Axion spectra and the associated x-ray spectra of low-mass stars

Abstract

Axion particles are among the best candidates to explain dark matter and resolve the strong CP problem in the Standard Model. If such a particle exists, the core of stars will produce them in large amounts. For the first time, we predict the axion spectra and their associated luminosities for several low-mass stars -one and two solar masses stars in the main sequence and post-main sequence stages of evolution. Equally, we also compute the x-ray excess emission resulting from the conversion of axions back to photons in the presence of a strong magnetic field in the envelope of these stars. Hence, a given star will have a unique axion spectrum and La axion luminosity. And if such star has a strong magnetic field in its stellar envelope, it will also show a characteristic x-ray spectrum and Laγ x-ray luminosity. Such radiation will add up to the x-ray electromagnetic spectrum and LX luminosity of the star. The present study focuses on axion models with an axion-photon coupling constant, 5 10 −11 GeV −1 , a value just below the most recent upper limit of 6.6 10 −11 GeV −1 found by CAST and IAXO helioscopes. The range of axion parameters discussed here spans many axion models' parameter space, including the DFSZ and KSVZ models. We found that axions with a mass in the range 10 −7 to 10 −5 eV and an axion-photon coupling constant of 5 10 −11 GeV −1 produce an axion emission spectra with an averaged axion energy that varies from 1 to 5 KeV, and an La ranging from 10 −6 to 7 10 −3 L⊙. We also predict that Laγ varies from 5 10 −8 to 10 −5 L⊙ for stars with an averaged magnetic field of 3 10 4 G in their atmospheres. Most of these Laγ predictions are larger than the LX observed in some stars. Therefore, such preliminary results show the potential of the next generation of stellar x-ray missions to constrain several classes of axion models.