⚛️ phenomenology

Detecting light axions from supernovae in nearby galaxies

This paper proposes that monitoring nearby galaxies for short gamma-ray bursts from supernovae, potentially triggered by deci-hertz gravitational-wave detections, could probe previously unexplored regions of axion-like particle parameter space by constraining the product of their nucleon and photon couplings to levels below the SN 1987A bound.

Original authors: Francesca Lecce (Bari U.,INFN, Bari), Alessandro Lella (Bari U.,INFN, Bari), Giuseppe Lucente (SLAC), Maurizio Giannotti (Zaragoza U.), Alessandro Mirizzi (Bari U.,INFN, Bari)

Published 2026-09-14

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CC BY 4.0

Original authors: Francesca Lecce (Bari U.,INFN, Bari), Alessandro Lella (Bari U.,INFN, Bari), Giuseppe Lucente (SLAC), Maurizio Giannotti (Zaragoza U.), Alessandro Mirizzi (Bari U.,INFN, Bari)

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

The universe is filled with invisible particles that might hold the key to some of its deepest mysteries. Among the most intriguing of these are axion-like particles, hypothetical cousins of the axion, a particle originally proposed to solve a specific puzzle about how matter behaves at the most fundamental level. While the standard model of physics explains much of what we see, it leaves gaps regarding dark matter and why certain forces act the way they do. Axion-like particles are a leading candidate to fill these gaps. They are expected to be incredibly light and to interact very weakly with ordinary matter, making them notoriously difficult to catch. However, the extreme environments of dying stars offer a unique laboratory. When a massive star collapses in a supernova explosion, its core becomes a furnace of such heat and density that it could spew out vast numbers of these elusive particles. If these particles exist and can turn into light under the right conditions, they might leave a trace that telescopes could finally see.

For decades, scientists have looked for this signal, but their search has been limited by a lack of opportunities and the wrong kind of tools. The most famous attempt relied on a supernova that exploded in 1987 in a nearby galaxy. At the time, detectors scanned for a burst of gamma rays that would have appeared if axions had been produced and then converted into light. None was found, which set a strict limit on how strongly these particles could interact with protons and photons. This result ruled out a large range of possibilities, but it left a vast, unexplored territory below that limit. The problem with looking for these signals in the future is that a supernova in our own Milky Way galaxy, which would be close enough to provide a clear signal, is a rare event, happening perhaps only once every few centuries. Looking further out to other galaxies seemed like a dead end because the signal would be too faint, and without a precise moment to start looking, the background noise of the universe would drown out any potential discovery.

A new study changes this picture by proposing a coordinated strategy that turns the entire sky into a hunting ground. The researchers, a team of physicists from Italy, Spain, and the United States, argue that we do not need to wait for a rare local event. Instead, we can watch for supernovae in nearby galaxies, which occur with much greater frequency—roughly seven times a year within a specific range of distance. The key to making this work is not just having powerful telescopes, but having the right kind of timing. In the past, astronomers had to guess when a star exploded based on how its light brightened over days, a process that created a wide window of uncertainty where background noise could hide a signal. The new approach relies on a combination of next-generation technology: a network of gamma-ray detectors that can see the entire sky at once, and a system that can pinpoint the exact moment a star collapses within seconds.

The study focuses on how these hypothetical particles would behave once they leave the dying star. As they travel through the magnetic fields of their host galaxy, there is a chance they will convert into gamma rays, a form of high-energy light. The researchers found that in certain nearby galaxies, the magnetic fields are much stronger than those in our own Milky Way. In these environments, the conversion of particles into light is far more efficient, boosting the signal enough to be detectable even from millions of light-years away. The team modeled three specific targets: the starburst galaxy M82, the spiral galaxy NGC 6946, and the Virgo Cluster. In these locations, the magnetic fields act like a powerful lens, concentrating the potential signal. By combining this enhanced conversion with a precise trigger, the search becomes much more sensitive.

The researchers calculated what would happen if a gamma-ray telescope, similar to the one currently orbiting Earth but with full-sky coverage, watched these galaxies for ten years. They simulated the detection of multiple supernovae, using the precise timing provided by future gravitational-wave detectors to narrow the search window to just ten seconds. This short window is crucial because it eliminates almost all background noise, allowing the telescope to look for a tiny, specific flash of light without confusion. The results show that over a decade of monitoring, this method could reach a sensitivity to the interaction between axions and protons that is significantly better than the limit set by the 1987 event. Specifically, it could probe a region of the parameter space that is currently unknown, potentially detecting particles with masses as low as one-billionth of a billionth of an electron volt.

Even if no signal is found, the study demonstrates that this approach would set the most stringent limits on these particles to date, pushing the boundaries of our knowledge far beyond what was previously possible. The authors emphasize that this is not a speculative dream but a realistic path forward, provided that the necessary technology comes online. The required gamma-ray detectors are already being designed, and the gravitational-wave observatories capable of providing the precise timing trigger are in development. By linking these tools together, astronomers could transform the steady stream of distant supernovae into a continuous, high-precision experiment. This would allow us to test the existence of these invisible particles in a way that was previously thought impossible, turning the distant, violent deaths of stars into a quiet, powerful voice for fundamental physics.

Technical Summary: Detecting Light Axions from Supernovae in Nearby Galaxies

Problem Statement
Axion-like particles (ALPs) coupled to nucleons are predicted to be copiously produced in the hot, dense cores of core-collapse supernovae (CCSNe). If these ALPs also couple to photons, they can convert into gamma rays within cosmic magnetic fields, potentially generating a short gamma-ray burst coincident with the neutrino signal. While a Galactic CCSN would provide an intense, easily detectable signal, such events are exceedingly rare (occurring roughly twice per century). Conversely, while extragalactic CCSNe occur frequently in nearby galaxies ( $\sim$ O(10) Mpc), previous searches have been limited by two major factors:

Flux Penalty: The inverse-square law reduces the flux from extragalactic sources by orders of magnitude compared to Galactic sources.
Timing Uncertainty: Without a neutrino trigger (which is undetectable from extragalactic distances), the core-collapse time must be inferred from optical light curves, leading to large observational time windows ( $\Delta t \sim 1$ day). This transforms the search from a background-free regime to a background-dominated one, severely degrading sensitivity.

Consequently, extragalactic SNe have remained largely unexplored for ALP detection, leaving a significant gap in parameter space below the constraints set by SN 1987A.

Methodology
The authors propose a multi-messenger strategy to overcome these limitations, combining full-sky gamma-ray monitoring with precise external time triggers. The methodology involves:

ALP Production: The study utilizes the 1D spherical symmetric Garching group's SN model (SFHo-s18.8) to calculate ALP fluxes generated via nucleon-nucleon (NN) bremsstrahlung. The resulting ALP spectrum is thermal with an average energy of $\sim 70$ MeV.
Conversion Environments: Unlike previous studies that assumed conversion only occurs in the Milky Way's magnetic field, this work models ALP-photon conversions within the host galaxies of nearby SNe. The authors analyze three specific environments:
- M82: A starburst galaxy ( $d \approx 3.5$ Mpc) with magnetic fields enhanced by starburst activity ( $\sim 0.1\text{--}1$ mG).
- NGC 6946: A spiral galaxy ( $d \approx 5.9$ Mpc) with magnetic fields of order $O(10)$ $\mu$ G.
- Virgo Cluster (M87): An elliptical galaxy ( $d \approx 16.9$ Mpc) benefiting from Mpc-scale intracluster magnetic fields providing long conversion baselines.
- Magnetic field and electron density models are derived from cosmological magnetohydrodynamical simulations (IllustrisTNG).
Detection Strategy: The sensitivity is calculated for a Fermi-LAT-like gamma-ray detector (e.g., the proposed GALAXIS mission) capable of full-sky coverage. The analysis varies the time window ( $\Delta t$ $Δ t$ ) for the search based on the precision of the core-collapse time trigger:
- $\Delta t = 1$ day (current optical survey accuracy).
- $\Delta t = 1$ hour (future optical survey accuracy).
- $\Delta t = 10$ s (optimistic scenario enabled by deci-hertz gravitational wave (GW) detectors like DECIGO or BBO, which can detect the neutrino-memory signal).
Statistical Analysis: Sensitivities are derived using the Feldman-Cousins prescription. The study evaluates the scaling of sensitivity with the number of stacked events ( $N_{SN}$ ) in both background-dominated and background-free regimes.

Key Contributions and Results
The paper demonstrates that extragalactic SNe can serve as powerful ALP laboratories if specific experimental conditions are met:

Enhanced Conversion Efficiency: The strong magnetic fields in nearby galaxies (particularly M82 and the Virgo Cluster) enhance ALP-photon conversion probabilities by 2–3 orders of magnitude compared to the Milky Way. This mitigates the flux penalty associated with large distances.
Impact of Time Triggers: The precision of the core-collapse time trigger is the dominant factor in sensitivity:
- With a $\Delta t = 1$ day window, the search is background-dominated, yielding a sensitivity to the coupling product $g_{ap} \times g_{a\gamma} \gtrsim 4.5 \times 10^{-24}$ GeV $^{-1}$ (for $m_a \lesssim 10^{-9}$ eV). This is only marginally better than the SN 1987A bound.
- With a $\Delta t = 10$ s window (enabled by GW triggers), the search enters a background-free regime. This improves the sensitivity to $g_{ap} \times g_{a\gamma} \gtrsim 2.3 \times 10^{-24}$ GeV $^{-1}$ for a single event.
Stacking Sensitivity: The authors show that stacking multiple events significantly improves constraints. In the background-free regime ( $\Delta t = 10$ s), sensitivity scales as $N_{SN}^{-1/2}$ . Stacking just 5 CCSNe within $\sim 20$ Mpc could reach a sensitivity of $g_{ap} \times g_{a\gamma} \approx 9.8 \times 10^{-25}$ GeV $^{-1}$ , a factor of $\sim 3.7$ improvement over the SN 1987A limit.
Feasibility: The paper estimates that roughly seven CCSNe occur per year within 20 Mpc. A decade-long monitoring program, combined with a deci-hertz GW detector network, would be sufficient to either detect a signal or set substantially stronger bounds on ALP couplings.

Significance and Claims
The authors claim that this work redefines the potential of extragalactic supernovae for axion physics. By moving beyond the reliance on rare Galactic events and leveraging the frequent occurrence of nearby extragalactic SNe, the proposed method offers a transformative tool for probing ALP parameter space.

Specifically, the paper asserts that:

The combination of full-sky gamma-ray monitoring and precise GW time triggers can probe a large, currently unexplored region of the parameter space below the longstanding SN 1987A bound.
The sensitivity reach ( $g_{ap} \times g_{a\gamma} \sim 10^{-24}$ GeV $^{-1}$ ) is achievable under realistic conditions within a decade.
This approach establishes a new frontier in multi-messenger astronomy, where the cumulative supernova activity of the local Universe becomes a precision tool for fundamental physics, potentially testing models of dark matter and the "axiverse" predicted by string theory.

The paper concludes that while current facilities lack the necessary coverage and timing accuracy, the realization of next-generation GW detectors and full-sky gamma-ray constellations would decisively enable this new era of extragalactic ALP astrophysics.

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