Constraining Axion Mass from Cooling of Neutron Star

Abstract

Neutron star cooling takes place generally via the neutrino and gamma emissions. Here additionally we studied the cooling of neutron star through axion emission via the nucleon-nucleon axion bremsstrahlung processes. We consider the nature of cooling of neutron star including the nucleon-nucleon axion bremsstrahlung process for both the degenerate and non-degenerate cases. The effect of axion cooling is demonstrated through the variation of luminosity with time and variation of surface temperature with time. The thermal evolution of a neutron star is discussed with two neutron star masses (1.4, 1.8 $\mathrm {M}_{\odot }$) and for each of the cases we consider three axion masses namely $m_{a}=10^{-5}\,\mathrm {eV},10^{-3}\,\mathrm {eV}, 10^{-2}$ eV. The results are compared with the case when the neutron star is cooled only by neutrino and gamma emissions and no axion induced cooling is considered. We use three data points from the observations of pulsars namely PSR B0656+14, Geminga, and PSR B1055-52 and compared these with the cooling curve obtained from our calculations. From these analyses, we derive an upper bound on axion mass limit to be $m_{a}\le 10^{-3}\,\mathrm {eV}$, which indicates that the axion decay constant $f_{a}\ge 0.6\times 10^{10}\,\mathrm {GeV}$.