⚛️ general relativity

Luminosity Signatures of Dark Sector Particles from Black Hole Evaporation in Neutron Stars

This paper proposes that microscopic black holes formed by dark matter accumulation in neutron stars can emit long-lived dark sector particles via Hawking radiation, which escape the star and decay into detectable high-energy neutrinos, photons, or charged particles, offering unique spectral and angular signatures to distinguish this mechanism from dark matter annihilation.

Original authors: Ioannis Dalianis, Anastasios Irakleous

Published 2026-09-22

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CC BY 4.0

Original authors: Ioannis Dalianis, Anastasios Irakleous

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep within the dense cores of neutron stars, the most compact objects in the universe, a hidden process might be taking place that bridges the gap between gravity and the quantum world. Neutron stars are the collapsed remnants of massive stars, packed so tightly that a single teaspoon of their material would weigh a billion tons. Because they are so heavy and dense, they act as powerful traps for dark matter, the invisible substance that makes up most of the matter in the universe. Over time, this dark matter can accumulate in the star's center, eventually collapsing under its own weight to form a microscopic black hole. According to the laws of quantum physics, such tiny black holes should not last forever; they should glow with a faint, theoretical light known as Hawking radiation, slowly evaporating until they vanish. However, this radiation has never been seen, and in the case of a black hole hidden inside a neutron star, the surrounding stellar material is so thick that it would swallow any ordinary particles the black hole emits, hiding the signal from our telescopes.

A new study by researchers Ioannis Dalianis and Anastasios Irakleous proposes a way to see this hidden event. They suggest that while the black hole's standard emissions are trapped, it might also spit out rare, long-lived particles that interact very weakly with matter. These particles could slip through the dense star like ghosts, escape into space, and then decay into high-energy neutrinos or photons that our detectors on Earth can catch. The researchers did not just look for these particles; they mapped out exactly how they would appear in the sky. They calculated the energy and the direction of these escaping particles, finding that their arrival pattern holds a specific signature. This signature depends on how long the particle lives before decaying and how far away the star is, rather than on the temperature of the black hole itself.

The team focused on four specific types of hypothetical particles that could be produced in this way: particles that interact only through gravity, dark photons, dark-Z bosons, and heavy neutral leptons. For each of these, they traced the journey from the moment the particle is born in the black hole's evaporation, through its escape from the neutron star, to its eventual decay far away from the star. They found that the decay products, such as neutrinos or light, do not arrive from a single point directly behind the star. Instead, because the particles travel a distance before decaying, the signal arrives from a slightly wider area, creating a small, calculable blur on the sky. The size of this blur is determined by the particle's lifetime and the distance to the star. If the particle lives longer, it travels further before decaying, and the signal spreads out more.

One of the most striking findings is that this spreading effect is surprisingly consistent. Even though the black hole gets hotter and emits particles with higher energies as it evaporates, the overall shape of the signal on the sky remains roughly the same. This happens because two opposing effects cancel each other out: as the particles move faster, they travel further before decaying, but they also beam their decay products more tightly in the forward direction. The result is a stable, predictable pattern that depends primarily on the particle's lifetime. The researchers compared this scenario to a different way such particles could be made: the collision of dark matter particles. They found that while the energy of the particles coming from black holes looks very different from those coming from collisions, the overall shape of the signal on the sky can look almost identical if you only look at the total light without separating it by energy.

However, the researchers showed that if you look at the signal in specific energy ranges, the two sources become distinct. The black hole produces a broad, continuous range of energies, while the dark matter collisions produce a sharp, single energy. By separating the incoming particles by their energy, astronomers could tell which process created them. This distinction is crucial because it would allow scientists to confirm the existence of microscopic black holes and, in doing so, observe Hawking radiation for the first time. The study suggests that current and future telescopes, particularly those looking for high-energy neutrinos and gamma rays, could use these specific energy and angle patterns to hunt for these events. While the signal from a single star might be too faint to see, the combined glow from many neutron stars across our galaxy could be strong enough to detect. This work provides a clear roadmap for how to look for these elusive signals, turning a theoretical prediction about black holes into a concrete target for observation.

Technical Summary: Luminosity Signatures of Dark Sector Particles from Black Hole Evaporation in Neutron Stars

Problem Statement
Microscopic black holes (MBHs) may form at the centers of neutron stars through the accumulation and gravitational collapse of captured dark matter (DM). While Hawking radiation from such objects offers a potential window into quantum gravity and physics beyond the Standard Model (BSM), direct observation is hindered by the dense stellar medium. Standard Model (SM) particles produced by Hawking evaporation are efficiently absorbed and thermalized within the neutron star. The paper addresses the possibility that a fraction of the Hawking luminosity is emitted into sufficiently weakly interacting, long-lived BSM particles (mediators, denoted as $S$ ). These mediators can escape the star and subsequently decay into observable SM particles (neutrinos, photons, or charged particles) outside the stellar environment. The specific challenge addressed is characterizing the joint energy–angular distribution of these secondary particles to distinguish this mechanism from other high-energy sources, particularly DM annihilation.

Methodology
The authors develop a theoretical framework to model the production, propagation, and decay of long-lived mediators emitted by MBHs in neutron stars.

Source Modeling: They assume MBHs form with initial masses $M_{BH} \lesssim 10^6$ kg, corresponding to Hawking temperatures $T_{BH} \gtrsim \text{TeV}$ . The Hawking emission spectrum is calculated including spin-dependent greybody factors, accounting for the rapid mass loss and temperature increase during the evaporation burst.
Escape and Decay Conditions: The study derives conditions for mediators to escape the neutron star (scattering optical depth $\tau_{scat} \lesssim 1$ ) and decay within an observable distance (boosted decay length $\ell_{dec} \gtrsim R_{NS}$ but $\ell_{dec} \lesssim D$ , where $D$ is the source distance).
Kinematic Derivation: A detailed derivation of the joint energy–angular distribution ( $d^2F_Y/dE_Y d\Omega_\psi$ $d^{2} F_{Y} / d E_{Y} d Ω_{ψ}$ ) is performed for two-body decays $S \to Y\bar{Y}$ $S \to Y \overset{ˉ}{Y}$ . This incorporates:
- Relativistic beaming of decay products in the mediator rest frame.
- The correlation between the mediator's energy, decay radius, and the observed angle of the daughter particle.
- The integration over the time-evolving Hawking spectrum and the exponential decay probability of the mediator.
Benchmark Scenarios: The mechanism is illustrated using four representative BSM realizations: gravitationally coupled scalars, dark photons ( $A'$ ), dark- $Z$ bosons ( $Z'$ ), and heavy neutral leptons (HNLs).
Comparative Analysis: The derived signatures are contrasted with monochromatic mediator production from DM annihilation ( $\chi\chi \to SS$ ).

Key Contributions and Results

Universal Angular Scaling: A central analytical result is that for ultra-relativistic mediators ( $R_{NS} \ll \ell_{dec} \ll D$ ), the normalized energy-integrated angular profile becomes approximately independent of the mediator energy and, consequently, the black hole temperature. This arises because the boost-enhanced decay length ( $\ell_{dec} \propto \gamma_S$ ) is compensated by the narrowing of the relativistic beaming cone ( $\alpha_{beam} \propto \gamma_S^{-1}$ ). The characteristic angular extent is controlled primarily by the combination $c\tau_S/D$ , where $\tau_S$ is the mediator's proper lifetime.
Degeneracy and Resolution: The paper demonstrates that the energy-integrated angular profiles of Hawking evaporation and monochromatic DM annihilation can be nearly degenerate if the total mediator yields and propagation parameters are matched. However, this degeneracy is broken when considering energy-resolved angular distributions.
- Hawking evaporation produces a broad, evolving energy spectrum, whereas DM annihilation yields a monochromatic spectrum with sharp kinematic endpoints.
- Restricting the analysis to specific energy bins reveals distinct angular morphologies, particularly near the kinematic endpoints of the monochromatic source, which are absent in the broad Hawking spectrum.
Secondary Spectra: The secondary particle spectra ( $dN_Y/dE_Y$ ) from Hawking evaporation follow a power-law scaling ( $E^{-3}$ ) at high energies, reflecting the underlying Hawking distribution, distinct from the box-like spectrum characteristic of two-body decays from monochromatic parents.
Viability of Mechanisms: The study confirms that all four benchmark scenarios (gravitational scalars, dark photons, dark- $Z$ , HNLs) possess parameter spaces where mediators can escape the neutron star and decay at astrophysically relevant distances (parsecs to kiloparsecs), producing observable neutrino or gamma-ray signals.

Significance and Claims
The paper claims to provide a complementary observational probe for Hawking radiation that addresses the spatial morphology of the signal, which has been less explored than total luminosity or energy spectra.

Distinguishing Production Mechanisms: The joint energy–angular distribution serves as a powerful discriminator between MBH evaporation and DM annihilation. While the energy spectrum alone distinguishes the production mechanism (broad vs. monochromatic), the energy-resolved angular distribution offers an additional probe of the underlying kinematics and production history.
Probing Mediator Properties: For a source at a known distance, the characteristic angular extent of the signal provides a direct measurement of the mediator's proper lifetime ( $\tau_S$ ), independent of the black hole's temperature evolution.
Observational Targets: The authors identify high-energy neutrino and gamma-ray searches as the primary observational channels. They note that while current neutrino telescopes (e.g., IceCube) may struggle to resolve the angular structure for typical parameters (point-like sources), gamma-ray instruments (e.g., H.E.S.S., LHAASO, CTAO) with finer angular resolution are well-suited to resolve the extended emission.
Model Independence: The derived angular scaling laws are presented as largely independent of the specific particle physics model, relying instead on general relativistic kinematics and the properties of long-lived mediators.

The work concludes that while absolute flux predictions depend on astrophysical uncertainties (DM density, neutron star population) and particle physics details (couplings, branching ratios), the derived energy–angular structure constitutes a robust, characteristic signature of long-lived particles escaping microscopic black hole evaporation.

Technical Summary: Luminosity Signatures of Dark Sector Particles from Black Hole Evaporation in Neutron Stars

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