Neutron star cooling via axion emission by nucleon–nucleon axion bremsstrahlung

Abstract

Neutron stars generally cool off by the emission of \(\gamma \) -rays and neutrinos. But axions can also be produced inside a neutron star by the process of nucleon–nucleon axion bremsstrahlung. The escape of these axions adds to the cooling process of the neutron star. We explore the nature of cooling of neutron stars including the axion emission and compare our result with the scenario when the neutron star is cooled by only the emission of \(\gamma \) -rays and neutrinos. In our calculations we consider both the degenerate and non-degenerate limits for such axion energy loss rate and the resulting variation of luminosity with time and variation of surface temperature with time of the neutron star. In short, the thermal evolution of a neutron star is studied with three neutron star masses (1.0, 1.4 and 1.8 solar masses) and by including the effect of axion emission for different axion masses ( \(m_{a}=10^{-5}, 10^{-3}\) and \(10^{-2}\,\mathrm{eV}\) ) and compared with the same when the axion emission is not considered. We compared theoretical cooling curve with the observational data of three pulsars PSR B0656 \(+\) 14, Geminga and PSR B1055-52 and finally gave an upper bound on axion mass limits \(m_{a}\le 10^{-3}\,\mathrm{eV}\) which implies that the axion decay constant \(f_{a}\ge 0.6\times 10^{10}\,\mathrm{GeV}\) .

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