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 (gae) and ALP-photon (ga gamma) couplings. For masses ma <= (3.5-5.5) x 10-11 eV, we find ga gamma x gae < (0.4-2.8) x 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 BT = 1.4 mu G). This corresponds to gae < (0.4-2.8) x 10-12 for ga gamma > 1.0 x 10-12 GeV-1. This analysis supercedes by over an order of magnitude the limit on gae x ga gamma placed by the CAST solar axion experiment and is among the strongest constraints on these couplings.