Neutron Star Cooling via Axion Emission by Nucleon-Nucleon Axion Bremsstrahlung

TL;DR

This paper investigates how axion emission influences neutron star cooling, comparing theoretical models with observational data to set upper bounds on axion mass and decay constant.

Contribution

It introduces a detailed model of neutron star cooling including axion emission, considering both degenerate and non-degenerate limits, and derives new constraints on axion properties.

Findings

Axion emission significantly affects neutron star cooling curves.

Upper bound on axion mass is established at $m_a \,\leq 10^{-3}$ eV.

Derived lower limit on axion decay constant is $f_a \geq 0.6 \times 10^{10}$ GeV.

Abstract

Neutron stars generally cools 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 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, 1.8 solar masses) and by including the effect of axion emission for different axion masses ($m_{a}=10^{-5}\rm{eV}, 10^{-3}\rm{eV}, 10^{-2}\rm{eV}$) 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 B056+14, Geminga and PSR B1055-52 and finally give an upper bound on axion mass limits $m_{a}\leq10^{-3}$ eV which implies that the axion decay constant $f_{a}\geq0.6\times10^{10}\hspace{1mm}\rm{GeV}$.