Betelgeuse constraints on coupling between axionlike particles and electrons

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_aγ) couplings. For masses m_a ≤ (3.5–5.5) × 10⁻¹¹ eV, we find g_aγ × g_ae < (0.4–2.8) × 10⁻²⁴ GeV⁻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 μG). This corresponds to g_ae < (0.4–2.8) × 10⁻¹² for g_aγ > 1.0 × 10⁻¹² GeV⁻¹. This analysis supercedes by over an order of magnitude the limit on g_ae × g_aγ placed by the CAST solar axion experiment and is among the strongest constraints on these couplings.