Detecting light axions from supernovae in nearby galaxies

Abstract

Axion-like particles (ALPs) coupled to nucleons can be efficiently produced in core-collapse supernovae (SNe) and then, if they couple to photons, convert into gamma rays in cosmic magnetic fields, generating short gamma-ray bursts. Though ALPs from a Galactic SN would induce an intense and easily detectable gamma-ray signal, such events are exceedingly rare. In contrast, a few SNe per year are expected in nearby galaxies within $\mathcal{O}(10)$ Mpc, where strong magnetic fields can enable more efficient ALP-photon conversions than in the Milky Way, offering a promising extragalactic target. This circumstance motivates full-sky gamma-ray monitoring, ideally combined with deci-hertz gravitational-wave detectors to enable time-triggered searches from nearby galaxies. We show that, under realistic conditions, a decade of coverage could reach sensitivities to ALP-photon coupling $g_{a γ} \gtrsim 10^{-16} \rm{GeV}^{-1}$ for ALP masses $m_a \lesssim 10^{-9} $ eV and assuming an ALP-nucleon coupling close to SN 1987A cooling bound. This sensitivity would allow one to probe a large, currently-unexplored region of the parameter space below the longstanding SN 1987A bound.

Figures (10)

• Figure 1: Sensitivities at $95\%$ CL in the $g_{ap}\times g_{a\gamma}$vs$m_a$ plane for future extragalactic SN occurring in M82 (red), NGC 6946 (yellow) or Virgo Cluster (black) for a Fermi-LAT-like gamma-ray detector, assuming an observation time window of $\Delta t=10$ s enabled by the core-collapse time trigger provided by deci-hertz GW interferometers. The dashed red curve illustrates the stacked sensitivity from 10 SNe in an environment similar to M82. The gray band displays the constraint from the non-observation of gamma rays from SN 1987A for ALPs converting in the Milky-Way, while the hatched region would be probed by conversions in the SN 1987A progenitor magnetosphere assuming a $100\,\rm G$ surface field Manzari:2024jns, but no measurements of such field are available Fiorillo:2025gnd. For comparison, we also show the sensitivity to a future Galactic SN at a distance of $d=10$ kpc (dashed black curve) by assuming the same magnetic field models used for the SN 1987A limit. Finally, we show in green the dominant astrophysical bounds in this mass range AxionLimitsReynes:2021bpeNing:2024ekyBenabou:2025jcv.
• Figure 2: Cumulative number of observed core-collapse supernovae in the period 2008-2025 (left vertical axis) from nearby galaxies in function of the distance. On the right vertical axis is also indicated the inferred SN rate per year.
• Figure 3: Sensitivity ($95\%$ CL) on $g_{ap} \times g_{a\gamma}$ as a function of the number of observed extragalactic SNe, $N_{\rm SN}$, yielding a gamma-ray flux similar to the one estimated in the case of M82. We assume different time windows to search for the ALP signal, namely $\Delta t=1$ d (dashed curve), $\Delta t=1$ h (dot-dashed curve), $\Delta t=10$ s (continuous curve). In the inset we show on a linear scale the sensitivity for $\Delta t = 10$ s scaling as $N_{\rm SN}^{-1/2}$. The $x$ and $y$ axes are the same as in the main panel.
• Figure S1: Time-integrated ALP emission spectrum from $NN$ bremsstrahlung (solid line), and pionic processes (dashed line). These spectra are obtained from fitting formulas in Eqs. (\ref{['eq:bremsfit']}) and (\ref{['eq:piofit']}) by setting $g_{ap} = 10^{-9}$ and $g_{an} = 0$. On the right axis the corresponding values of the photon flux from M82 reaching the Earth by considering ALP--photon conversion in the subhalo and along the line of sight that yield the average conversion probability. The ALP--photon conversion probability is estimated at $g_{a\gamma} = 10^{-12}\,\rm{GeV}^{-1}$.
• Figure S2: Magnetic-field profiles (upper panels) and free-electron density profiles $n_e$ (lower panels) as a function of the distance from the galactic center for our fiducial line of sight. The left panel refers to M82 (solid line) and NGC 6946 (dashed line), while results for the Virgo Cluster are reported in the right panel.