with repulsive self-interactions ($\lambda > 0$). The vortex winding number is fixed at $n = 1$. The colored regions (purple, pink, red, orange, green, blue) correspond to the same parameter ranges shown in Fig. \ref{['PeVBHFig']}.
• Figure 3: Contour plot of spin-down emission power ($P = 10^{37}\,\mathrm{erg/s}$) for rotating axion-like particle (ALP) stars, computed using a 6th-order expansion of the potential. Each panel displays solutions at fixed stellar radius, with vortex winding number $n=1$. The semi-transparent colored regions follow the same parameter conventions as Fig. \ref{['FigBSRep']}. We focus exclusively on the dense axion star branch, as the dilute branch solutions predominantly reside in the super-spinning regime ($a>1$) relevant for PeVatron production. The yellow region specifically denotes QCD axion solutions. The black region denotes the non-existence of the real solution, where the quantity in the root of Eq. \ref{['eqAS']} becomes negative.
• Figure 4: Contour plot of the spin-down emission power $P = 10^{37}\,\mathrm{erg/s}$ for rotating Q-balls with vortex winding number fixed at $n=1$. The transparent colored regions maintain the same parameter correspondence as in Fig. \ref{['FigBSRep']}. The transparent purple shade identifies Higgs-like Q-ball solutions with characteristic field amplitude $\phi_0 \sim m$, while the yellow region represents QCD axion Q-balls modeled using a sixth-order potential expansion. This comprehensive representation captures the essential features of Q-ball emission across different theoretical scenarios while maintaining consistent comparison with previous BS results.
• Figure 5: Emission power $P$ as a function of dimensionless spin parameter $a$ for rotating BSs, ALP stars and Q-balls composed of millicharged dark matter. All configurations share common parameters $M = 10^{30}$ g and $q = 10^{-35}$. The orange band represents black hole vortices coupled to millicharged DM, exhibiting a continuous spectrum characterized by the effective flux parameter $n_{\rm eff}$. The purple band indicates the observational window for detected PeVatron emission at $P \approx 10^{37}$ erg/s. This comparative analysis highlights the distinct spin-dependent emission characteristics across different exotic compact object scenarios.