We present upper limits on the axionlike particle (ALP) photon coupling constant $g_{a\gamma \gamma }$ from a broadband spectral analysis of the Coma cluster (Abell 1656) radio halo. We have used flux density measurements spanning from 30.9 MHz to 6.6 GHz, which probe ALP masses from 0.26 to $55.9\mathrm{\mu }\mathrm{eV}$, the widest continuous mass coverage from any single cluster to date. We model the synchrotron foreground with the physically motivated in situ reacceleration spectrum, which provides a substantially better description of the strongly curved Coma halo spectrum than the simple power law adequate for the Bullet cluster. We adopt an aperture-matched profile-likelihood analysis as our primary statistical estimator, fitting the full 23-point radio spectrum simultaneously at each tested axion mass. For the robust CMB-only and $\mathrm{CMB}+\text{extragalactic}$ radio background photon baths, the $2\sigma$ upper end points at $m_a=55.9\mathrm{\mu }\mathrm{eV}$ are both $\simeq 4.35\times {10}^{-8}{\mathrm{GeV}}^{-1}$. Under the conditional $g_{a\gamma \gamma }$ derived under a specific, conditional modeling framework of the intracluster medium (ICM) kinetic-temperature prescription for the stimulating photon bath, the corresponding limit strengthens to $g_{a\gamma \gamma }<7.33\times {10}^{-12}{\mathrm{GeV}}^{-1}$, lying below the CAST helioscope and horizontal-branch stellar bounds at the high-frequency end of the Coma spectrum, while the lowest-frequency bins remain weaker than these stellar-evolution limits. At $m_a>40\mathrm{\mu }\mathrm{eV}$, the conditional ICM-enhanced limits are comparable to the sensitivity reached by the MADMAX prototype, while providing continuous broadband coverage across this mass range. We further forecast the sensitivity achievable with the Square Kilometre Array (SKA) in its high-frequency bands. Under the same conditional ICM photon-bath prescription, an ideal thermal-noise-limited SKA midfrequency telescope (SKA-MID) band 5b observation with 100 h of integration on Coma would reach $g_{a\gamma \gamma }\sim 7.5\times {10}^{-13}{\mathrm{GeV}}^{-1}$ across $70–130\mathrm{\mu }\mathrm{eV}$, extending broadband sensitivity into a mass range that remains sparsely constrained by present experiments. A systematic comparison across 14 galaxy clusters demonstrates that the luminosity distance to the observer is the dominant figure of merit, with the nearby Ophiuchus cluster ($z=0.028$) emerging as the premier SKA-MID target in our forecast analysis, at a projected limit of $g_{a\gamma \gamma }\sim 2.70\times {10}^{-13}{\mathrm{GeV}}^{-1}$.

Close

PrevNext

View figure in article

• Particle astrophysics
• Particle dark matter