Betelgeuse constraints on coupling between axionlike particles and electrons

Abstract

Axionlike particles (ALPs) can be produced by thermal processes in a stellar interior, escape from the star and, if sufficiently light, be converted into photons in the external Galactic magnetic field. Such a process could produce a detectable hard x-ray excess in the direction of the star. In this scenario, a promising class of targets is the red supergiants, massive stars which are experiencing the late part of their evolution. We report on a search for ALP-induced x-ray emission from Betelgeuse, produced via the combined processes of bremsstrahlung, Compton and Primakoff. Using a 50 ks observation of Betelgeuse by the NuSTAR satellite telescope, we set 95% C.L. upper limits on the ALP-electron (${g}_{ae}$) and ALP-photon (${g}_{a\ensuremath{\gamma}}$) couplings. For masses ${m}_{a}\ensuremath{\le}(3.5--5.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\text{ }\mathrm{eV}$, we find ${g}_{a\ensuremath{\gamma}}\ifmmode\times\else\texttimes\fi{}{g}_{ae}<(0.4--2.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$ (depending on the stellar model and assuming a value of the regular Galactic magnetic field in the direction transverse to Betelgeuse of ${B}_{T}=1.4\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{G}$). This corresponds to ${g}_{ae}<(0.4--2.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ for ${g}_{a\ensuremath{\gamma}}>1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$. This analysis supercedes by over an order of magnitude the limit on ${g}_{ae}\ifmmode\times\else\texttimes\fi{}{g}_{a\ensuremath{\gamma}}$ placed by the CAST solar axion experiment and is among the strongest constraints on these couplings.