Search for Axion-Like Particles from Nearby Pre-Supernova Stars

Abstract: Axion-like particles (ALPs) are hypothetical pseudoscalar bosons that arise in many extensions of the Standard Model and are well-motivated dark matter candidates. Nearby massive stars in the late stages of stellar evolution provide a promising environment for enhanced ALP production due to their high core temperatures and densities. We search for a combined signal of ALP-induced hard X-ray and soft $γ$-ray emission from 18 nearby pre-supernova stars using the full public 22-year INTEGRAL/SPI dataset, construct individual stellar spectra and link them in a coherent analysis. A maximum-likelihood approach is used to extract fluxes in the 20--2000 keV energy range. Stellar evolution models are employed to obtain the expected spectral shapes of ALP production processes peaking between 50--500 keV, depending on stellar mass and evolutionary stage. We construct a joint likelihood that incorporates uncertainties in stellar parameters to derive combined constraints on the coupling constants $g_{aγ}$ and $g_{ae}$ as a function of the ALP mass $m_a$. The hard X-ray and soft $γ$-ray fluxes of all selected stars are consistent with zero within uncertainties. We provide upper limits on the continuum emission and on the 511 keV and 1809 keV line fluxes. The combined upper limit on $g_{aγ} \times g_{ae}$ is $(0.008 - 2) x 10^{-24}$ GeV$^{-1}$ (95% C.I.) while the ALP-photon coupling is constrained to $g_{aγ} = (0.13 - 1.26) x 10^{-11}$ GeV$^{-1}$ (95% C.I.) for $m_a\leqq10^{-11}$ eV, depending on the time to core collapse and magnetic field assumptions. Conservative limits of $(0.27 - 1.25) x 10^{-24}$ GeV$^{-1}$ (95% C.I.) are obtained assuming all but one star are in the early He-burning phase. These results rank among the strongest limits on ALP couplings to date and demonstrate the importance of soft $γ$-ray observations for probing ALPs and massive star evolution.